Polyembryony is a
reproductive phenomenon that leads to the development of multiple embryos
within a single seed, resulting in genetically similar offspring. The most
common type of polyembryony is nucellar embryony, which occurs when the ovule's
maternal tissue gives rise to additional embryos. However, some plant species
are capable of producing multiple embryos through the cleavage of pre-embryos.
The degree of polyembryony is influenced by environmental factors and the
source of pollen, but it is primarily controlled by a dominant gene with a
heterozygous allele.
Distinguishing between
nucellar and zygotic seedlings based on morphological markers can be
challenging, but biochemical and molecular markers such as RAPD markers have
been used to identify them. Recent developments in molecular biology have led
to the discovery of genes such as msg-2 and SERK, which are linked to somatic
embryogenesis in many plant species.
Although polyembryony
is valuable for generating high-quality, true-to-type plants, it can also
hinder hybridization programs since it reduces the production of zygotic
seedlings. Polyembryony occurs when two or more embryos develop in a developing
ovule. In contrast, monoembryony refers to the production of strictly sexual
seed parents containing a single embryo.
Polyembryony is not an
abnormal feature but rather a desirable character exhibited by approximately
255 genera belonging to 153 families. Polyembryony is a type of apomixis that
results in the autonomous development of multiple embryos through asexual
reproduction, and the resulting progeny are genetic replicas of the mother
plant. Understanding the anatomical structure of the ovule is crucial to
comprehend the origin and formation of multiple ovules in angiospermic seeds.
The gynoecium is the
female reproductive organ in a flower, and the carpel is its functional unit.
The ovary swells to form the ovule, which ultimately becomes the seed. A fully
developed ovule comprises an embryo sac, nucellus almost entirely enclosed by
one or two integuments, a small opening at the apical end, and the micropyle,
the main entry point for pollen tubes into the embryo sac. The base of the
ovule is called the funiculus, which attaches to the ovary wall through the
placenta. Ovules are categorized into five types based on the position of the
funiculus and micropyle. Anatropous ovules are common in horticultural crops,
with the micropyle close to the funiculus and the ovule appearing inverted. In
horticultural crops, the embryo sac, nucellus, and chalaza are surrounded by
two integuments, making the ovule bitegmic.
The female gametophyte
known as the embryo sac is where male gametes fuse to form the zygotic embryo
and endosperm. Within the 7-celled embryo sac, two polar nuclei and an egg
apparatus containing an egg cell and two synergids are found at the micropylar
region, and three antipodal cells are located at the chalazal end. The egg
apparatus and antipodal cells are haploid and uninucleate, while the central
cell (polar nuclei) is diploid or binucleate. Double fertilization is the norm
in angiosperms, where one male gamete fuses with the egg cell to form the
embryo (2n) and the other fuses with the polar nuclei to form the endosperm
(3n). In polyembryonate species, nucellar cells (2n), integuments (2n),
synergids (n), or antipodal cells (n) can form additional embryos.
Polyembryony in plants
can occur in different ways, illustrated in Figure 3. Adventitious
polyembryony, which originates from maternal tissue, is the most common method.
Cleavage of the fertilized embryo and the formation of additional embryo sacs
with haploid cells participating in embryo formation are less frequent. The
four different ways in which polyembryony can occur are:
i. Embryos formed by
the 2n sporophytic/maternal tissue of the ovule
ii. Embryos formed by
cells of the embryo sac other than the egg cell
iii. Development of
more than one embryo sac within the same ovule
iv. Cleavage of
proembryos.
Polyembryony is a
phenomenon in which multiple embryos arise from a single fertilized ovule or
from somatic cells of the nucellus or integument of the embryo sac. Adventive
embryony, which is triggered by the activation of sporophytic cells, is the
most common form of polyembryony. The resulting embryos are called adventive
embryos, and nucellar embryony, where the embryo develops from the nucellus, is
the most common feature in families of horticultural importance.
Polyembryony can occur
in various ways, including cleavage of the fertilized embryo, formation of
additional embryo sacs, and activation of sporophytic cells of the ovule.
Cleavage polyembryony is common in Orchidaceae, Poaceae, and gymnosperms.
Embryos may arise from other cells of the embryo sac, such as synergids,
antipodal cells, and endosperm cells, but the most common source is the
synergids.
Polyembryony can be
classified based on the source of origin, frequency of occurrence, and ploidy
level. Plants that are strictly monoembryonic have a frequency of multiple
embryos of less than 6%, nearly monoembryonic plant species have a frequency of
6-10%, and polyembryonic plant species have a frequency of multiple embryo
formation of more than 10% and are called polyembryonate.
Polyembryony can also
be classified into true and false categories based on embryogenesis. In true
polyembryony, two or more embryos arise in the same embryo sac from nucellus,
integument, synergid, etc. In false polyembryony, more than one embryo sac is
formed in an ovule, which is followed by the formation of multiple embryos.
Nucellar embryony is a
common form of polyembryony in plants, characterized by the initiation of
embryonic development from somatic cells of the nucellus tissue. These nucellar
embryos develop alongside the zygotic embryo, giving rise to seedlings that are
genetically identical to the female parent.
Adventive embryogenesis
is a four-step process involving the formation, differentiation, division, and
development of adventive embryos. Adventive Embryo Initial Cells (AEICs)
generally appear before pollination and divide before the division of the
zygote but after endosperm division in fertilized seeds. Recent studies show
that the initiation of adventive embryos in Citrus occurs autonomously and is
not affected by pollination, fertilization, or the development of zygotic
embryos or endosperm. However, the development of adventive embryos is greatly
influenced by endosperm development.
Different types of
seeds are produced in a genotype that produces polyembryonic seeds by nucellar
embryony. These seeds contain normal zygotic embryos and nucellar embryos.
Nucellar embryos can be found near or away from the micropyle, and their number
varies depending on the species and variety of the fruit tree. Nucellar embryos
develop at a slower rate than zygotic embryos and are found at the micropylar
end of the embryo.
Polyembryony is common
in fruit trees such as citrus, mango, Syzygium sp., kiwi, almond, Fragaria sp.,
and peach. The occurrence of polyembryony varies among species and varieties,
indicating a genetic basis for this phenomenon. Factors such as minor genes,
pollen sources, and environmental conditions can affect the number and type of embryos
produced, and nucellar embryo development is often dependent on the survival
and development of the zygotic embryo, as well as on the process of
fertilization and pollination.
Several theories have
been proposed to explain polyembryony, but the genetic theory is the most
accepted. The presence of polyembryony is determined by the gene, and in citrus
and mango, it is usually controlled by a dominant gene with a heterozygous
allele. Monoembryonic citrus species have a homozygous recessive gene that synthesizes
a potent inhibitor of embryogenesis. The degree of polyembryony varies in
polyembryonate offspring obtained by crossing monoembryonic and polyembryonic
parents, indicating the presence of minor genes affecting the degree of
polyembryony. Modifier or duplicate genes may also be present in some crosses,
leading to a variation in progeny ratios.
The degree of
polyembryony varies across species and varieties, as well as with environmental
factors such as location, position of fruits on a tree, and other factors. The
visual recognition of nucellar and zygotic seedlings at a juvenile stage can be
difficult, and selecting the wrong seedling is more likely, especially when the
male and female parents are similar. The difference between zygotic and nucellar
seedlings can only be made after fruit.
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