Delayed sperm incorporation into parthenogenetic mouse eggs: Sperm nucleustransformation and development of resulting embryos

In this study we examined the effect of experimentally induced asynchrony b etween male and female pronuclei on male pronucleus formation and developme ntal potential of the resulting mouse embryos. We demonstrate that when the interval between oocyte activation and sperm incorporation is up to 1.5-2 hr, the spermatozoa transform into normal pronuclei. These male pronuclei c an replicate their chromosomes during the first embryonic cell cycle and ar e transcriptionally competent. During the first cleavage these "delayed" ma le pronuclei condense into discrete mitotic chromosomes and when resulting embryos are transplanted into oviducts of pregnant females at least same of them can develop to term. In contrast, when sperm nuclei are introduced in to parthenogenetic eggs 3 hr or more after activation, their transformation into pronuclei is significantly impaired, and they neither replicate nor t ranscribe. During the first mitosis they form a group of condensed chromati n, which is displaced into one of the resulting blastomeres leading to form ation of haploid/diploid mosaic embryos. These mosaic embryos have poor dev elopmental potential: only a few can reach blastocyst stage in vitro and no full-term development of such embryos was observed after transfer into pre gnant females. We conclude that the cytoplasmic factors that make possible the transformation of a sperm nucleus into a functional male pronucleus exh aust within 1.5-2 hr after fertilization and that the male genome which had skipped the first cell cycle cannot become a functional partner in the emb ryonic genome. (C) 1999 Wiley-Liss, Inc.