INDUCTION OF PATERNAL GENOME LOSS BY THE PATERNAL-SEX-RATIO CHROMOSOME AND CYTOPLASMIC INCOMPATIBILITY BACTERIA (WOLBACHIA) - A COMPARATIVE-STUDY OF EARLY EMBRYONIC EVENTS

Paternal genome loss (PGL) during early embryogenesis is caused by two different genetic elements in the parasitoid wasp, Nasonia vitripenni s. Paternal sex ratio (PSR) is a paternally inherited supernumerary ch romosome that disrupts condensation of the paternal chromosomes by the first mitotic division of fertilized eggs. Bacteria belonging to the genus Wolbachia are present in Nasonia eggs and also disrupt paternal chromosome condensation in crosses between cytoplasmically incompatibl e strains. Cytoplasmic incompatibility Wolbachia are widespread in ins ects, whereas PSR is specific to this wasp. PGL results in production of male progeny in Nasonia due to haplodiploid sex determination. The cytological events associated with PGL induced by the PSR chromosome a nd by Wolbachia were compared by fluorescent light microscopy using th e fluorochrome Hoescht 33258. Cytological examination of eggs fertiliz ed with PSR-bearing sperm revealed that a dense paternal chromatin mas s forms prior to the first metaphase. Quantification of chromatin by e pifluorescence indicates that this mass does undergo replication along with the maternal chromatin prior to the first mitotic division but d oes not replicate during later mitotic cycles. Contrary to previous re ports using other staining methods, the paternal chromatin mass remain s condensed during interphase and persists over subsequent mitotic cyc les, at least until formation of the syncytial blastoderm and cellular ization, at which time it remains near the center of the egg with the yolk nuclei. Wolbachia-induced PGL shows several marked differences. M ost notable is that the paternal chromatin mass is more diffuse and te nds to be fragmented during the first mitotic division, with portions becoming associated with the daughter nuclei. Nuclei containing portio ns of the paternal chromatin mass appear to be delayed in subsequent m itotic divisions relative to nuclei free of paternal chromatin. Crosse s combining incompatibility with PSR were cytologically similar to Wol bachia-induced PGL, although shearing of the paternal chromatin mass w as reduced. Wolbachia may, therefore, block an earlier stage of patern al chromatin processing in the fertilized eggs than does PSR. (C) 1995 Wiley-Liss, Inc.