Difference Between Apomixis and Polyembryony

Key Difference – Apomixis vs Polyembryony
 

Flowering plants produce seeds to sustain their generations. Seeds are produced as a result of sexual reproduction in most plants. However, in certain plants, seeds are formed without fertilization of egg cells. This process is known as apomixis. Apomixis is defined as the asexual formation of seeds from unfertilized egg cells, avoiding the processes of meiosis and fertilization. Polyembryony is another phenomenon associated with seeds. Formation of more than one embryo from a single zygote in a seed is known as polyembryony. The key difference between apomixes and polyembryony is that apomixes produce seeds without fertilization while polyembryony produces more than one embryos in a single seed by fertilized egg cell (zygote).

CONTENTS
1. Overview and Key Difference
2. What is Apomixis
3. What is Polyembryony
4. Side by Side Comparison – Apomixis vs Polyembryony
5. Summary

What is Apomixis?

Seed development is a complex process in sexual reproduction of seed plants. It happens via flower formation, pollination, meiosis, mitosis and double fertilization. Meiosis and fertilization are the most important steps in seed formation and sexual reproduction. During those steps, a diploid mother cell (megaspore) undergoes meiosis to produce a haploid cell (megaspore) and then to produce an egg cell. Later the egg cell fuses with a sperm to produce a diploid zygote which develops into an embryo (seed).

However, some plants are able to produce seeds without being subjected to meiosis and fertilization. These plants bypass several important steps of sexual reproduction. In other words, sexual reproduction can be short-circuited in some plants to produce seeds. This process is known as apomixis. So apomixes can be defined as a process that produces seeds without meiosis and fertilization (syngamy). It is a type of asexual reproduction which mimics sexual reproduction. It is also known as agamospermy. Most apomicts are facultative and show both sexual and asexual seed formations.

Apomixis can be classified into two major types named gametophytic apomixes and sporophytic apomixes based on the way the embryo develops. Gametophytic apomixes occur via gametophyte and sporophytic apomixes occur directly via diploid sporophyte. Normal sexual reproduction produces seeds which give genetically diverse offspring. Due to the lack of fertilization in apomixis, it results in a genetically uniform seedling progeny of the mother.

Apomixis is not commonly observed in most plants. It is absent in many important food crops as well. However, due to its advantages, plant breeders try to harness this mechanism as a technology to produce high yielding secure foods for consumers.

There are advantages and disadvantages in apomixis process. Apomixis produces seedling progeny identical to the mother parent. Hence, apomixes can be used to produce genetically identical individuals effectively and rapidly. The characteristics of mother plants can also be maintained and exploited by apomixis for generations. Hybrid vigor is an important characteristic which gives the heterosis. Apomixis helps to conserve hybrid vigor for generations in crop varieties. However, apomixis is a complex phenomenon which does not have a clear genetic basis. The maintenance of the apomictic seed stocks is difficult unless linked with a morphological marker during the development.

What is Polyembryony?

Embryogeny is the process which forms embryo from zygote (fertilized egg). The embryo is the seed part which becomes the future offspring. The formation of more than one embryo from a single fertilized egg in a single seed is known as polyembryony. This phenomenon was discovered by Leeuwenhoek in 1719.

There are three types of polyembryony: simple, cleavage, and adventive polyembryony. Formation of embryos due to fertilization of more than one egg cell is known as simple polyembryony. Formation of embryos by saprophytic budding is known as adventive polyembryony. Formation of embryos due to cleavage of growing embryo is known as cleavage polyembryony.

Polyembryony is shown by certain plant species such as onion, groundnut, lemon, orange, etc.

What is the difference between Apomixis and Polyembryony?

Apomixis vs Polyembryony
Apomixis is a form of asexual reproduction which develops seeds without fertilization (without fusion of gametes).	Poyembryony is a phenomenon which describes the formation of more than one embryo from a zygote (single fertilized egg).
Fertilization
Apomixis does not involve fertilization.	Polyembryony is a result of fertilization.
Zygote Formation
Zygote is not produced during apomixis.	Zygote is produced before polyembryony.
Seeds
Seedlings are genetically identical.	Since all embryos are produced from a single zygote, seedlings are uniform.
Similarity to Mother Plant
They are clones of the mother plant.	They are not genetically identical to the mother plant.
Examples
Some species of Asteraceae and grasses are examples.	Onion, groundnut, mango, lemon, and orange are examples.

Summary – Apomixis vs Polyembryony

Apomixis and polyembryony are two terms related to reproduction of seed plants. Apomixis is the formation of seeds without fertilization. It produces seedling progeny identical to the mother parent. Polyembryony is the presence or formation of more than one embryo in a seed by a fertilized egg cell (Zygote). It develops uniform seedlings similar to asexual reproduction. This is the difference between apomixes and polyembryony.

References
1. Ross A. Bicknella, and Anna M. Koltunow. “Understanding Apomixis: Recent Advances and Remaining Conundrums.” The Plant Cell. N.p., 01 June 2004. Web. 21 May 2017
2. “Apomixis and Polyembryony in Flowering Plants.” YourArticleLibrary.com: The Next Generation Library. N.p., 22 Feb. 2014. Web. 21 May 2017. <http://www.yourarticlelibrary.com/biology/apomixis-and-polyembryony-in-flowering-plants/11762/>

Image Courtesy:
1. “Citrus fruits” By Scott Bauer, USDA –  Agricultural Research Service, the research agency of the United States Department of Agriculture (Public Domain) via Commons Wikimedia
2. “Taraxacum officinale dew” By Jojo assumed. Own work assumed (based on copyright claims). (CC BY-SA 3.0) via Commons Wikimedia