NB Cherfas,
B Gomelsky,
N Ben-Dom,
G Hulata,
Y Peretz
An unusually high rate of spontaneous diploidization of a maternal chromosome-set (SDM) was revealed in two successive generations of induced gynogenesis in ornamental (koi) common carp. The first gynogenetic generation (GG1) was obtained from a normal koi female and the second one (GG2) from one of her gynogenetic daughters. In both cases gynogenesis was obtained from unshocked eggs inseminated with genetically inactivated sperm (UV-irradiated at 800 j per m 2 ). The gynogenetic nature of the progeny was confirmed by a color gene marker and the all-female sex composition. Diploid gynogens amounted to 19% (GG1) and 34% (GG2) of the total number of embryos starting gynogenetic development. About 7000 larvae were obtained in GG2. The genetic characteristics of GG2 were investigated by electrophoretic analysis of five isozymeas and transplantation tests No segregation was observed in any of the isozymes, but all transplants were rejected. Thus, GG2 can not be regarded as a clone. The cytological mechanism responsible for SDM in koi is yet unclear. Based on the negative transplantation results and some other preliminary data, pre-synaptic chromosome-set endoreduplication in early oogenesis, as well as other known meiotic transformations leading to genetic homogeneity, can be excluded. Two possible mechanisms, i.e. suppression of the 1st or 2nd meiotic divisions, are currently under examination. Gynogenetic experiments were carried out, in which eggs were subjected to heat shock at 0.15-0.20 τ o after insemination (anaphase II in the common carp). In two different gynogenetic progenies originating from shocked eggs, about 12% of the investigated larvae were tetraploids (as judged by nucleoli count), while no tetraploids were found in un-shocked gynogenetic groups. This result is in agreement with the possible suppression of the 1st meiotic division, although there is no direct evidence for that. The high rate of SDM in two successive gynogenetic generations suggests a hereditary control for this phenomenon in koi. This is believed to be the first experimental evidence for inheritance of SDM in a bisexual fish species. It is especially relevant to the question of natural polyploidy in fish, suggesting that the ability to produce diploid eggs can arise in pure species, and does not always require pervious hybridization as suggested.
