Apomixis

1.0Introduction

Apomixis is a form of asexual reproduction that mimics sexual reproduction. It originated from the Greek words "apo," meaning without, and "mixis," meaning mixing. This unique reproductive strategy is observed in certain species of Asteraceae and grasses.

In botany, the term apomixis was defined by Winkler (1908) as a replacement of normal sexual reproduction by asexual reproduction (without fertilization or asexual seed formation). The term apomixis is commonly used as a synonym of Agamospermy.

Apomixis in plants stands out for its ability to produce seeds without the fusion of gametes, mimicking sexual reproduction while maintaining genetic uniformity. Unlike sexual reproduction, where offspring inherit genetic material from two parents, apomixis results in genetically identical plants emerging from a single parent. These offspring, termed clones, are each referred to as a ramet.

The hallmark of apomixis is its rapid multiplication and the production of offspring that closely resemble the parent plant. Seeds formed through apomixis, known as apomictic seeds, develop without fertilization, showcasing the replacement of normal sexual reproduction with asexual reproduction. 

Apomixis is prevalent in the plant kingdom, found across 40 angiosperm families and approximately 400 to 450 species. Notably, it is most frequently observed among members of the Asteraceae, Poaceae, Rosaceae, and Rutaceae families.

2.0Types of Apomixis

Diplospory

Diplospory is a reproductive method where the megaspore mother cell directly generates an embryo sac without undergoing meiosis. This resulting embryo sac is diploid, forming a diploid embryo without the need for fertilization from a diploid egg within this embryo sac. Examples of plants exhibiting diplospory include Parthenium and Taraxacum (Asteraceae).

Apospory

Apospory refers to forming a gametophyte directly from a sporophyte without undergoing meiosis. This phenomenon, first discovered by Rosenberg in Hieracium plants, involves the direct formation of an embryo sac or female gametophyte from any diploid cell of the sporophyte, excluding the megaspore mother cell. This results in the gametophyte always remaining diploid. Examples of plants exhibiting apospory include Hieracium, Ranunculus, and Rubus.

Adventive Embryony

Adventive embryony involves the formation of an embryo from any diploid cell within the ovule, such as cells of the nucellus or integuments. These diploid cells behave similar to a zygote. Adventive embryony can be derived from the nucellus in plants like Citrus, Mangifera (Mango), Opuntia, and Onion, while it can arise from the integuments in Spiranthus australis. 

Sporophytic budding, a process where the embryo forms outside the embryo sac and is subsequently pushed into it, is a type of adventive embryony.

P. Maheshwari classifies apomixis into the following types

• Recurrent apomixis: Recurrent apomixis is a phenomenon characterized by the development of an additional embryo sac directly from the megaspore mother cell or archesporial cell, bypassing meiosis. As a result, the entire embryo sac becomes diploid, leading to a diploid egg cell. Remarkably, the embryo then develops directly from this diploid egg cell without needing fertilization. Recurrent apomixis encompasses various processes, including somatic apospory, diploid parthenogenesis, and diploid apogamy. Notably, diploid parthenogenesis and apogamy typically occur only within aposporic (somatic) embryo sacs. Therefore, it is the occurrence of somatic or diploid apospory that defines recurrent apomixis.
• Non-Recurrent Apomixis: In nonrecurrent apomixis, the megaspore mother cell undergoes typical meiotic divisions, forming a haploid embryo sac megagametophyte. The resulting haploid plants have half the chromosome count of the mother plant. The embryo may originate from either the egg through haploid parthenogenesis or from another gametophyte cell via haploid apogamy.
• Vegetative Apomixis: Vegetative apomixis involves the production of offspring through structures other than flowers, such as bulbils or vegetative propagules, which often germinate while still attached to the parent plant. This mechanism is observed in plants like Agave and certain grass species.

3.0Applications of Apomixis 

Apomixis plays several crucial roles in plant reproduction and agriculture:

• Genetic Stability: Apomixis enables clonal reproduction, preserving desirable traits across generations without sexual recombination, ensuring uniformity in crop performance.
• Crop Improvement: It facilitates the rapid fixation of superior genotypes, aiding breeding programs in developing high-yielding, disease-resistant, or stress-tolerant varieties.
• Expansion of Breeding Strategies: It offers an alternative to traditional breeding methods, allowing for the development of novel crop varieties suited to various conditions or preferences.
• Production of Hybrid seeds: Apomixis solves this problem by ensuring that hybrid traits remain stable in the progeny. By converting hybrids into apomicts, farmers can continuously use the same hybrid seeds for successive crops without purchasing new seeds each year. This potential of apomixis in the hybrid seed industry has spurred active research in laboratories worldwide to unravel its genetics and transfer apomictic genes into hybrid varieties.