This article gives an overview of
potato genetics and reproductive biology and their implication in conventional
potato breeding. The commonly cultivated potato (Solanum tuberosum ssp. tuberosum
L.) is an autotetraploid (2n=4x=48, 4EBN) with tetrasomic inheritance. Both
interlocus (epistasis) and intralocus (heterozygosity) interactions occur and
the more they are, the greater the heterosis. Favorable traits are fixed in F1
generation due to clonal propagation. Potato exhibits inbreeding depression
characteristic to cross pollinated species yet most of the seeds obtained from
open-pollinated fruits are the result of self-fertilization; flowering and
fruiting are mainly affected by genotype, day length, and temperature.
Continued self-pollination results in inbreeding depression; this results in
reduction of germination percentage, plant vigor, flowering, male fertility,
and open-pollinated fruit set. Hybrids are generally more vigorous than
open-pollinated seeds; the open-pollinated seeds are generally selfs. The
cultivated potato has many wild relatives; which provide genetic diversity as well
as genes for valuable production and quality traits. However, introgression of
useful genes from these wild relatives is difficult due to fertilization
barriers. These barriers include incompatibility between pollen and pistil,
male sterility resulting from interactions between nuclear and cytoplasmic
genes, and differences in endosperm balance number (EBN), or effective ploidy.
These are overcome through use of fertility restorer gene, sexual
polyploidization and somatic polyploidization through protoplast fusion. The
principal method of potato breeding is the conventional hybridization followed
by recurrent selection in the clonal generations. The choice of parents is
determined by the breeding objective; crossability and unrelatedness should
also be considered. The simplest method for predicting the value of cross combinations
is to evaluate progenies at seedling stage. Conventional potato breeding takes
long (about 10 years) before a cultivar is released, mostly due to the slow
multiplication rate of the crop. This time can be reduced through use of marker
assisted selection in identifying parents with desirable traits and selecting
superior clones genotypically at seedlings stage.
Introduction
Worldwide, the cultivated potato of
commerce is Solanum tuberosum ssp. tuberosum L. The Andean
mountains of Peru and Bolivia is the primary center of genetic variability of
tuberous cultivated potatoes, where it has been cultivated for over 2,400 years
(Acquaah 2007). More than 200 potato varieties were developed by the Aymara
Indians on the Titicaca plateau, 10 to 20 degrees south and 3,000 to 4,600
meters above sea level (Sleper and Poehlman 2006). The secondary center of
diversity of cultivated potatoes is in southern South America, particularly in
Chile (Bukasov 1966). From here, the potato was introduced to Europe between
1565 and 1580, by the Spaniards, from where it was introduced into Germany in
the 1620’s and then into France after the seven year war, and thereafter, to
the rest of Europe (Hijmans 2001; Acquaah 2007). It was introduced to Virginia,
USA in 1621 (Sleper and Poehlman 2006; Acquaah 2007). In Africa, it was brought
later by the colonialists. Potato is a crop of major economic importance worldwide
(Tsegaw 2005; FAO 2008). On a global scale, potato is the third most cultivated
food crop after wheat and rice (FAO 2008). Potato is the most important among
root and tuber crops, with an annual production of approximately 330 million tons
grown on about 19.7 million hectares (FAO 2010); it is followed by cassava,
sweet potato, and yam (FAO 2004, 2008). Potato is grown in more than 150
countries worldwide from latitudes 65ºN to 50ºS, and from sea level to 4,000 meters
above sea level (Acquaah 2007). The world average potato production is 17 t
ha–1, while direct consumption as human food is 31.3 kg per capita (kg yr–1)
(FAO 1995, 2004). Potatoes can be grown wherever it is neither too hot (ideal
average daily temperature below 21ºC) nor too cold (above 5ºC), and there is
adequate water from rain or irrigation. On regional basis, Asia and Europe are
the major potato producing regions, accounting for more than 80% of world
production, while Africa produces the least, accounting for about 5% (FAO
2008).
There are approximately 200 wild Solanum
species distributed from the south-western United States to Chile and
central Argentina, and are concentrated in Peru and Bolivia (Spooner and
Hijmans 2001). The geographical distributions of many species overlap (Spooner
and Hijmans 2001). These species are able to maintain their genetic uniqueness
in the absence of geographical separation due to internal reproductive barriers
(Dvøra’ 1983; Singh et al. 1989). These barriers include incompatibility between
pollen and pistil, male sterility resulting from interactions between nuclear
and cytoplasmic genes, and differences in endosperm balance number (EBN), or
effective ploidy (Camadro et al. 2004). The wild Solanum relatives
provide genetic diversity as well as genes for valuable production and quality
traits. In order to utilize the wealth of genetic resources in potato, breeders
and geneticists must first understand the causes of interspecific hybridization
failure. They can then devise strategies to overcome these barriers. This chapter gives an overview of genetics
and reproductive biology of Solanum tuberosum L. and their
implication in potato breeding. Commonly cultivated potato herewith will be
referring to Solanum tuberosum ssp. tuberosum L.
Potato Genetics
The genus Solanum contains over
2000 species, of which only 150 are tuber bearing (Plaisted 1980; Sleper and Poehlman
2006). The common potato, Solanum tuberosum L. belongs to the
tuber-bearing section Petota: this section is subdivided into 21 series
containing 228 wild and 7 cultivated species (Hawkes 1994). Solanum
tuberosum L. belongs to series Tuberosa; this series contains 14
wild and 7 cultivated species (Matsubayashi 1991). The seven cultivated species
are Solanum ajanhuiri, S. chaucha, S. curtilobum, S. juzepczukii, S.
phureja, S. stenotomum and S. tuberosum (Hawkes 1990).
All species of the section Petota have
the same basic chromosome number (x = 12) and they constitute a polyploid
series ranging from diploids (2n=2x = 24) to hexaploids (2n=6x=72) (Douches and
Jastrzebski 1993; Carputo et al. 2003; Carputo and Barone 2005). Five genomes
(A, B, C, D and P) are recognized in the tuber-bearing species of section Petota
(Plaisted 1980; Matsubayashi 1991). All diploid tuber-bearing species
comprise one major genomic group A; no diploid species have ever been identified
with B, C, D and P genomes (Matsubayashi 1991). Two major mechanisms have been proposed
to explain the origin of polyploidy: chromosome doubling of the somatic cells
and formation of unreduced gametes (sexual polyploidization) (Gavrilenko 2007).
Almost all polyploids in nature appear to have originated through sexual
polyploidization (Harlan and Wit 1975). This is particularly true for the
species of section Petota, many of which often form both 2n pollen
and 2n eggs (Watanabe and Peloquin 1991). The 2n gametes provide opportunities
for gene flow between species with different ploidy levels and/or different
endosperm balance number (EBN) (Den Nijs and Peloquin 1977). Thus, in addition
to causing polyploidization, the ability to form 2n gametes also
facilitated interspecific hybridization which has played an important role in
the evolution of wild and cultivated potatoes and in the formation of
polyploidy complexes in the section Petota (Gavrilenko 2007). The
section Petota contains both alloploids and autoploids; the seven cultivated
species are autoploids (Matsubayashi 1991; Gavrilenko 2007). Solanum
tuberosum L. is an autotetraploid with AAAA genome and displays tetrasomic
inheritance ratios (Bradshaw and Mackay 1994). Generally, strict
allotetraploids and allohexaploids species are sexually fertile, show regular
meiosis, are self-compatible and display disomic inheritance (Hawkes 1990;
Douches and Jastrzebski 1993). Allotriploid, allopentaploid and autotetraploid
species have irregular meiosis, are sterile or have very low levels of fertility
(Gavrilenko 2007). Nearly all of the diploid species are out-breeders, with a
single S-locus, multiallelic, gametophytic self-incompatibility system (Dodds
1965; Hawkes 1990). One hundred and eighty one tuber-bearing species of Solanum
have known ploidy levels: 76% are diploids, 3% triploids (2n=3x=36), 12%
tetraploids (2n=4x=48), 2% pentaploids (2n=5x=60) and 7% hexaploids (2n=6x=72)
(Hawkes 1990; Spooner et al. 2004). The three commonly cultivated diploid
species are Solanum stenotomum, Solanum phureja, and Solanum
ajanhuiri (Sleper and Poehlman 2006). Solanum stenotomum is adapted
to high altitude areas in southern Peru and Bolivia. Solanum phureja is
widely used in bridge-crossing, and as a source of resistance to bacterial wilt
(Fock et al. 2000, 2001). It is adapted to lower altitudes in the frost-free
Andean valleys of Venezuela, Colombia, Ecuador and northern Peru (Hawkes 1956).
It lacks tuber dormancy and can be replanted immediately in areas where continuous
cropping is possible (Plaisted 1980). There are two cultivated triploid
species, Solanum chaucha, and Solanum juzepczukii, one tetraploid
species S. tuberosum, and one pentaploid species Solanum curtilobum (Hawkes
1990). A common wild allohexaploid, Solanum demissum, is the source of
the major R genes that confer resistance to late blight of potatoes (Carputo et
al. 2003; Sleper and Poehlman 2006).
The commonly grown potato, Solanum
tuberosum L., is an autotetraploid (2n=4x=48, 4EBN) species
that displays tetrasomic inheritance (Bradshaw and Mackay 1994; Sleper and
Poehlman 2006). The species has a monophyletic origin, which means that it
developed out of one wild plant, and hence it has a narrow genetic diversity.
There are two major subspecies of Solanum tuberosum; andigena or
Andean, and tuberosum or Chilean (Raker and Spooner 2002). The Andean
potato is adapted to the short-day conditions prevalent in the equatorial and
tropical regions where it originated (Raker and Spooner 2002). It is indigenous
to the Andean region from Venezuela to northern Chile and Argentina (Hawkes
1990). The Chilean potato is adapted to the long-day conditions prevalent in
the higher latitude region of southern Chile, especially Chiloé Island and
Chonos Archipelago, where it is thought to have originated (Hawkes 1990;
Hijmans 2001). The genetic relationship between subspecies tuberosum and
andigenum is unresolved (Raker and Spooner 2002). The transition from
subspecies andigena to subspecies tuberosum apparently resulted
from transporting material from the short-day environment of the
Peruvian/Bolivian Andes to the long days conditions. This transportation
accompanied by adaptation is believed by Hawkes (1990) to have occurred twice:
the first event would have taken place in Chile where original subspecies andigena
material, brought here by migrating Indian tribes from the Andes, underwent
adaptation to long-days and cool climatic conditions. The second time this
development took place was in Europe after the Spaniards introduced potato
there. Grun (1990), suggested that tuberosum was distinct from andigenum
based on cytoplasmic sterility factors, geographical isolation, and
ecological differences. Solanum tuberosum ssp. tuberosum contains at
least seven different cytoplasmic sterility factors (Sps, Sms, Ins, TAs, ASFs,
VSAs and Fms) that conditions sterilities in presence of dominant chromosomal
genes (Sp, SM, In, TA, ASF, VSA and Fm) that occur in ssp. andigena (Grun
et al. 1977). The cytoplasmic sterility sensitivity factors that are typical of
ssp. tuberosum have not been found to occur in ssp. andigena. In
addition, the two subspecies are well separated geographically in their native
areas with ssp. andigena at high altitudes in the northern and central
Andes Mountains and ssp. tuberosum at sea level in southern Chile. There
are also well developed internal isolation barriers separating the two
subspecies because the hybrids ssp. tuberosum x ssp. andigena are
usually male sterile and often female sterile (Grun et al. 1977; Grun 1979).
Hawkes (1990) distinguished the two subspecies on the grounds that subspecies tuberosum
has fewer stems, more horizontal foliage, less-dissected leaves, wider
leaflets, and thicker pedicels than andigenum. In addition, subspecies andigenum
has five chloroplast genotypes (A, C, S, T, and W) while subspecies tuberosum
has only three (A, T, and W) (Hosaka and Hanneman 1988).
In Solanum tuberosum there can
be four different alleles at a locus (Ross 1986). The tetraploid nature of
cultivated potato can be exploited by the breeder to improve desirable
characteristics. Because of the potato’s autotetraploid nature, intralocus
interactions (heterozygosity) and interlocus interactions (epistasis) occur,
and are important when selecting breeding procedures to improve certain traits.
Heterozygosity in potato is attributed to self-incompatibility and has most
likely been enhanced by millions of years of asexual propagation by tubers.
Selection based on maximum heterozygosity, rather than additive genetic
variance, is critical in potato breeding, especially for quantitative traits.
It is assumed that increased heterozygosity leads to increased heterosis
(Bradshaw and Mackay 1994; Sleper and Poehlman 2006). Heterosis in potato is
when the progeny surpasses the value of the best parent or the parental mean. The
exploitation of heterosis is by far the most important goal in potato breeding.
The inheritance of heterosis is by minor genes or by the side effects of the
major genes. Their action can proceed in an additive (general combing ability)
or in a non-additive manner (specific combing ability): in most case both
operate (Ross 1986). Heterosis in potato is based mainly on non-additive
interactions of genes and it comprises intralocus (over dominance) as well as
interlocus (epistasis) interaction between genes and alleles (Ross 1986).
Asexually propagated species such as potatoes have evolved taking advantage of
non-additive or epistatic gene action (Sleper and Poehlman 2006). The level of
heterozygosity in potatoes is influenced by how different the four alleles are
within a locus; the more diverse they are, the higher the heterozygosity and
the greater the number of interlocus (epistatic) interactions and hence the
greater the heterosis (Ross 1986; Bradshaw and Mackay 1994; Sleper and Poehlman
2006). To see how increased heterozygosity can lead to more epistatic
interactions, it is necessary to identify the allelic conditions possible in an
autotetraploid (Caligari 1992; Sleper and Poehlman 2006). Five tetrasomic
conditions are possible at an individual locus in an autotetraploid (Table 1).
It is hypothesized that the
tetraallelic condition provides the maximum heterosis because more interlocus
interactions are possible for this tetrasomic condition than for the other
configurations (Ross 1986; Sleper and Poehlman 2006). For example, in the tetraallelic
condition, the six first-order interactions are: a1a2, a1a3, a1a4, a2a3,
a2a4, a3a4. The four second-order interactions are: a1a2a3, a1a2a4, a1a3a4,
a2a3a4. The one third-order interaction is a1a2a3a4. There are a total of 11
different interactions possible for the tetraallelic condition. This is in
contrast to the monoallelic condition, which has no interactions. The highest
level of heterosis will occur as the frequency of tetraallelic loci increase.
The greatest number of interlocus interactions will also occur as the frequency
of tetraallelic loci increase. In breeding potatoes for higher tuber yields,
inter- and intralocus interactions have been shown to be important; procedures
that maximize the frequency of tetraallelic loci should be considered in
breeding potato for increased yields (Ross 1986; Bradshaw and Mackay 1994;
Sleper and Poehlman 2006). Therefore, the segregation of heterotic seedlings in
a population is likely to be greatest when three conditions are fulfilled: 1)
the parents possess as low a coefficient of inbreeding as possible, 2) as many
loci as possible have different alleles and, 3) the parents belong to different
gene pools which improves the chances of allelic diversity, i.e., wide hybridization
(should be as unrelated as possible) (Ross 1986). In potatoes, heterosis is of
direct relevance for improving traits under consideration as it gets fixed in
F1 generation (Upadhya and Cabello 2000) due to vegetative propagation of the
crop. Because potato is a highly heterozygous crop, an increase in heterozygosity
results in heterosis. Distantly related genotypes are more complementary and
they produce heterotic progenies (Ross 1986).
Reproductive Biology
in Cultivated Potato
It is difficult to classify the
cultivated Solanum tuberosum L. as to the extent of natural cross
pollination. It exhibits inbreeding depression characteristic to cross
pollinated species yet most of the seeds obtained from open-pollinated fruits
are the result of self-fertilization (Plaisted 1980). Potato is predominantly
self-pollinated, although some cross pollination is often accomplished by bumblebees
(Caligari 1992; Acquaah 2007). Wind pollination plays a minor role in nature.
Outcrossing is enforced in cultivated (and most wild) diploid species by a
single S-locus, multiallelic, gametophytic self-incompatibility system (Dodds
1965). While self-incompatibility does not operate in S. tuberosum, 40%
(range 21–74%) natural cross-pollination was estimated to occur in ssp.
andigena in the Andes (Brown 1993) and 20% (range 14–30%) in an artificially
constructed andigena population (Glendining 1976).
Flowering
Potato has a terminal inflorescence
consisting of 1 to 30 (but usually 7 to 15) flowers, depending on cultivar
(Plaisted 1980; Acquaah 2007). The flower is 3–4 cm in diameter and contains five
sepals and petals, and a bilobed stigma (Acquaah 2007). The five petals give
the flower a star shape (Plaisted 1980; Caligari 1992). The petals vary in size
with the cultivar, and the color varies from white to a complex range of blue,
red, and purple (Sleper and Poehlman 2006; Acquaah 2007). The petals are united
and tubular. The stamens are attached to the corolla tube and bear erect
anthers (Almekinders and Struik 1996). The anthers are bright yellow except for
those produced on male sterile plants, which are light yellow or yellow-green
in color; the stigma protrudes above a cluster of large, bright yellow anthers
(Sleper and Poehlman 2006).
Flowers open starting with those
nearest the base of the inflorescence proceeding upwards at a rate of about 2–3
flowers per day (Acquaah 2007). At the peak bloom, there are usually 5–10 open
flowers (Caligari 1992; Acquaah 2007). Flowers stay open for only 2–4 days,
while the receptivity of the stigma and duration of pollen production is about
two days (Sleper and Poehlman 2006). Flowers open mostly in the early morning,
although a few may continue to open throughout the day (Sleper and Poehlman 2006).
Genotype, day length, light intensity and temperature are the main factors that
determine flowering and fruiting in potato (Turner and Ewing 1988; Sleper and
Poehlman 2006). Flowering in potato is best when long days (around 16 hours),
abundant moisture, and cool temperatures prevail (Almekinders and Struik 1996;
Sleper and Poehlman 2006). Photoperiod of 12–14 hours and night temperatures of
15–20°C has been shown to favor flower production and berry setting in potato
(Almekinders 1992; Gopal 2006). Short days during flowering may lead to
abscission of the floral bud, giving the impression that the cultivars do not
flower well (Almekinders and Struik 1996). Therefore, in tropics and
sub-tropics, conditions conducive for flowering and fruiting are available only
at high altitudes (>1500 m above sea level) (Gopal 1994). Turner and Ewing
(1988) found that reducing light intensity completely suppressed flower
development; all floral buds aborted at an early stage of development. A wide
genetic variability has been observed for days to flowering, duration of
flowering, flowering intensity, and berry setting (Gopal 2006). Gopal (1994)
conducted a survey of the flowering behavior, male sterility, and berry setting
in 676 accessions of tetraploid Solanum tuberosum from 25 countries. He
conducted his studies outdoors at Kufri in India (32ºN, 70ºE, 2500 masl) during
summer (12.1–14.1 hours of day length). He found that flowering intensity
ranged from the dropping of floral buds just after initiation to profuse
blooming. Majority of the accessions (58.3%) bloomed profusely, but 20.4% did
not bloom at all. Time to flowering ranged from 6 to 15 weeks and duration of
flowering from 1 to 10 weeks. Pollen stainability (as an indicator of fertility)
ranged from 0 to 90%, with 23% of the accessions being completely male sterile.
Berry setting ranged from zero to more than five berries per plant, with none
in 31.8% of the flowering accessions. Therefore, only 54.3% of the accessions were
found to be fertile in all respects and could be used as male and female parents.
He also found that premature bud abscission was the major cause of sterility.
In addition to genotype, temperature, and photoperiod, other factors such as
inflorescence position (Almekinders and Wiersema 1991), plant/stem density
(Almekinders 1991), competition between flower and tuber (Thijn 1954),
precipitation and date of planting (Jauhnl 1954) and soil nutrient level
(Bamberg and Hanneman 1988) are also known to influence production of flowers
and fruits in potato. Increasing stem density has been shown to decrease the
number of flowers per plant, berry set and seed production from every
inflorescence (Mok 1985; Almekinders 1991; Almekinders and Wiersema 1991).
Increasing plant density was shown to increase the proportion of primary
flowers in the total number of flowers per plant and reduced the proportion of
flowers on lateral stems (Almekinders 1991). However, increasing plant density
was shown to increase the number of flowers and seed weight per m2 (Mok 1985).
Nitrogen enhances the export of cytokinins from the roots to the shoots
resulting in delayed senescence of the plants. Berries therefore have a longer
time to mature on the mother plants and a better chance for high quality seed
production (Van Staden et al. 1982). Studies have shown that periodic
supplemental nitrogen applications to the soil during seed development at rates
higher than recommended for tuber production enhance flowering and delay plant
maturity thereby prolonging the berry development period (CIP 1985). Flower
production was increased by more than three times when supplemental N rates up
to a total of 240 kgha–1 were applied at weekly intervals. In addition, significant
increases in 100-seed weight were obtained with supplemental application of N
(Pallais 1986). In a subsequent experiment, the highest 100-tps weight was
found in plants receiving N at 600 kg ha–1. Therefore N rates greater than those
required for tuber production enhance quality TPS production.
Some techniques that are used by
breeders to enhance flowering and seed set include shading of glasshouses to
reduce temperature below 22ºC, girdling or constriction of the stem, and
grafting of young potato shoots onto tomato or other compatible Solananceous
plants (Caligari 1992; Sleper and Poehlman 2006). However, the last method
gives weaker growth (Gopal 1994). Another method includes growing the seed
tuber on a brick and the brick is covered with sand and peat. The roots grow
over the brick and when they have penetrated the soil on which the brick is
lying, the covering sand is washed away. The tips of the stolons, which would
otherwise produce new tubers, are then removed. This promotes vigorous stem
growth and enhances flowering (Gopal 1994). Other methods which have been
reported include foliar spray with GA3 (El-Gizawy et al. 2006).
Pollination in
Cultivated Potato
Controlled pollination may be done in
the field or in the greenhouse under controlled conditions. Crosses made in the
field are liable to suffer losses from wind, rain, heat and drought and most
breeders therefore prefer to work in the greenhouse. The best time for crossing
is early morning when temperatures are not high (Acquaah 2007). Prior to
crossing, flower buds that are mature and plump with the petals ready to
separate are selected for emasculation (Acquaah 2007). If pollinations are done
in the field, it is important to emasculate just prior to crossing as the wind
can break off the stigma before pollination occurs if they are emasculated too
far ahead of pollination. Normally, the unopened flowers of the female parent
must be emasculated one to two days before crossing to avoid self-contamination.
For emasculation, choose the bud that has developed the petal color but is unopened.
Pull back the petals carefully to expose the immature anthers; pull-off all the
anthers carefully with a blunt scalpel or tweezers. To facilitate emasculation
of the selected buds, and to prevent contamination of the emasculated flowers
by the open flowers, the remaining buds and open flowers in the inflorescence
should be removed (Sleper and Poehlman 2006; Acquaah 2007). Removing the extra
flowers increases the chance that pollination will be successful and reduces
competition for photoassimilates (Almekinders and Struik 1996). In the male
plant, look for a newly opened flower and pull off the anthers. Slit open the
anthers using a blunt scalpel to collect pollen; dab the pollen onto stigma of
the emasculated female flower and label it; pollinate again the following day
(Plaisted 1980). Flowers with plump
bright yellow anthers and brown tips are most apt to be good sources of pollen.
Pollen is most abundant in the morning (Plaisted 1980). Potato pollen is robust
if it kept cool and dry. It can be kept for a month if store at 2.5ºC or a year
at –24ºC (Blomquist and Lauer 1962). Pollen viability is prolonged if stored
under dry conditions such as in a desiccator containing silica gel (Howard
1958). Pollen stored at room temperature and humidity losses viability after
one day.
Alternatively, open flowers are
collected from the male plant and laid out to dry overnight (Almekinders and
Struik 1996). In the following morning, the pollen is collected from them by
shaking into gelatine capsules or small tubes (Sleper and Poehlman 2006). To
pollinate, the stigma is dipped into the pollen and then the pollination tag is
attached. The emasculated flowers do not need covering to avert contamination.
Germination of the pollen is completed after 30 minutes, and the ovary is
fertilized within 12 hours (Bradshaw and Mackay 1994). The berries appear
within a few weeks and to prevent losses, they are bagged with nylon netting of
large mesh. Potato fruits (berries) contain about 50 to 500 seeds with an
average of 200 seeds (CIP 1984). Pollinations can also be done on flowers
attached to stems that have been cut and placed in jars of water with an
anti-bacterial agent to reduce contamination (Peloquin and Hougas 1959;
Wolfgang et al. 2009).
Infertility Problems
in Cultivated Potato
Biological seed production in potatoes
is low due to: failure of plants to flower, dropping of buds and flowers either
before or after pollination, low pollen production, failure to produce viable
pollen, male sterility, and self-incompatibility (Sleper and Poehlman 2006).
Because open pollinated berries in potato result predominantly from self-pollination,
formation of open pollinated berries indicates that the genotype bearing them
is both male and female fertile, self-compatible, and that after fertilization,
flowers do not drop, but develop into fruits. Non-formation of berries, on the
other hand, can be due to any one or more of the causes listed above. Berry
setting is the ultimate test of fertility if these berries carry seed in them,
which is generally the case (Gopal 1994). Hence, from a practical point of
view, genetic blockage at any stage from floral bud initiation to seed set
should be considered as sterility.
Male sterility
In male-sterile plants, flowers do not
produce functional anthers or viable pollen but the ovaries function normally.
Male sterility may be controlled by the action of the nuclear genes (genetic
male sterility) or by the cytoplasm (cytoplasmic male sterility) (Sleper and
Poehlman 2006). Male sterility due to deleterious nuclear genes is a very
common and serious constraint in potato breeding (Gopal 2006). The deleterious
recessive alleles can accumulate in tetraploid potato cultivars because they
are more easily masked than in diploids. The failure to produce pollen may be
an inherent characteristic with sterility being dominant over fertility.
Presence of a tetrasomic gene, which is lethal when present in a homozygous
condition, or partly lethal when present in a heterozygous condition has also
been reported (Sleper and Poehlman 2006). Though both pollen and ovule
sterility can occur, pollen sterility ranging from partial to complete absence
of pollen grains is very common in potato (Pushkarnath and Dwivedi 1961).
Almost one third of the potato cultivars derived from Solanum tuberosum ssp.
tuberosum do not form berries (Ross 1986). Male sterility in potatoes is
probably controlled by more than one gene, with partial dominance or by
nuclear-cytoplasmic interactions (Howard 1978) and depends partially on environmental
factors. In cultivated S. tuberosum potato seven types of cytoplasmic
factors exists each of which condition, in combination with a specific
genotype, a specific type of sterility (Grun 1970). These cytoplasmic factors
are: indehiscence (Ins), sporads (Sps), shriveled microspores (SMs), anther
style fusion (Afs), thin anthers (TAs), females sterility (Fms), and deformed
flower (dfs) (Gopal 2006). Variations, depending on the stage at which
development is blocked have also been observed within a particular type of
sterility. A block can occur at any place in the development process and
depending on the particular gene-plasmon interaction involved, different kinds
of blockage may occur (Grun 1970, 1990). The prevalence of sterility to such a
large extent limits the use of many germplasm clones as parents, but at the
same time this is advantageous because males sterile clones do not require
emasculation when used as females in controlled pollinations (Gopal 2006).
Because the marketable product in potato is not seed, there is no selection
pressure for high fertility in breeding programs. In fact, fruit development
may partition resources away from tuber yield, so breeders may inadvertently
select against high fertility (Jansky and Thompson 1990). Levels of cytoplasmic
genetic male sterility are frequently variable, presumably due to genetic and
environmental influences (Hanneman Jr. and Peloquin 1981). Breeders can,
therefore, overcome this type of sterility by either carrying out reciprocal
crosses or selecting parents that do not contain sensitive cytoplasm or
dominant nuclear sterility genes (Iwanaga et al. 1991; Tucci et al. 1996).
Fertility restorer genes have been found for the latter (Jansky 2009). For
example, there is a dominant gene (Rf) that restores fertility to plants
that contain the dominant male sterility gene (Ms) in the presence of
sensitive cytoplasm (Iwanaga et al. 1991).
Incompatibility
Crossability in a broad sense can be
defined as any natural or artificial fusion of two genetically different cells
leading to hybrid progeny. Incompatibility is a form of infertility caused by
the failure of plants with normal pollen and ovules to set seed due to some
physiological hindrance that prevents fertilization. Incompatibility may be
caused by failure of pollen tube either to penetrate the stigma or to grow
normally the full length of the style so that fertilization may occur (Sleper
and Poehlman 2006). Incompatibility restricts self-fertilization and inbreeding
and fosters cross-fertilization and outbreeding. Incompatibility can be
bilateral (in both directions) or unilateral (in only one direction); both
self- and cross- incompatibility governed by a series of allelomorphs are
widely prevalent in potato. Self and cross incompatibility are closely
interrelated: self-compatible species (SC) will cross with other
self-compatible species; self-incompatible (SI) species will cross with other
self-incompatible species; self-compatible species will cross as females with
self-incompatible species, but self-incompatible species as females will not
cross with self-compatible species (Hanneman Jr. 1999). This SC x SI and SI x
SC reciprocal differences rule was termed unilateral incompatibility (Lewis and
Crowe 1958). Unilateral incompatibility is a phenomenon in which SC species can
be crossed as a female, but not as a male, to SI species (Abdalla and Hermsen
1972). The cultivated diploids are obligate out-breeders due to self-incompatibility
governed by the S locus system (Simmonds 1997; Hosaka and Hanneman 1998). This
incompatibility system does not operate in tetraploids. If effectively
pollinated, the sequence of decreasing seed fertility (i.e., seed production
per plant) goes: diploids>andigena>tuberosum. The least
fertile class is certainly the 4x Solanum tuberosum subspecies tuberosum.
Experience supports the assertion as to the superior fertility of andigena
over tuberosum group. There is a widespread view that the cross tuberosum x
andigena gives progeny superior to the reciprocal andigena x
tuberosum. However, the matter is still unresolved (Simmonds 1997). In
general the tetraploids bear fewer seeds per berry but larger than the
diploids. Seed size in any potato has a large maternal element in its
determination even though seed numbers per berry are bi-parentally controlled
(Simmonds 1995). While self-incompatibility does not operate in tetraploid Solanum
tuberosum, 40% (21 to 74%) natural cross pollination was estimated to occur
in subsp. andigena in the Andes (Brown 1993) and 20% (14 to 30%) in an
artificially constructed andigena population (Glendining 1976). The
cultivated Solanum tuberosum is a tetraploid in which recombination
among all the four homologs is possible (Bradshaw and Mackay 1994). Such
species do not exclusively self-fertilize in their natural habitats (Brown
1993) and they maintain high levels of heterozygosity across generations
(Hosaka and Hanneman 1998).
Endosperm balance
number hypothesis
In angiosperms, double fertilization
results in the production of an embryo and endosperm, both of which are
critical for the development of viable seed. Successful development of embryos
and seeds requires proper endosperm development (Sleper and Poehlman 2006). The
endosperm (3n) is formed as a result of fertilization of the polar nuclei or
central nucleus (2n) by a male nucleus (n). The embryo (2n)
results from the fertilization of an egg (n) by a male nucleus (n). The
endosperm balance number (EBN) hypothesis states that normal endosperm
development occurs when the ratio of maternal to paternal EBN contribution to
their progeny is 2:1 (Johnston et al. 1980). Any deviation from this ratio (2
EBN maternal: 1 EBN paternal) will result in no seed set. Intraspecific
intraploidy crosses in potato typically produce viable seeds containing
well-developed endosperm. Conversely, in most interploidy crosses, unviable
seeds are produced due to endosperm failure (Brink and Cooper 1947). However,
endosperm may also fail to develop adequately in some intraploidy, interspecific
crosses, while some interploidy crosses succeed. A 2:1 maternal: paternal ratio
of endosperm balance factors, rather than genomes, is necessary for normal
endosperm development in potato (Johnston et al. 1980). The EBNs are
independent of ploidy levels but have been described as the “effective ploidy”
of the parent (Hanneman Jr. 1999). The nature of these endosperm balance
factors has yet to be elucidated although genetic models have been proposed
(Ehlenfeldt and Hanneman Jr. 1988a; Camadro and Masuelli 1995). Solanum species
have been assigned endosperm balance numbers (EBN) based on their ability to
hybridize with each other (Hanneman Jr 1994). Barring other crossing barriers, viable
seeds will be produced from crosses between plants with matching EBN values.
This will produce a 2:1 maternal: paternal ratio of endosperm balance factors
after fertilization of the central cell to produce endosperm. The most common
ploidy, EBN combinations in potato are 6x (4EBN), 4x (4EBN), 4x (2EBN), 2x
(2EBN) and 2x (1EBN) (Hawkes and Jackson 1992). Breeders use EBN values to
determine whether interspecific crosses will succeed. The EBN concept also
allows them to design strategies to access wild germplasm by manipulating EBN
(Johnston et al. 1980). Endosperm balance number can be increased through
somatic doubling (Ross et al. 1967; Sonnino et al. 1988) or the production of 2n
gametes. Endosperm balance number can be reduced through anther culture or
parthenogenesis (Veilleux 2005). Furthermore, embryo rescue can be used to
secure a hybrid where embryo abortion is due to a defective endosperm (Jansky
2006). As the largest compatibility group is EBN = 2, it is now common for
potato breeders to secure tetraploid hybrids from 4x (S. tuberosum)
× 2x (2x S. tuberosum × wild species) crosses in which an
unbalanced endosperm prevents the development of triploid embryos.
Another problem in pollinating potato
is poor nicking (i.e., unsynchronized flowering of the parents). This can be
prevented by planting both the male and female parents in a greenhouse. The
luxuriant growth of plants in the greenhouse ensures a long period of pollen
production, which can be stored under appropriate conditions for later
pollination. Pollen can be stored desiccated in the refrigerator for 1 to 2
weeks and in the freezer for 6 months to one year (Sleper and Poehlman 2006).
Overcoming
Crossability Problems Sexual Polyploidization (Production of 2n gametes)
Specific genes have been identified in
wild and cultivated species that produce unreduced male gametes by at least
three different mechanisms (Mok and Peloquin 1975b). The three distinct meiotic
mutations during microsporogenesis are parallel spindles (ps) (including
also fused and tripolar spindles), premature cytokinesis-1 (pc-1), and
premature cytokinesis-2 (pc-2), all inherited as simple Mendelian
recessives (Mok and Peloquin 1975a). When these mutations are present at the
end of meiosis, dyads with two 2n microspores are formed. The ps is
the most common mutation leading to 2n pollen production in the potato
(Watanabe and Peloquin 1991, 1993). The ps gene causes the spindles of
the two second meiotic metaphases, which normally are perpendicular to one
another in the same cell, to be parallel. The chromosomes of the two second
meiotic metaphase plates, under the influence of ps gene go to two
instead of four poles resulting in restitution of the 2n chromosome
number. Although this restitution occurs in the second meiotic division, it is
referred to as first division restitution (FDR) because it brings back together
in one nucleus most of the genes, proximal to the kinetochore, that were
separated in the first meiotic division. All loci from the centromere to the first
crossover that are heterozygous in the parent will be heterozygous in the
gametes, and half the heterozygous parental loci beyond the first crossover
will be heterozygous in the gametes (due to small chromosome size, there is
normally only one cross-over per chromosome arm). The result of FDR is
formation of unreduced and highly heterozygous male gametophytes that grow
vigorously down the style (Simon and Peloquin 1976). The heterozygosity of the
male unreduced gametes in turn results in vigor of the offspring plants (Mok
and Peloquin 1975a). Several mutations leading to 2n egg formation have
also been found (Werner and Peloquin 1991). Meiotic mutations affecting
megasporogenesis and resulting in formation of unreduced eggs is most commonly
the omission of the second meiotic division, which is genetically similar to
SDR (Werner and Peloquin 1987). The recessive gene (os) controls the
formation of unreduced gametes by this mechanism. While the genetic consequence
of 2n pollen formation in potato is typically FDR that of 2n egg
formation is second division restitution (Werner and Peloquin 1990). The
combined presence of os gene producing unreduced eggs and ps genes
producing unreduced male gametes has resulted in production of 4x following
2x x 2x crosses. Formation of unreduced gametes by FDR and SDR
allows the transfer of large portions of intralocus (heterozygous) and
interlocus (epistasis) interactions from the 2x parent to the resulting
4x progeny. This is in contrast with normal meiosis in 2x parents
which would transfer little or no intralocus and interlocus interactions. The
genetic consequences of FDR 2n gametes are very different from those of
SDR 2n gametes. In an FDR 2n gamete, all loci from the centromere to the
first crossover on each chromosome have the same genetic constitution as the
parent of that gamete. That is, all dominance (intralocus) interactions up to
the first crossover are maintained in the gametes. Even in the chromosomal
region beyond the first crossover, half of the loci that were heterozygous in
the parent will remain so in 2n gametes. Since potato chromosomes are small,
there is typically only one crossover per chromosome (Yeh et al. 1964; Carputo
et al. 2003). Consequently, FDR 2n gametes provide a unique and powerful
method of transmitting blocks of advantageous dominance (intralocus) and
epistatic (interlocus) interactions to polyploid offspring even following
meiosis, which usually breaks up such interactions. In contrast, SDR 2n gametes
contain non-sister chromatids from the centromere to the first crossover. It
has been estimated that FDR can transfer 80% of the heterozygosity and a
significant portion of epistasis from parent to progeny. The SDR is less efficient
and transfers less than 40% of the heterozygosity of the 2x female
parent to the 4x progeny. Both FDR and SDR allow breeders to transfer
desirable linkage groups and gene interactions intact from parent to offspring
without having them broken up through the normal meiosis (Sleper and Poehlman
2006). This is important because the potato is clonally propagated; once
heterosis if fixed in F1, it is not broken up again. Formation of unreduced
gametes by either male or female parent is called unilateral sexual
polyploidization (USP); simultaneous occurrence of unreduced gametes in the
male and female parent is bilateral sexual polyploidization (BSP) (Sleper and
Poehlman 2006). Unilateral sexual polyploidization offers a modified form of
conventional breeding that can maximize the effects of heterosis. Exceptionally
high tuber yields have been observed in tetraploid (2n = 4x = 48) progenies
obtained from 4x X 2x mating in potatoes (Hanneman Jr and Peloquin 1967, 1968;
Mok and Peloquin 1975a). The progeny of 4x X 2x crosses are typically vigorous and
relatively uniform for high tuber yield, which may at first seem surprising,
considering the heterozygosity of the parents. The heterotic response is most
commonly observed when the tetraploid is used as the female parent and the
diploid parent produces 2n pollen by FDR (Jansky 2006). In addition,
families from 4x X 2x (FDR 2n pollen) crosses out-yield 4x X 2x (SDR 2n pollen)
and 4x X 4x families by about 50% (Mok and Peloquin 1975b). Because intralocus
and interlocus interactions contribute to high yield in potato, this significant
increase in yield by 4x X 2x (FDR 2n pollen) hybridization is most
likely due to the increase in transmission of heterozygosity and epistasis by
2n FDR gametes (Mendiburu and Peloquin 1977). However, high tuber yield must be
accompanied by acceptable tuber quality. Previous reports showed that a large
number of haploid (2x) X diploid S. chacoense hybrids used in USP
schemes produced tetraploid offspring with good tuber appearance and size,
along with high yield (Schroeder and Peloquin 1983). Bilateral sexual
polyploidization provides an alternative sexual polyploidization strategy. In
this scheme, both parents are diploid and produce 2n gametes. The
potential advantage of bilateral sexual polyploidization is that highly
heterotic offspring can be produced by crossing diverse diploid parents. The
disadvantage is that both parents must produce 2n gametes. In addition,
since the meiotic mutations that produce 2n gametes exhibit variable
expressivity (n and 2n gametes are produced by the same plant),
both diploid and tetraploid offspring will be produced. Tetraploid progeny from
bilateral sexual polyploidization are highly heterotic and typically out-yield
their diploid full-sibs (Mendiburu and Peloquin 1977) and even tetraploid
commercial cultivars (Werner and Peloquin 1991c). The yield gains from
bilateral sexual polyploidization are typically higher than those from
unilateral sexual polyploidization, presumably due to the contributions of
heterozygosity from both parents (Werner and Peloquin 1991c).
Endosperm balance number can be reduced
through anther culture or parthenogenesis. The production of haploids through
anther culture is possible, but can be difficult because it requires the
presence of genes for androgenic competence, which are not always found in
potato cultivars (Sonnino et al. 1989). In contrast, it is relatively easy to
produce parthenogenetically derived haploids from Tuberosum Group tetraploids by
crossing them to selected pollinators (Hougas et al. 1958). These 2x (2EBN)
haploids cross readily to 2x (2EBN) wild species, often producing hybrids with
good yield, adaptation and fertility (Yerk and Peloquin 1986; Hermundstad and
Peloquin 1986). Seed set when haploids are crossed to wild species is similar
to that when cultivars are intercrossed (Budin and Gavrilenko 1994). The
production of haploids is dependent on a pollination mechanism which permits
fertilization of the central cell but not the egg, allowing the egg to develop
into a plant through parthenogenesis (Hougas et al. 1958). Haploids (2n=2x=24)
of the common potato (2n=4x=48) have been obtained in large numbers from a wide
range of parental clones (Hougas et al. 1964). They have been successfully
hybridized with cultivated and wild, 24-chromosome, tuber-bearing Solanum species
from Mexico and South America.
Polyploidization through
Somatic (protoplast) fusion
Diploid hybrids can be somatically
doubled through chemical means such as colchicine (Ross et al. 1967) or through
tissue culture (Sonnino et al. 1988) to bring them to teraploid level. However,
tetraploids produced by this method do not exhibit a yield increase because new
interlocus and intralocus interactions are not created (Rowe 1967; Maris 1990;
Tai and Jong 1997). Somatic doubling can produce only one type of heterozygote
(duplex- AAaa) and a maximum of two alleles per locus. A common somatic fusion strategy
fuses protoplasts of tetraploid cultivars with those of sexually incompatible
diploid wild species. The resulting hexaploid hybrids are often fertile and
crossable with the tetraploid cultivars (Carputo et al. 1997). The pentaploid
offspring are also fertile and tetraploid clones are recovered after a few
backcrosses. Alternatively, diploid cultivated potato clones can be fused with
diploid wild species to produce tetraploid hybrids (Rokka et al. 1994; Carputo
et al. 1997). However, hexaploid somatic hybrids from 4x + 2x fusions are
typically more successful in crosses to cultivars than are tetraploid hybrids
from 2x + 2x fusions (Helgeson et al. 1988). Most somatic fusions have been
carried out to capture disease resistance genes, but somatic fusion hybrids
with improved salt tolerance have also been developed (Bidani et al. 2007).
While chromosomes from both parents are typically found in somatic fusion
hybrids, the genetic contributions of some wild species are lost more rapidly
than others in backcross generations (Naess et al. 2001). Somatic fusion has
allowed the production of fertile hexaploid hybrids between tetraploid S.
tuberosum (EBN = 4) and diploid EBN = 1 species, such as the
non-tuber-bearing species S. brevidens that has tuber soft rot and early
blight resistances (Tek et al. 2004) and S. bulbocastanum that has a
major gene for broad spectrum resistance to late blight (Naess et al. 2000).
Conventional Potato
Breeding
Conventional potato breeding involves
initial crossing of parents possessing complementary traits followed by
selection in the subsequent clonal generations (Sleper and Poehlman 2006;
Bradshaw and Bonierbale 2010). Because the crop is generally vegetatively
propagated, favorable traits are fixed in the F1 generation. The clones are
highly heterozygous and exploit heterosis. However, progenies produced by selfing
of clones reveal strong inbreeding depression (Arndt and Peloquin 1990). For
effective potato breeding, there is need to understand the crop’s reproductive
biology as well as the breeding procedures.
Selection of Parents
and Prediction of Cross Outcome
Making crosses between pairs of parents
with complementary features has traditionally been, and still is the main route
for the development of new cultivars (Caligari 1992; Sleper and Poehlman 2006;
Acquaah 2007). The parents are usually chosen on the basis of their phenotypes
(Caligari 1992; Bradshaw and Mackay 1994). The aim is to generate genetic
variation on which to practice phenotypic selection over a number of vegetative
generations for clones with as many desirable characteristics as possible for release
as new cultivars (Caligari 1992; Sleper and Poehlman 2006; Acquaah 2007). The
choice of parents depends largely on the aims and objectives of the breeder
(Caligari 1992).
An important criterion for the choice
of parents is their crossability and unrelatedness (Wolfgang et al. 2009).
Often the parents are chosen due to their performance per se.
Theoretically, this cannot be secure in clonal crops like potatoes; clonal
varieties are highly heterozygous hybrids and polyploids, so that segregation
in crossings is almost unpredictable (Wolfgang et al. 2009). Suggestions that
have been made for better assessment of parents is the offspring performance
from test crosses; the other suggestion is to work on reduced polyploidy level,
which has been especially proposed for breeding tetraploid potatoes (Ross
1986). In general, parents should have a good combing ability and good
performance over all traits. In potatoes, it has been observed that SCA is
nearly as large as GCA, and in some cases SCA has been observed that is clearly
larger than GCA (Tai 1976; Killick 1977). In situations where not much is known
about the performance of a cross, the number of cross combinations should be
increased to the maximum of the breeder’s capacity and the number of genotypes
per cross should be kept small (Wolfgang et al. 2009). This is based on
selection theory which shows that “if the breeder has no prior knowledge on the
cross, the breeder has to make as many crosses as possible”; this also
minimizes the risk of raising genotypes with poor performance (Wricke and Weber
1986). Potatoes are highly heterozygous so that dominance and epistatic effects
contribute considerably to clone performance. Therefore, it should be assumed
not much is known about the value of a cross combination until it has been made
and the progeny tested.
Crossing success can be predicted based
on endosperm balance number (EBN), or effective ploidy, of the parents if they
are known. Once the cross is successful, the simplest method for predicting the
value of cross combinations is to evaluate progenies at seedling stage (Neele
and Louwes 1989; Neele et al. 1991). If a close relation between seedling
performance and performance in subsequent field generations exists, as found by
Brown and Caligari (1989) for tuber yield and plant appearance and by Neele and
Louwes (1989) for crisp quality and dry-matter content, progeny selection could
be carried out at the seedling stage. Progeny tests offer the means to replace
phenotypic recurrent selection with a much more efficient, multitrait, genotypic
recurrent selection program, in which the generation cycle time can be reduced
by several years, because parents with good GCA can be recognized shortly after
each round of hybridization. Then their progeny can be used for subsequent
crossing cycles and selection, whilst cultivars are being produced from
resowings of the best progenies (Bradshaw and Mackay 1994). Progeny means is
therefore a reliable approach in identifying superior cross combinations (Brown
and Caligari 1989; Gopal 1997).
For highly heritable traits, the mid-parent
value is a good predictor of the mean performance of the offspring, and a few
carefully chosen crosses can be made (Bradshaw et al. 2000). However, with an
only moderately heritable trait such as yield, offspring mean is less
predictable, and more crosses need to be made to ensure that they include the
best possible cross for the trait (Bradshaw 2007). For such a trait breeders
still have rely on phenotypic data and the concepts of quantitative genetics to
determine crossing strategies (Bradshaw 2007).
Mid parent values for yield and quality
traits can complement the results of the seedling progeny tests (Maris 1989).
The mid-parent value is the predictor that is generally used in potato breeding
programs because the method is quick, cheap, and easy. No time is lost with the
production of hybrid seed and seedlings, and the data are available from experiments
already performed with the clones of interest. If additional information is required,
the cost of trials is not likely to interfere with a large number of entries to
be tested (Neele et al. 1991). Neele et al. (1991) found the seed potato
harvest prediction by the mid-parent value to be very good for most characters,
with many correlation coefficients between progeny mean and mid-parent value
exceeding r = 0.8. Moderate correlation coefficients were noted for foliage
weight, number of stems, tuber shape and number of tubers. However, at ware
potato harvest, the correlation coefficients were lower; tuber yield in
particular was poorly predicted by the mid-parent value. Maris (1989) obtained
moderate to good correlation coefficients (r=0.51 to r=0.85) between the
mid-parent value and the actual progeny performance for various agronomically
important characters. For yield and number of tubers, however, the correlations
were moderate (r=0.51 and r=0.59 respectively). Brown and Caligari (1989) were
not able to accurately predict the progeny performance by the mid-parent value.
This suggests that a prediction based on the mean of the parental values might
have limited value and might not result in progenies with the best prospects.
Implications of
Genetics in Potato Breeding
The 2n gametes represent unique
tools for genetic studies of potatoes. This is important given that the
tetrasomic inheritance of the tetraploid potato makes such studies very difficult.
The 2n gametes have been used to determine gene-centromere map
distances, such as those of isozymes and the yellow tuber flesh color loci
(Douches and Quiros 1987), and those of genes conferring resistance to viruses
and nematodes (Wagenvoort and Zimnoch-Guzowska 1992). They have also been
employed to infer the physical location of QTLs controlling total tuber yield
(Buso et al. 1999). A very important feature of 2n gametes is that they
make the potato the best organism in which to manipulate all sets of
chromosomes for breeding purposes. They allow breeders to broaden the genetic
diversity, introducing both new genes for the improvement of traits of interest
and allelic diversity to maximize heterozygosity in tetraploid varieties. In
the potato, as in other polysomic polyploids, the genetic variance for several
important polygenic traits (e.g., tuber yield), is almost entirely
non-additive, depending on intralocus (heterozygosity) and inter-locus
(epistasis) interactions (Mendoza and Heynes 1974). Thus, breeding for
polygenic traits should be oriented towards maximizing heterozygosity and
maintaining valuable epistatic combinations. The significance of 2n gametes
in these crossing schemes lies not only in the possibility of returning to the
4x level, but also in their ability to transmit non-additive genetic
effects (heterozygosity and epistasis) from the 2x parent to the 4x offspring.
This aspect is important given that n gametes from diploids are only
capable of transmitting additive effects, whereas n gametes from
tetraploids can only transmit a certain number of first-order intralocus
interactions; epistasis is not transmitted due to the disruptive effects of
meiosis. Due to the high level of intra- and inter-locus interactions
transmitted by FDR gametes and on the wide natural occurrence of 2n pollen,
much emphasis has been given to the 4x x 2x—FDR breeding approach
to produce superior potato genotypes. Besides the advantages of the allelic
interactions transmitted, FDR 2n gametes are highly uniform, and thus
unilateral sexual polyploidization is expected to produce vigorous and fairly
homogeneous progeny in crosses with tetraploids (Ortiz 1997). The availability
of so many species with a 24-chromosome complement, the ease with which most of
these species can be crossed with S. tuberosum haploids, and the
widespread occurrence of meiotic mutations leading to 2n gametes,
strongly favor sexual polyploidization crossing-schemes for germplasm
introgression from species which are crossable with S. tuberosum haploids.
Dihaploids obtained from autoteraploid
potato could exhibit disomic inheritance and a smaller population is required
to generate a particular genotype than with the tetraploids. Such dihaploids
can be more easily bred for recessive traits than their polyploid parents. The
selected germplasm can then return to the original polyploidy level by somatic
polyplodization using colchicines or sexual polyplodization using 2n gametes
(Sleper and Poehlman 2006). Dihaploids may also be used to transfer genes from polyploidy
to a related diploid species and vice versa. Somatic fusion can be applied in
combining disease resistance genes from two sexually incompatible parents. When
protoplasts of diploids carrying a major gene for PVX resistance (Hines and
Marx 2001) were fused with those of diploids carrying a major gene for PVY
resistance (Ry), most of the hybrids expressed both genes (Thach and
Wenzel 1993). Similarly, when clones carrying genes for resistance to different
pathotypes of potato cyst nematode were fused, some of the hybrids were
resistant to both pathotypes (Rasmussen et al. 1996). Foliar and tuber late
blight resistance have also been combined in somatic fusion hybrids (Rasmussen
et al. 1998). The fusion of diploid S. verrucosum protoplasts with those
of cultivated potato clones combined PLRV resistance from S. verrucosum with
adaptation and tuber yield from the cultivated potato donor (Carrasco et al.
2000). Resistance to bacterial wilt in the tetraploid potato, has frequently
been sought from the diploid S. phureja which produces unreduced
gametes; another diploid, S. stenotomum has been used at experimental
level through somatic fusion (Fock et al. 2000, 2001). The potential use of
somatic hybridization has been demonstrated by the successful introduction of
traits of resistance to viruses (Gibson et al. 1988; Valkonen and Rokka 1998),
to extreme climatic conditions such as frost (Preiszner et al. 1991), to fungi
(Mattheij et al. 1992) and to insects (Serraf et al. 1991) into the cultivated
potato. Resistance against bacterial wilt has successfully been transferred
from S. commersonii (Laferriere et al. 1999) and S. phureja (Fock
et al. 2000) into potato through somatic hybridization.
Marker Assisted
Selection in Potato Breeding
In potato, molecular markers have been
used for construction of genetic linkage maps (Bonierbale et al. 1988; Gebhardt
et al. 1991; Bonierbale et al. 1994), trait tagging (Gebhardt et al. 1993;
Bryan et al. 2002), finger printing analysis (Milbourne et al. 1997), phylogeny
studies (Raker and Spooner 2002), and characterization of accessions from
germplasm banks (Ghislain et al. 2006). Powell et al. (1991) have suggested
using genetic distance based on molecular markers to select diverse parents
capable of producing high performing progeny. Molecular markers have been
identified for resistance against late blight (Colton et al. 2006), nematodes
and viruses in potatoes (Gebhardt et al. 2006). There are three major types of
genetic markers: (Harris 1976) (1) morphological (also ‘classical’,
’phenotypic’ or ‘visible’) markers which themselves are phenotypic traits or
characters; (2) biochemical markers, which include allelic variants of enzymes
called isozymes; and (3) DNA (or molecular) markers, which reveal sites of
variation in DNA sequence (Winter and Kahl 1995). The major disadvantages of
phenotypic and biochemical markers are that they may be limited in number and
are influenced by environmental factors or the developmental stage of the plant
(Winter and Kahl 1995). In addition, biochemical markers are expensive. Despite
these limitations, these markers have been extremely useful to plant breeders (Weeden
et al. 1994; Eagles et al. 2001). Molecular markers are the most widely used
mainly due to their abundance. They are also environmentally neutral and
independent, and therefore more robust and unbiased compared to phenotypic
descriptors. They arise from different classes of DNA mutations such as
substitution mutations (point mutations), rearrangements (insertions or
deletions) or errors in replication of tandemly repeated DNA (Paterson 1996).
The most widely used molecular markers in potatoes are restriction fragment
length polymorphism (RFLP), random amplified polymorphic DNA (RAPD), amplified
fragment length polymorphism (AFLP), and simple sequence repeats (SSR) or microsatellites
(Collard et al. 2005). Single nucleotide polymorphisms (SNP) are the latest
markers (Hamilton et al. 2011).
Conclusions and
Recommendations
Potato is easy to breed because it is
clonally propagated; once variation is released through crossing, there is no
problem with stabilizing (fixing) any desirable combination that arises, as any
clone can be multiplied unchanged by asexual reproduction. However, generating
TPS is a problem due to various infertility systems operating in potatoes;
these can be overcome through use of 2n gametes and somatic fusion.
Conventional potato breeding takes long, about 10 years before a cultivar is
released, mostly due to the slow multiplication rate of the crop. This time can
be reduced by use of progeny tests to discard whole families before starting
the within family selection; use of modern methods of rapid multiplication may
shorten the time even further. A big impact on the efficiency and rate of
progress would be the identification of superior clones genotypically as
seedlings in the glasshouse. This will require molecular marker assisted
selection or preferably direct recognition of the desired allele at a genetic
locus. Molecular makers have been used extensively in potatoes in genetic
studies; they could be used in identifying parents with desired traits and
hence shorten the breeding cycle.
Abbreviations
EBN: Endosperm balance number
GCA: General combining ability
SCA: specific combining ability
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