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What are apomixis and polyembryony?

What are apomixis and polyembryony?

Reproduction is the process of producing new offspring. It is a feature of the living beings as a means to maintain the species on Earth. There are two types of reproduction:

Sexual Reproduction: It is defined as the mode of reproduction where a gamete containing one set of chromosomes (also known as a haploid) combines with another gamete forming a diploid embryo. For example, animals sexually reproduce by the combination of a haploid egg cell (called the ovum) and a haploid sperm cell to produce a diploid zygote.  Sexual reproduction is a common method of reproduction in multicellular eukaryotes, viz., plants, fungi, and mammals. There are two major types of sexual reproduction and they are syngamy and conjugation.

Asexual Reproduction: This type of reproduction does not involve the fusion of gametes or the interchanging of chromosomes. Offspring produces by asexual reproduction from either unicellular or multicellular organisms inherit its genes from a single parent. The different types of asexual reproduction are binary fission, budding, vegetative propagation, spore formation (sporogenesis), fragmentation, parthenogenesis, and apomixis.

Apomixis is one of the asexual reproductions in flowering plants where the formation of the seed occurs from the maternal tissue of the ovule, an embryonic development altogether avoiding the process of meiosis and fertilization. Since differing from the sexual embryo, the apomictic embryo contains no paternal contribution and making it genetically identical to the maternal plant. Apomixis naturally occurs in 326 genera representing angiosperms, the majority belonging to Poaceae, Asteraceae, and Rosaceae. It is synonymous with agamospermous because seeds are only found in angiosperm and gymnosperm taxa. This reproduction may originate gametophytically or saprophytically based on forming embryo sac tissue of the female gametophyte. Mechanisms of apomixis composed of three developmental components:

Apomeiosis: Apomixis initiated with the generation of cells capable of forming an embryo without meiosis. This stage is used to describe both apospory and diplospory.

Parthenogenesis: At this stage, there is a spontaneous development of the zygote without fertilization.

Endosperm Development: The formation of endosperm is initiated by mitotic divisions of the primary endosperm nucleus. The endosperm tissue surrounds and nourishes the embryo till the embryo matures and ready to germinates.

Polyembryony

Polyembryony is the formation of more than one embryo from a single fertilized gamete. This is possible because more than one archegonia are fertilized, and many zygotes are formed, which develop into embryos and only one of them successfully develop into a complete embryo.  It was firstly discovered in the year 1719 by Antonie van Leeuwenhoek in Citrus plant seeds and found in both plants and animals. Examples of polyembryony in the animal kingdom include the formation of two identical twins in humans and another regular occurrence is the usual birth of four offspring by the nine-banded armadillo. In plants, this fertilization occurs in one or more than one embryonic sac or the existence of an embryo outside the sac.

Polyembryony can be experimentally Induced by polyembryony or naturally occurring True and False polyembryony.

  1. Induced Polyembryony: Experimentally, the development of an embryo can be made in a culture medium. Such induced embryos are also known as somatic embryos or adventitious embryos.
  2. True Polyembryony: Embryos are produced in a single embryo sac with the additional embryos created either by cleavage of a zygote or from antipodal cells and synergies.
  3. False Polyembryony: In this fertilization, there are two or more nuclei of two or more embryos fusing for the development of a monosporic embryo sac.

Polyembryony is classified into five different types:

  1. Cleavage Polyembryony: This type of polyembryony involves the splitting of a single embryo into several identical parts with each part possessing the capability of developing into a fully mature embryo. Cleavage polyembryony commonly occurs in gymnosperms with the example of Pinaceae, Taxodiaceae, Cupressaceae, and Podocarpaceae.
  2. Simple Polyembryony: Simple Polyembryony occurs due to the fertilization of many archegonia eggs giving rise to many embryos, but out of many, only a single embryo attains maturity. An example of this phenomenon is reported in Cycas circinalis where two archegonia of the same ovule independently grow into two embryos and also into two seeds. Simple and Cleavage polyembryony is common in the species Cupressus.
  3. Rosette Polyembryony: In some gymnosperms, cleavage of suspensor cells occurs. Since each suspensor cell has embryonic potential that mostly is suppress by the embryo. Sometimes, groups of meristematic cells are located at the base of these suspensor cells. This group of cells budding on the suspensor cells are called rosette embryos which they do not further develop.
  4. Mixed Polyembryony: This polyembryony occurs due to the presence of more than one pollen tube entering the ovule that leads to the fertilization of synergids and antipodal cells.
  5. Adventive Polyembryony: Polyembryony, when the embryo is formed from the diploid sporophytic cells such as nucellus and integuments of the ovule which sometimes may lead to more than one embryo forming within a seed, is called adventive polyembryony.

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