The apomixis is a form of asexual reproduction of certain species through seeds. Its result is a genetically identical generation to the mother plant. The etymological meaning of apomixis comes from the Greek ” apo ” which means – lack or absence – and ” mixis ” which indicates – mixture or union. Indeed, in apomixis the union of the male and female gametes does not occur for the formation of the embryo.
Apomictic plants do not express the adaptive advantages – from an evolutionary point of view – that sexual reproduction provides. However, apomixis is a mechanism that allows the maintenance of genotypes adapted to specific environmental conditions.
In apomixis the reproductive mechanism bypasses the sexual process and allows the plant to multiply through seeds. In this process , meiosis , the formation of embryos from fertilization and the creation of viable endosperm, does not occur .
The seeds of apomictic plants are formed from the maternal tissue of the ovule, bypassing meiosis and fertilization. This type of reproduction is frequent in most of the species of the Poaceae family , as well as in the genera Asteraceae , Rosaceae and Rutaceae .
If this capacity could be transferred to crops of agronomic interest such as corn and wheat, it would become a useful piece of genetic improvement, since its use would favor the quantity and quality of food obtained from superior genotypes.
Three different mechanisms are known through which plants reproduce by apomixis. Gametophytic apomixis due to diplosporia and aposporia, and sporophytic apomixis or adventitious embryo.
Diplosporia is an asexual reproduction mechanism or apomixis where the embryo originates from a non-reduced embryo sac. As a result, the new embryo has the same chromosomal number as the mother plant of origin.
It is a process that occurs when the mother cell of the embryo sac or female gametophyte develops directly from the embryo. Also known as diploid parthenogenesis, it is characterized by the presence of a diploid embryo.
Aposporia is an apomictic or asexual reproduction mechanism where the embryo sac originates from somatic cells. The embryo sac originates from some somatic cell located in the integument or nucela that surrounds the stem cell of the embryo sac.
In this case, a gametophyte develops, but meiosis does not occur; the embryo is also diploid. In this process, the reduction of the chromosome number does not occur, which is complemented by the parthenogenesis or apomictic development of the ovum.
Called nucellar embryo or sporophytic apomixis, it is a type of asexual reproduction by seeds or apomixis common in citrus. In this case, no formation of an embryo sac is observed, since the embryo develops from a diploid sporophyte.
In fact, the embryo originates from a somatic cell at the level of the ovum of the mother plant. Later it develops by consecutive mitotic divisions, neither the meiosis process nor the formation of the female gametophyte occurs.
Apomixis is the result of the modification of certain stages of embryonic processes that are fundamental for sexual reproduction. In this case, the reduction of the chromosome number and the meiotic process, including the random union and the fusion of gametes.
Indeed, during apomixis these embryonic changes manage to disable the meiotic process and its products. Likewise, they avoid or replace the fertilization process through parthenogenetic development.
In apomixis there are four embryonic processes that distinguish it from sexual reproduction:
It is a process that occurs when sporophytic structures are formed without meiotic reduction or degeneration of the macrospora -megaspore-. It constitutes the simplification of the meiotic process, and they occur in both diplosporia and aposporia.
Development of the embryo sac
In apomixis, cytologically non-reduced cells (2 n ) have the ability to develop the embryo sac. In the case of aposporic apomictic species, the embryo sac develops from the inner part of the seminal primordium or nucela.
Embryonic process that results in the formation of the embryo directly from the egg cell, without prior fertilization. That is, the apomictic development of the ovule for the formation of a new plant from an unfertilized ovum.
Process related to those apomictic plants that require pollination, despite the fact that they develop without the fertilization of the mother cell. The endosperm is formed from the fusion of the male gamete with the polar nuclei of cells of the embryo sac.
In fact, in processes of gametophytic apomixis, the fusion of the female and male gametes or double fertilization is suppressed. However, even though the fertilization of the polar nuclei is canceled, the endosperm develops independently.
Apomixis is an efficient technique to produce seeds and new species in a short time. In effect, it allows creating new hybrid varieties with better yields and higher phenotypic quality.
Through apomixis, the loss of certain specific characters in hybrids is prevented. Being a functional mechanism for the production of disease-free plants and obtain higher yields and productivity of crops.
The Taraxacum officinalis (Dandelion) is one of the most common examples of apomictic plants. In this regard, apomixis occurs frequently in plants of the families Poaceae -grasses-, Rosaceae and Composite -asteraceae-.
In the composites or asteraceae, apomixis is the inescapable form of reproduction of most species. On the contrary, in Poaceae and Rosaceae, apomixis alternates with sexual reproduction -apomixis facultative-.
Specifically, apomixis occurs in several genera; Achillea, Arnica, Brachycome, Crepis, Conyza, Erigeron, Eupatorium, Hieracium, Parthenium and Taraxacum .
In poaceae, apomixis was initially identified in the genus Poa , later it was described in various paniceas and andropogoneas. Among the genera of Poaceae, Bothriochloa, Capillipedium, Cenchrus, Dichanthium, Heteropogon, Paspalum, Setaria, Sorghum and Themeda can be noted .
Weeping grass ( Eragrostis curvula ) is a food source that allows increasing the production of beef. One of its forms of reproduction is through diplosporic apomixis, which can be compulsory or facultative.
Other examples of apomictic plants are located in the genera Sorbus -serbales- and Crataegus -thorn- of the Rosaceae family. As well as the species Rubus fruticosus (bramble) and the genus of flowering plants Hieracium belonging to the Asteraceae family.
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