EVIDENCE FOR A GENOMIC IMPRINTING SEX DETERMINATION MECHANISM IN NASONIA-VITRIPENNIS (HYMENOPTERA, CHALCIDOIDEA)

Five different models have been proposed for the sex determination mec hanism of Chalcidoidea (Hymenoptera). Except for the most recently pro posed model (genomic imprinting sex determination; GISD), each of thes e models has required complicating additions to explain observed pheno mena. This report provides the first experimental test of the GISD mod el while simultaneously examining the four previously proposed models of sex determination. This test utilizes the parasitic wasp Nasonia vi tripennis, crossing polyploid females with males harboring the paterna l sex ratio chromosome (PSR). The results of this study support the GI SD model as the mechanism of sex determination in Chalcidoidea. Specif ically, crosses demonstrate that sex determination is independent of e mbryonic heterozygosity, ploidy, and gametic syngamy but is directly c orrelated with the embryonic presence of correctly imprinted chromosom es of paternal origin. These crossing experiments also provide informa tion about the poorly characterized mechanisms of PSR, a supernumerary chromosome that induces paternal autosome loss in early embryos. The results demonstrate that the poor transmission of PSR through females is not a result of the ploidy of the host but of an alternative sex-de pendent process. Crossing data reveal that PSR consistently induces th e loss of the entire paternal complement that it accompanies, regardle ss of whether this complement is haploid or diploid.