The paternal-sex-ratio (PSR) chromosome in natural populations of Nasonia (Hymenoptera : Chalcidoidea)

Selfish genetic elements may be important in promoting evolutionary change. Paternal sex ratio (PSR) is a selfish B chromosome that causes all-male fa milies in the haplodiploid parasitic wasp Nasonia vitripennis, by inducing paternal genome loss in fertilized eggs. The natural distribution and frequ ency of this chromosome in North American populations of N. vitripennis was investigated using a combination of phenotypic and molecular assays. Sampl ing throughout North America failed to recover PSR except from populations in the Great Basin area of western North America. Extensive sampling of Gre at Basin populations revealed PSR in frequencies ranging from 0 to 6% at di fferent collection sites, and extended its distribution to Idaho and Wyomin g. Intensive sampling in upstate New York did not detect the chromosome. Fr equencies of the maternal-sex ratio distorter (MSR), son killer (SK) and vi rgin females ranged from 0 to 12%. Paternal sex ratio may be restricted to the Great Basin because its spread is hampered by geographical barriers, or because populations in other areas are not conducive to PSR maintenance. H owever, it cannot be ruled out that PSR occurs in other regions at very low frequencies. The apparent limited distribution and low frequency of PSR su ggest that it will have relatively little impact on genome evolution in Nas onia.