Abstract
Cytological investigation was made on the early development of gynogenetic diploids as faras the stage of two balstomeres in the loach Misgurnus anguillicaudatus. Diploid gynogenesis was induced by the cold-shock treatement (CST; 0°C, 30 min) of eggs after the insemination of UV-irradiated (9, 000erg/mm2) sperm. Before CST, oocyte was penetrated by a single spermatozoon and resumed second meiosis to reach the anaphase. During CST, meiosis did not proceed beyond the anaphase II, and the aster and spindle fibers disappeared except for the connecting fibers between the daughter chromosome groups at the spindle poles. The sperm nucleus remained as a condensed mass. After CST, the two haploid sets of oocyte chromosomes reunited and developed into a diploid female pronucleus. On the other hand, the spermnucleus, swelling to be transformed into a male pronucleus with aster fibers, approached andcame in contact with the female pronucleus. Then it underwent pycnosis without amphimixis and did not participate in the first karyokinesis, remaining on the equatorial plane attached toapart of the spindle as a dense chromatin body (DCB). Until the second karyokinesis, DCB was recognized in the cytoplasm of one of the two blastomeres. Thus direct evidence andsome aspects of mechanisms of the induction of gynogenetic diploids under the present condi-tions were submitted.
