Sex determination in parasitoid wasps studied by a genome-wide set of SNP markers
Differentiation between males and females is a basal asset of sexually reproducing organisms. Yet, sex determination systems that initiate this differential development are highly diverse across the animal kingdom. In the insect order Hymenoptera, all sexually reproducing species are haplodiploid, with males normally developing from unfertilized, haploid eggs and females from fertilized, diploid eggs. However, sex is not determined by fertilization or ploidy per se. Perhaps the most widespread mechanism of sex determination within the Hymenoptera is complementary sex determination (CSD), which is characterised by the presence of diploid males. Diploid males develop from fertilised eggs that are homozygous at a sex locus. Heterozygosity at this locus leads to female development. Despite the clear relationship between CSD, mating structure and inbreeding, the rate of diploid male production and mating structure have rarely been measured within the same population under natural conditions.
We examine the production of diploid males and mating structure in a natural population of Cotesia vestalis (Braconidae), which is a solitary endoparasitoid of diamondback moth larvae Plutella xylostella. Complementary sex determination in C. vestalis is likely based on two unlinked sex loci as suggested by the rate of diploid male production in laboratory crosses. We thus expected to find low levels of diploid males and a low level of inbreeding in the field population of C. vestalis. Because very few molecular markers were available for C. vestalis, we developed a set of single nucleotide polymorphism (SNP) assays by de novo Illumina sequencing and assembly of the entire C. vestalis genome using pooled DNA of C. vestalis females.