SEX DETERMINATION IN THE HYMENOPTERA - A REVIEW OF MODELS AND EVIDENCE

The haploid males and diploid females of Hymenoptera have all chromosomes in the same proportions. This rules out most familiar sex-determining mechanisms, which rely on dosage differences at sex determination loci. Two types of model - genic balance and complementary sex determination (CSD) - have been invoked for Hymenoptera. Experimental studies provide no good evidence for genic balance models, which are contradicted by the detection of diploid males in 33 disparate species. Furthermore, recent advances have shown that sex determination in the best-studied diploid animals does not depend on genic balance, removing the original justification for hymenopteran genic balance models. Instead, several Hymenoptera have single-locus CSD. In this system, sex locus heterozyotes are female while homozygotes and hemizygotes are male. Single-locus CSD does not apply to several inbreeding species and this probably reflects selection against the regular production of diploid males, which are sterile. A multilocus CSD model, in which heterozygosity at any one of several sex loci leads to female development has also been proposed. To date, multilocus CSD has not been demonstrated but several biases against its detection must be considered. CSD can apply to thelytokous races as long as the cytogenetic mechanism permits retention of sex locus heterozygosity. However, some thelytokous races clearly do not have CSD. The distribution of species with and without CSD suggests that this form of sex determination may be ancestral in the Hymenoptera. However, phylogenetic analyses are hindered by the lack of data from several superfamilies and the fact that the internal phylogeny of the Hymenoptera remains controversial.