apomixis

Reproduction in flowering plants without the fusion of gametes. The term covers all types of asexual reproduction, including vegetative reproduction in which a plant is propagated by organs other than the sex organs in the flower, and agamospermy, in which seeds are produced without fertilization.

Because the plant embryos grow from egg cells without being fertilized by pollen, the seeds that are formed are clones of each other and of the mother plant. Genetically modified plants that reproduce using apomixis are therefore much less likely to lead to accidental gene transfer to wild populations than those that reproduce sexually. Apomixis is especially common in the grass, rose, and aster families.

Agamospermy is a degenerate form of sexual reproduction and the fruits and seeds that are produced have a perfectly normal appearance and are dispersed in the normal way. It is therefore difficult to detect when a plant is reproducing agamospermically without studying it under controlled conditions. If pollination is prevented and yet viable seeds are still produced, agamospermy must be operating. Another indication of agamospermy is that the offspring are all identical to the seed parent and do not vary from one another as sexually produced offspring do.

The mechanism of agamospermy varies, but three types can be recognized: parthenogenesis is the development of an embryo from an unfertilized egg cell which has not undergone the normal process of meiosis to halve the chromosome number; apogamy is very similar to parthenogenesis but the cell that forms the embryo is not the egg cell itself; adventitious embryony occurs when the embryo arises from the part of the ovule known as the nucellus or from the surrounding envelopes (integuments). A further complication is that some plants must be pollinated before apomictic seed formation can occur; the pollen stimulates seed development but does not actually fertilize the egg cell.