SPERM NUCLEAR TRANSFORMATIONS IN CYTOPLASMIC EXTRACTS FROM SURF CLAM (SPISULA-SOLIDISSIMA) OOCYTES

Following their incorporation into oocytes, sperm nuclei (SN) of the s urf clam, Spisula solidissima, undergo an initial expansion, followed by condensation and then a dramatic enlargement during their developme nt into male pronuclei. These changes are temporally correlated with a lterations in the maternal chromatin: germinal vesicle breakdown (GVBD ), meiotic maturation, and female pronuclear development, respectively . To analyze possible changes occurring in SN at fertilization, surf c lam oocyte extracts; prepared before and after parthenogenetic activat ion, were examined for their ability to affect SN in vitro. Sperm head s were incubated in extracts for variable periods up to 5 hr. Extracts prepared from oocytes following GVBD (15 min postactivation) induced an expansion in similar to 90% of SN by 60 min incubation. However, wh en SN were incubated in extracts from unactivated or 4-min-activated o ocytes only similar to 30% underwent expansion. Ultrastructural examin ation of specimens taken at increasing periods of incubation in oocyte extracts revealed that SN expansion in vitro resembled chromatin deco ndensation in vivo. SN incubated 1 to 5 hr in extracts prepared from o ocytes following GVBD consisted of decondensed chromatin surrounded to varying degrees by membranous cisternae. Staining with anti-lamin ant ibody was variable: some specimens (60-70%) were positive while others (30-40%) were weak to negative. In contrast, all decondensed SN incub ated in extracts from postmeiotic oocytes (65 min postactivation) were delimited by an intact nuclear envelope possessing nuclear pores and reactive to anti-lamin antibody. Decondensation of SN in 15- or 65-min extracts was blocked by EDTA, 2,6-dimethylami-nopurine, histone, and protamine. The presence (65-min extract) and absence (unactivated, 4- and 15-min extracts) of sperm nuclear envelope assembly in vitro is co nsistent with events in vivo, where such a structure forms after meiot ic maturation in concert with the development of the female pronucleus . (C) l994 Academic Press, Inc.