Estimation of allele numbers at the sex-determining locus in a field population of the turnip sawfly (Athalia rosae).

Hymenopteran insects (sawflies, ants, bees, and wasps) have an unusual genetic system called haplodiploidy, where parthenogenetically produced haploid eggs become males, and fertilized, diploid eggs become females. Several hypotheses have been proposed to explain the mechanism of such sex determination, including control at a single polymorphic locus. From experiments of mother-son mating and using a genetic marker, we show that a single multiallele locus controls sex determination in the turnip sawfly (Athalia rosae). We estimated the number of alleles at this single locus in a field population by analyzing the rate of diploid males in the field and the rate of diploid males by random crossing in the laboratory. Only one diploid male was discovered in 1306 diploid larvae collected in the field. However, the number of alleles calculated by random crossing in the laboratory was 45-50. We suggest that the effective population size may be much larger than that from the areas where we collected larvae, and that there are mechanisms for avoiding inbreeding, including protogyny, dispersion, and sperm displacement by second-mated males.