Apomixis is a term commonly used with flowering plants to indicate that they have reproduced asexually through seeds. The plants that grow from these seeds are identical to the mother plant. This is of great use for seed production and plant breeding. Over 400 plant species produce apomictic seeds, including dandelions and blackberries.
Facultative apomixis indicates the situation where apomixis occurs part of the time, but sexual reproduction can still happen. This can be very advantageous evolutionarily. For instance, Kentucky bluegrass periodically produces new strains through sexual reproduction. The best of them are propagated through apomixis, and the plant has many strains that are well-adapted to localized regions.
Arctic plants benefit greatly from apomictic reproduction. The extreme conditions there make pollination by insects difficult, so it is hard to transfer genetic material from plant to plant. Given the difficult conditions, the plants might benefit from having a few more greatly specialized strains than from the constantly evolving populations that would occur with sexual reproduction.
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There is a lot of interest in learning how to genetically manipulate apomictic plants for agricultural reasons. There are, however, technical difficulties in doing so. Plants that are apomictic frequently exhibit polyploidy, meaning they have more than two copies of their chromosomes. This can make gene transfer difficult.
An advantage to using apomictic seed would be that small farmers would be able to produce their own seeds of elite cultivars. Viral diseases can spread through vegetatively propagated plants, and this could be minimized by cloning with seed cultures. One could also take advantage of locally-adapted varieties that are resistant to extreme climatic conditions or pathogens.
There are two main forms of apomixis. The first involves the gametophyte, which is the plant’s reproductive haploid multicellular structure. Being haploid means that it only contains one set of chromosomes. The gametophyte produces gametes — mature reproductive cells that normally would unite with another of the opposite sex to undergo sexual reproduction. In gametophytic apomixis, however, an unfertilized egg cell gives rise to an embryo. This is similar to parthenogenesis in animals, which is reproduction in females, without sex.
The second form is known as sporophytic apomixis. In this case, the embryos are formed like buds directly from integument tissue. This is part of the inner tissue of the ovule, the structure that holds the embryo sac. The embryo sac continues with its development, while the sporophytic embryo will go ahead to form apomictic seeds. This process is also known as adventitious embryony. Except for commonly occurring in citrus plants, sporophytic apomixis is a rare process in higher plants.