Complementary sex determination in hymenopteran parasitoids and its implications for biological control

Abstract In haplodiploid Hymenoptera, unfertilized eggs produce haploid males while fertilized eggs lead to diploid females under most circumstances. Diploid males can also be produced from fertilization under a system of sex determination known as complementary sex determination (CSD). Under single-locus CSD, sex is determined by multiple alleles at a single sex locus. Individuals heterozygous at the sex locus are female while hemizygous and homozygous individuals develop as haploid and diploid males, respectively. In multiple-locus CSD, two or more loci, each with two or more alleles, determine sex. Diploid individuals are female if one or more sex loci are heterozygous, while a diploid is male only if homozygous at all sex loci. Diploid males are known to occur in 43 hymenopteran species and single-locus CSD has been demonstrated in 22 of these species. Diploid males are either developmentally inviable or sterile, so their production constitutes a genetic load. Because diploid male production is more likely under inbreeding, CSD is a form of inbreeding depression. It is crucial to preserve the diversity of sex alleles and reduce the loss of genetic variation in biological control. In the parasitoid species with single-locus CSD, certain precautionary procedures can prevent negative effects of single-locus CSD on biological control.