Apomixis is a type of asexual plant reproduction that produces seeds without meiosis or fertilization.
The term comes from Greek origin: “apo” meaning “away from” and “mixis” meaning “mixing. ” Thus, it is all about avoiding genetic recombination and sexual reproduction processes. Thus, the offspring are genetically identical clones of the parent plant and maintain the parental genotype across generations.
Apomixis in contrast to sexual reproduction, where meiosis produces gametes with half the chromosome number and fertilization returns the diploid state. In apomixis, the normal sexual pathways are replaced or modified to avoid these steps.
Types of apomixis based on embryo development
Based on embryo development, apomixis are of two types: Gametophytic apomixis and Sporophytic apomixis.
Gametophytic apomixis
In this type of apomixis, an embryo sac is mitotically formed from a diploid cell in the ovule without the process of meiosis. This type of embryo development takes place without fertilization. Such a kind of embryo sac development is known as apomeiosis. Another term for the same is gametophytic apomixis. In this type, gametophytic apomixis is often observed in herbaceous and tree species bearing dehiscent fruits.
Example: Gametophytic apomixis are seen in families such as Poaceae, Asteraceae, and Ranunculaceae.
Sporophytic apomixis
Sporophytic apomixis is a form of apomixis in which embryos are formed from nucellus or integument cells of diploid somatic cells of the ovule. Consequently, the developing seeds are true genetic clones of parent plants. Generally, it is found in many plants such as Citrus and mango.
Types of apomixis based on frequency
Obligate apomixis– On some flowering plants, sexual reproduction is absent and seed formation takes place by apomixis. This kind of apomixis that completely replaces sexual reproduction in some plants is called obligate apomixis. Example- Paspalum notatum.
Facultative apomixis– In some flowering plants sexual reproduction also occurs in addition to meiosis. This type of apomixis is called facultative apomixis. Example- Hippophae rhamnoides.
Types of apomixis classified by Maheshwari (1954)
Maheshwari (1954) classified apomixis into four types: Recurrent apomixis, Non-recurrent apomixis, Adventitive embryony, and Vegetative apomixis.
Recurrent apomixis
In this type, the embryo sac develops from the archesporial initial (diplospory) or cells of the nucellus (somatic apospory).
Diplospory– It is the process in which a diploid embryo sac with diploid embryo is formed by the division of the megaspore mother cell either using mitosis or by interrupting meiosis. Both parental genes and those of the offspring are identical.
Depending upon the degree of irregularity in meiosis, it is classified into four types.
- Antennaria type – Two unreduced nuclei undergo two mitotic divisions to form an eight-nucleate embryo sac of Polygonum type. This is also observed in Datura.
- Taraxacum type – The first meiotic division results in a restitution nucleus, meiosis II gives rise to unreduced cells, and finally an eight-nucleate embryo sac. The egg forms an embryo as in Taraxacum.
- Ixeris type – A megaspore with a restitution nucleus is bi-nucleate and hence after three mitotic divisions results in the formation of an eight-nucleate embryo sac as in Ixeris.
- Allium type – A pre-meiotic endo-mitotic doubling makes meiotic prophase start with a double chromosome number so that after meiosis an unreduced embryo sac with 8 nuclei is formed as seen in Allium.
Apospory– The embryo sacs are produced from the cells of the nucellus or integument. The embryo sac initially comes from the somatic cells of the chalaza or nucellus. The most common example of somatic apospory is that of Hieracium. The megaspore mother cell divides through meiosis and gives rise to the megaspore tetrad. At this time, a somatic cell positioned at the chalazal end of the ovule gets enlarged and vacuolated. It eventually forms an aposporic embryo sac. The embryo sac is diploid and the unfertilized eggs of such embryo sac give rise to diploid embryos and are seen in species like Malus, Ranunculus, etc. When the megaspore mother cell does not undergo meiosis but functions directly as the embryo sac initial it is referred to as gonial apospory.
Non-recurrent apomixis
In this type of apomixis, a haploid embryo develops directly from a haploid egg cell (n). It may be of 3 types- haploid parthenogenesis, and haploid apogamy.
Haploid parthenogenesis refers to the formation of an embryo from the unfertilized egg of an embryo sac. It is mainly seen in Solanum species.
Haploid apogamy is the development of an embryo from any cell of the embryo except the egg cell. This kind of apogamy can be seen in species like Lilium.
Adevntitive embryony
It is the type of apomixis in which the embryo develops from nucellus or integuments of the ovule. In these cases, the embryos that develop from the fertilized egg either degenerate or compete with adventive embryos. Normally, adventive embryony results in the development of more than one embryo in a seed. For example, this type of apomixis is seen on Citrus.
Vegetative apomixis
In vegetative apomixis, the inflorescence produces vegetative bulbils or buds instead of flowers. In some plants such as Poa bulbosa and certain Allium, Agave, and grass species, inflorescence produces vegetative buds or bulbils instead of flowers which contribute to the vegetative propagation of these plants.
Advantages of apomixis in plant reproduction and agriculture
Apomixis offers many advantages in plant reproduction, both in natural ecosystems and in agriculture. The main advantages include the following:
1. Genetic Uniformity: Apomixis does not involve meiosis or fertilization; it results in offspring genetically identical to the parent plant. The desirable traits are not lost from generation to generation and are very important in crop species where high yields or disease resistance must be retained.
2. Cost-Effective Reproduction: Apomixis does not require pollination and fertilization, thereby saving dependence on pollinators and environmental conditions, and can ensure reproducibility even in isolated or unfavorable conditions.
3. Maintenance of Hybrid Vigor: In hybrid plants, apomixis enables heterozygosity and heterosis to be maintained over several generations without any genetic segregation associated with sexual reproduction.
4. Seed production: The advantage of apomixis is seed production without a compatible mate or pollinators, which can be useful in monoculture farming of those species that have very few reproductive options.
5. Advanced Breeding Programs: Apomixis can hasten the plant breeding process by stabilizing desirable traits and accelerating the process by cutting out the time and resources needed for cross-breeding and selection.
6. Adaptation to stressful environments: Those species that undergo apomictic reproduction easily conquer challenging and variable environments where sexual variability is not favorable to the system or is uncertain.
Apomixis differs from sexual reproduction
Apomixis is a type of reproduction where seeds are produced without fertilization. As a result, the offspring will be genetically identical to the parent.
Sexual reproduction, on the other hand, forms seeds after meiosis and gamete fusion between male and female gametes, thus providing offspring with genetic variation. Though sexual reproduction promotes genetic variation that ensures adaptability, apomixis guarantees that desirable traits will be carried through generations.
Apomixis is particularly advantageous in stable environments where maintaining consistent traits is crucial, whereas sexual reproduction is essential in dynamic ecosystems requiring genetic diversity for survival.
Limitations and challenges of apomixis in plant breeding
Some of the limitations of apomixis in plant breeding are as follows-
Apomixis does not include recombination and thus all the offspring are genetically identical. If any harmful genetic mutation occurs then it cannot be prevented from accumulating.
Apomixis does not involve genetic variability, recombination, and gene flow which never results in population diversity.
The plants produced by apomixis cannot adapt to new and changing environments.
Apomixis is limited to certain specific crops and has low frequency.
Recent research in apomixis
Apomixis has been in the spotlight in recent research and has huge potential applications in plant breeding. Some notable advances include-
Synthetic apomixis has been recently induced in F1 generation hybrid rice by inducing mutations in BABY BOOM1 (BBM1) gene in egg cells. This technique will allow clonal propagation and retention of desirable features through generations.
The molecular basis and genetic mechanisms of apomixis have been studied to understand the key genes controlling this process and their roles in enhancing genetic crop improvement.
Genome editing technologies such as CRISPR-Cas9 have given new tools to manipulate these genes to introduce apomixis in major crops. Further, the regulation of epigenetic control and hormonal signaling mechanisms for the induction of apomixis has been highlighted. This leads to deeper knowledge about the mechanisms controlling and harnessing apomixis in plant breeding.
Future prospects
Apomixis has the potential to change agriculture from a non-renewable system to a more sustainable one and it could solve some of the biggest challenges in agriculture.
Apomixis can retain the heterosis of crops in hybrid plants through generations without the need for repeated purchase of hybrid seeds which reduces the cost and makes high-yielding seeds easily available.
Stabilization of desirable traits such as drought tolerance, and disease resistance, by apomixis can help in the development of crops that perform well in changing climate and environmental conditions.
Apomixis provides a way to conserve endangered plant species and rare genotypes for biodiversity.
The application of apomixis in staple foods would be a way of promoting food security.
References
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