What is Polyembryony?

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.