Single-locus complementary sex determination in Diadegma chrysostictos (Gmelin) (Hymenoptera : Ichneumonidae)

Following the establishment of isofemale lines and subsequent inbreeding, t he ichneumonid parasitoid wasp Diadegma chrysostictos (Gmelin) was shown by segregation of polymorphic alloenzyme loci to have single-locus complement ary sex determination (sl-CSD). This and the biparental nature of diploid m ales was confirmed using two independent Mendelian recessive phenotypic mar kers. The existence of diploid males, sl-CSD, and the abrogation of diploid males following outbreeding was further confirmed by flow cytometry, a pot entially general method that is independent of the maternal sex allocation or the need for genetic markers. Estimates of the number of sex alleles in several British populations demonstrated 17-19 alleles in Britain, with a d ecline toward the northerly limit of the parasitoid's range, varying from 1 6 in the south of England to 4-5 in central Scotland, in broad agreement wi th the rate of attainment of a male-biased sex ratio when used to establish en masse laboratory cultures. These data represent the second confirmation of the existence of sl-CSD in the Ichneumonidae (and the first in the Camp opleginae subfamily), lending further support to the notion that sl-CSD was the ancestral condition in the Aculeata/Ichneumonoidea clade.