Apomixis is a type of asexual reproduction in plants, where seeds are produced without fertilization or meiosis.
- Hans Winkler (1908) first introduced the term apomixis for the substitution of the sexual process by asexual reproduction without nuclear fusion.
- It is derived from the Greek words “apo” and “mixis,” where “apo” means “away from” and “mixis” means “act of mixing.”
- Thus, in apomixis, the embryo develops from a diploid cell, bypassing the fundamental aspect of sexual reproduction (fertilization and meiosis).
- The offspring produced by apomixis are genetically identical to the parent plant and are referred to as apomictic.
- Apomixis has been reported in over 400 species across 35 families of flowering plants, including Asteraceae and Gramineae.
Types of Apomixis
Apomixis is generally classified based on of embryo development, frequency, and occurrence.
Based on Embryo Development
On the basis of embryo development, apomixis are of two types: gametophytic apomixis and sporophytic apomixis.
Gametophytic Apomixis
In this type, the embryo sac is formed by mitosis from a diploid cell in the ovule, bypassing the process of meiosis. This type of embryo development occurs without fertilization. Such a process is known as apomeiosis or, alternatively, gametophytic apomixis.
Example: Gametophytic apomixis is commonly observed in families like Asteraceae, Ranunculaceae, Gramineae, etc.
Sporophytic Apomixis
In this type of apomixis, embryos develop from the nucellus or integument cells of diploid somatic cells in the ovule and follow the typical sexual pathway. The diploid cells may develop multiple globular embryos.
Example: Sporophytic apomixis is commonly observed in citrus plants.
Based on Frequency
On the basis of frequency, apomixis can be classified into two types.
Obligate Apomixis
In this type of apomixis, the plant produces only apomictic embryos, as sexual reproduction is completely absent. Seed formation occurs through apomixis.
Example: Obligate apomixis is seen in species like Paspalum notatum.
Facultative Apomixis
In this type, the plant can produce both apomictic and normal embryos, as sexual reproduction occurs in addition to meiosis.
Example: Facultative apomixis has been observed in Hippophae rhamnoides.
Apomixis Classified by Maheshwari (1954)
According to Maheshwari (1954), apomixis is categorized into four types: recurrent apomixis, non-recurrent apomixis, adventitive embryony, and vegetative apomixis.
Recurrent Apomixis
In recurrent apomixis, the embryo sac is diploid and develops from the megaspore mother cell (MMC) without undergoing meiosis. The embryo sac can originate from either archesporial initial (diplospory) or cells of the nucellus (somatic apospory).
Diplospory
Here the diploid embryo sac is developed from the megaspore mother cell by mitotic division or by incomplete meiosis. Depending on the degree of irregularity in meiosis, diplospory is divided into four types.
Antennaria Type: Two unreduced nuclei divided by two mitotic divisions, resulting in an eight-nucleate embryo sac (e.g., Datura).
Taraxacum Type: The first meiotic division ends in a restitution nucleus; subsequently, meiosis II forms unreduced cells, finally developing an eight-nucleate embryo sac (e. g., Taraxacum).
Ixeris Type: A binucleate megaspore with a restitution nucleus is divided by three mitotic divisions and forms an eight-nucleate embryo sac (e.g., Ixeris).
Allium Type: A premeiotic endomitotic doubling results in meiotic prophase initiating with a doubled chromosome number, ultimately leading to the formation of an unreduced, eight-nucleate embryo sac (e.g., Allium).
Somatic Apospory
The diploid embryo sacs are produced from nucellar cells or integument rather than from megaspores. The somatic cells of the chalaza or nucellus function as the embryo sac initial. The megaspore mother cells divide by meiosis to form a megaspore tetrad. A somatic cell of the chalazal end of the ovule gets vacuolated and enlarged. This cell finally developed into an aposporic embryo sac (e.g, Hieracium).
In species like Ranunculus, the unfertilized eggs in the diploid embryo sac produce diploid embryos. Sometimes the megaspore mother cell does not undergo meiosis but functions directly as the embryo sac initial. It is known as gonial apospory. The embryo sac initial bypassing the meiosis and divides by three mitotic divisions, forming an eight-nucleate embryo sac (e.g., Antennaria alpina).
Non-recurrent Apomixis
In this type of apomixis, the haploid embryo develops directly either from an egg cell or from other haploid cells of the embryo sac without fertilization. In this case, meiosis occurs in a normal sexual cycle, resulting in the formation of an embryo sac. However, due to various reasons, fertilization fails to take place.
It is generally two types: haploid parthenogenesis and haploid apogamy.
Haploid parthenogenesis is the development of an embryo from the unfertilized egg cell of the embryo sac (e.g., Solanum sp.).
Haploid apogamy is the formation of an embryo from some other cell of the embryo sac apart from the egg (e.g., Lilium martagon).
Adventive Embryony
Here the embryo develops from the nucellus or integument of the ovule rather than the embryo sac. The diploid cell gives rise to an embryo within the embryo sac; the zygotic embryo degenerates or competes with the apomictic embryo.
Adventive embryony generally produces multiple embryos within a single seed. Example: Citrus, Nigritella.
Vegetative Apomixis
In vegetative apomixis, the plants produce vegetative bulbils or buds in the inflorescence instead of flowers. The bulbils often germinate while still on the parent plant. It is more of a type of vegetative reproduction. So many researchers do not consider this process as apomixis. Example: Allium, Agave, Poa, etc.
Significance of Apomixis in Plant Reproduction
Apomixis offers several advantages in plant reproduction. Some key benefits are:
- Apomixis produces offspring genetically identical to the parent plant as it does not involve meiosis or fertilization. Thus, apomixis is useful in maintaining the genetic purity from generation to generation.
- In hybrid plants, apomixis prevents the loss of specific characters in the hybrid. So heterozygosity and heterosis are to be retained across several generations without any segregation and recombination of chromosomes associated with reproduction.
- Pollination and fertilization are not required in apomixis. It helps plants to reproduce in extreme or isolated environments where sexual reproduction might be limited.
