Key Difference – Apomixis vs Parthenogenesis
Flower formation, meiosis, mitosis and double fertilization are the major components of the seed formation pathway. In normal sexual reproduction cycle, production of haploid gametes and fusion of male and female gametes serve as the major steps which lead to the formation of the embryo that becomes the seed. Seeds germinate and produce new plants and continue the life cycles. However, in some plants seed formation occurs asexually without following the above mentioned major two steps meiosis and fertilization. It is known as apomixis. In certain plants and animals, new individuals are produced directly from unfertilized ovules. The process is known as parthenogenesis. The key difference between apomixis and parthenogenesis is that apomixis is the process which produces seeds without fertilization while parthenogenesis is a general term that describes the process which produces offspring directly from unfertilized egg cells.
CONTENTS
1. Overview and Key Difference
2. What is Apomixis
3. What is Parthenogenesis
4. Side by Side Comparison – Apomixis vs Parthenogenesis
5. Summary
What is Apomixis?
Seed development is a complex process which completes several major steps during the sexual reproduction of seed plants. It happens via flower formation, pollination, meiosis, mitosis, double fertilization, etc. Meiosis and fertilization are the most important steps in seed formation and sexual reproduction since diploid megaspore mother cell should undergo meiosis to produce haploid megaspore and finally to produce egg cell. Egg cell should be fused with sperm cell to produce a diploid zygote which develops into the embryo (seed). However, in some plants, several major steps of the sexual reproduction are bypassed in the formation of seeds. In other words, the sexual reproduction can be short-circuited in some plants to produce seeds. This process is known as apomixis. Apomixes can be defined as a process that produces seeds without meiosis and fertilization (syngamy). It is a type of an asexual reproduction which mimics sexual reproduction. It is also known as agamospermy.
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 via directly from diploid sporophyte. Normal sexual reproduction produces seeds which give genetically diverse offspring. Due to the lack of fertilization in apomixis, it results in genetically uniform seedling progeny to the mother.
Apomixis is not observed in most plants. It is absent in many important food crops. However, due to its advantages, plant breeders try to harness this mechanism as a technology to produce high yielding secure foods for consumers.
Advantages and Disadvantages of Apomixis
There are advantages and disadvantages in apomixis process. Genetically identical individuals can be produced effectively and rapidly by apomixes since it results in seedling progeny identical to mother parent. The characteristics of mother plants can also be maintained and exploited by the apomixis for generations. Hybrid vigor is an important characteristic which causes 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 apomictic stocks is difficult unless linked with a morphological marker during the development.
Most apomicts are facultative which show both sexual and asexual seed formations.
Figure 01: Vegetative Apomixis shown by Poa bulbosa
What is Parthenogenesis?
Parthenogenesis is a type of reproduction commonly shown in organisms, mainly by some invertebrates and lower plants. It can be described as a process in which unfertilized ovum develops into an individual (virgin birth) without fertilization. Therefore, it can be considered as a method of asexual reproduction. However, it is also possible to define it as an incomplete sexual reproduction since only the fusion of two gametes is absent in the sexual reproduction process. Parthenogenesis can be artificially stimulated even in mammals to produce an individual without going through fertilization. During the parthenogenesis process, the unfertilized egg is developed into a new organism; the resulting organism is haploid and it cannot undergo meiosis. They are mostly genetically identical to the parent. There are several types of parthenogenesis: facultative parthenogenesis, haploid parthenogenesis, artificial parthenogenesis and cyclic parthenogenesis.
In nature, parthenogenesis takes place in many insects. For example, in bees, the queen bee can produce either fertilized or unfertilized eggs; unfertilized eggs become male drones by parthenogenesis.
Figure 02: Male drone bee
What is the difference between Apomixis and Parthenogenesis?
Apomixis vs Parthenogenesis |
|
| Apomixis can be defined as the mechanism which produces seeds asexually. | Parthenogenesis can be defined as the process which develops individuals directly from the unfertilized eggs or ovules. |
| Progeny | |
| It produces genetically identical seedling progeny or mother clones. | It produces genetically identical female progeny |
| Shown by | |
| Apomixis is shown by some plants. | Parthenogenesis is shown by plants and animals. |
Summary – Apomixis vs Parthenogenesis
Apomixis and parthenogenesis are two modes of asexual reproduction. Apomixis produces seeds without meiosis and fertilization and results in mother clones. Parthenogenesis produces new individuals directly from unfertilized egg cells. This is the difference between apomixis and parthenogenesis.
Reference:
1. Anna Koltunow. “Apomixis.” Encyclopedia of Life Sciences. N.p., July 2012. Web. 03 May 2017
2. Hand, Melanie L., and Anna M. G. Koltunow. “The Genetic Control of Apomixis: Asexual Seed Formation.” Genetics. Genetics Society of America, June 2014. Web. 03 May 2017 [http://www.genetics.org/content/197/2/441]
Image Courtesy:
1. “Poa bulbosa, vegetative apomixis” By Nadiatalent – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Drohn im Flug 08-3” By Waugsberg – own work (CC BY-SA 3.0) via Commons Wikimedia
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