Apomixis

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apomixis

[‚ap·ə′mik·səs]
(embryology)
Parthenogenetic development of sex cells without fertilization.

Apomixis

 

various forms of asexual reproduction by animals and plants. In a more common, narrow sense, apomixis is the formation of an embryo without fertilization. In apomixis the embryo develops not from a zygote, but directly from an unfertilized egg cell (parthenogenesis) or—in higher plants—from the cell of the gametophyte, from the embryonic sac (apogamy), and even from the somatic cells of a seed bud. Apomixis is known in the form of parthenogenesis in worms, insects, fish, and reptiles. However, it is more widespread among plants. Apomixis, or asexual seed reproduction (agamospermy), is especially frequent among angiosperms. There are several thousand apomictic species among 300 genera of angiosperms, belonging to 80 families. These include such common, widespread plants as grasses (60 genera), the composite family (28 genera), the rose family (15 genera), and the rue family (13 genera).

Apomixis can be autonomous—in which case both the embryo and endosperm form without fertilization—and mentoral (pseudogamic or stimulatory)—in which case the embryo is formed from an unfertilized egg cell, but its development is stimulated by the fertilization of the embryonic sac, which gives rise to the endosperm. Apomixis may be induced experimentally by some factors (induced apomixis). Apomixis sometimes manifests itself sporadically in certain organisms (facultative apomixis) or is the basic and even sole means of reproduction (obligatory apomixis).

As a rule, apomictic species occupy vast areas without showing any sign of becoming extinct (many species of hawkweed, dandelion, lady’s mantle, cinquefoil, meadow grass, European dewberry, and others). Apomixis is successfully utilized in the breeding of citrus fruits, figs, fodder grasses, and other plants. It can be used in producing hybrid corn seeds and other crops from apomictic haploids by means of doubling the number of their chromosomes. Especially important is the use of apomixis with fruit bushes and other woody bushes for which the development of homozygous strains by lengthy self-pollination in six to seven generations is practically impossible. Apomixis can be used to enhance crossbred vigor since it gives rise to a relatively constant progeny which conserves the particularities of the primary form. In the USA and Britain this is the basis of producing on an industrial scale uniform and hardy stock grown from apomictic seedlings of certain apple species. In California, seedlings from apomictic seed buds are used to replace degenerating and weakened clones of citrus trees, which are usually reproduced vegetatively.

REFERENCES

Khokhlov, S. S. Perspektivy evoliutsii vysshikh rastenii. Saratov, 1949.
Khokhlov, S. S. “Apomiksis: klassiflkatsiia i rasprostranenie u pokrytosemennykh rastenii.” In Uspekhi sovremennoi genetiki, vol. 1. Moscow, 1967.
Maheshwari, P. Embriologiia pokrytosemennykh. Moscow, 1954. (Translated from English.)
Poddubnaia-Arnol’di, V. A. Obshchaia embriologiia pokrytosemennykh rastenii. Moscow, 1964.
Petrov, D. F. Geneticheski reguliruemyi apomiksis. Novosibirsk, 1964.

S. S. KHOKHLOV