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double fertilization[¦dəb·əl ‚fərd·əl·ə′zā·shən]
a sexual process in angio-spermous plants, in which both the egg cell and the central cell of the embryo sac are fertilized.
Double fertilization was discovered by the Russian scientist S. G. Navashin in 1898 in two species of plants—the lily Lilium martagon and the fritillaria Fritillaria orientalis. Both of the sperms brought into the embryo sac through the pollen tube participate in double fertilization. The nucleus of one sperm fuses with the nucleus of the egg cell; the nucleus of the other sperm fuses with the polar nuclei or with a secondary nucleus of the embryo sac. The fertilized egg cell develops into an embryo, and the central cell develops into an endosperm. In embryo sacs with a tricellular egg apparatus the contents of the pollen tube are usually released into one of the synergids, which then decomposes (the remains of the synergid nucleus and the vegetative nucleus of the pollen tube are visible in it); the second synergid subsequently dies off. Next, both sperms, together with the altered cytoplasm of the pollen tube, move to the slitlike interstice between the egg cell and the central cell. Then the sperms dissociate: one of them penetrates the egg cell and enters into contact with its nucleus; the other penetrates the central cell, where it contacts the secondary nucleus or one, and sometimes both, of the polar nuclei. The sperms lose their cytoplasm while still in the pollen tube or when they penetrate the embryo sac; sometimes sperms in the form of unaltered cells are observed even in the embryo sac. In double fertilization the nuclei of the embryo sac are in the interphase and are usually much larger than the nuclei of the sperms, whose form and condition may vary. In crepis and some other Compositae the sperm nuclei have the form of a doubly coiled or twisted chromatin filament; in many plants they are elongated, sometimes twisted, and more or less chromatinized and have no nucleoli; usually sperms consist of rounded interphase nuclei with nucleoli and are sometimes structurally indistinguishable from female nuclei.
It has been proposed (by E. N. Gerasimova-Navashina) that, based on the nature of the union of male and female nuclei, two types of double fertilization may be distinguished: premitotic and postmitotic. In premitotic double fertilization the sperm nucleus becomes immersed in the female nucleus and its chromosomes unspiral; the union of both sets of chromosomes occurs in the interphase (in the zygote). In postmitotic double fertilization the male and female nuclei, retaining their membranes, enter the prophase, at the end of which they begin to unite; interphase nuclei containing chromosome sets of both nuclei are formed only after the first mitotic division of the zygote. In double fertilization two hap-loid nuclei fuse in the egg cell; hence the nucleus of the zygot is diploid. The number of chromosomes in the nuclei of the endosperm depends on the number of polar nuclei in the central cell and on their ploidy; the majority of angiosperms have two haploid polar nuclei and the endosperm is triploid. Xenia—the manifestation of the dominant characters of the endosperm of the paternal plant in the endosperm of hybrid seeds—is a consequence of double fertilization. If several pollen tubes penetrate into the embryo sac, the sperms of the first of them participate in double fertilization, while the sperms of the rest degenerate. Instances of dispermy, that is, fertilization of an egg cell by two sperms, are very rare.
REFERENCESNavashin, S. G. Izbr. trudy, vol. 1. Moscow-Leningrad, 1951.
Maheshwari, P. Embriologiia pokrytosemennykh. Moscow, 1954. (Translated from English.)
Poddubnaia-Arnol’di, V. A. Obshchaia embriologiia pokrytosemennykh rastenii. Moscow, 1964.
Steffen, K. “Fertilisation.” In P. Maheshwari (editor), Recent Advances in the Embryology of Angiosperms. Delhi, 1963.
I. D. ROMANOV