Complementary sex determination in the genus Diadegma (Hymenoptera : Ichneumonidae)

In the evolution of sexual reproduction we would expect to see a close asso ciation between mating systems and sex determination mechanisms. Such assoc iations are especially evident in the insect order Hymenoptera which shows great diversity with respect to both of these characteristics. The ancestra l sex determination mechanism in this order is thought to be single-locus c omplementary sex determination (sl-CSD), which is inbreeding sensitive, and where inbreeding results in the production of sterile diploid males rather than daughters. Presently, however, there is insufficient data to give str ong support to the hypothesis that sl-CSD is truly the ancestral condition in the Hymenoptera, principally because of the difficulty of reliably deter mining the degree of male ploidy. Here we show that six ichneumonid parasit oids from the polyphyletic genus Diadegma are subject to sl-CSD, using neur onal cell DNA flow cytometry to distinguish ploidy levels. The presence of sl-CSD in these six species, together with earlier evidence from the author s for D. chrysostictos, provides considerable support for the notion that s l-CSD was ancestral in the Aculeata/Ichneumonoidea clade, which contains al l eusocial Hymenoptera. Moreover, because flow cytometry discriminates reli ably between haploid and diploid males, and is independent of the maternal sex allocation or the need for genetic markers, it has considerable potenti al for the determination of ploidy more generally.