IMPROVEMENT OF MALE PRONUCLEAR FORMATION DURING CROSS-FERTILIZATION BETWEEN CHINESE-HAMSTER SPERMATOZOA AND SYRIAN-HAMSTER OOCYTES BY NOCODAZOLE, AND CHROMOSOME ANALYSIS OF HYBRID ZYGOTES

During cross-fertilization between Chinese hamster spermatozoa and Syr ian hamster oocytes, incorporated sperm heads frequently fail to devel op into male pronuclei, whereas the group of oocyte chromosomes develo p into female pronuclei. The present study applies this cross-fertiliz ation system to the cytogenetic investigation of mammalian hybrid embr yos. Immediately after insemination, oocytes were exposed to 0.1 mu g/ ml nocodazole for 1 hr (1 hr group) or 2 hr (2 hr group), then further cultured. Although the rates of sperm penetration in the 1 hr (48.0%) and 2 hr (75.8%) groups were significantly lower than that in the con trol group (89.8%), the ratios of male pronuclear formation were highe r in both exposed groups (79.4% and 74.2%, respectively) than in the c ontrol group (10.6%). These results were apparently due to sperm head decondensation induced during the meiotic arrest of oocytes at metapha se II by nocodazole. Chromosomes of hybrid zygotes obtained after noco dazole exposure were analyzed at the first cleavage metaphase. The inc idence of structural chromosome aberrations in the Chinese hamster gen ome of hybrid zygotes was high in the control (42.1%) and 1 hr (48.8%) groups. This incidence was reduced to 14.4% in the 2 hr group. Becaus e the lag of sperm head decondensation behind the second meiotic divis ion of oocytes was greater in the control and 1 hr groups than in the 2 hr group, untimely sperm head decondensation may be implicated in oc currence of structural chromosome aberrations in the male genomes of h ybrid zygotes. Mol. Repuod. Dev. 52.117-124, 1999. (C) 1999 Wiley-Liss , Inc.