[Gynogenesis in vertebrates]

After a rapid description of five gynogenetic unisexual species, this paper reviews the methods used for inducing gynogenesis in bisexual species. 1) The frequency of male genome extrusion, very low after intraspecific mating, can be appreciably increased in some particular interspecific combinations; cold-shocking the eggs at the time of fertilization can also result in gynogenetic development. In the case, of mammals, the male pronucleus can be microsurgically removed, but in most cases (amphibians, fish), haploid gynogenesis is induced by mutagenic treatment of the sperm (gamma, X, UV irradiation; chemicals) before fertilization. 2) Viable gynogenesis depends on the possibility of diploidizing the female genetic set; several factors (oocyte aging, genetic factors, induction of ovulation) may be responsible for the high frequency of "spontaneous" diploidization reported in some studies. But diploid gynogenesis is generally obtained by induced retention of the second polar body or by induced suppression of the first cleavage, achieved by heat or pressure treatment of eggs (fish and amphibians) or by the use of antimitotic chemicals (mammals). Diploid gynegenesis results in high inbreeding levels and original sex ratios. In the discussion, we have tried to show that the objectives which stimulated its induction in amphibians have been frequently unattained or reached in other ways. In commercial fish species, gynogenesis may in the future contribute to the solution of major problems such as genetic improvement and control of reproduction.