Breeding System and Sex Determination in the Hymenoptera

Inbreeding, or mating among relatives, is widely considered to have harmful genetic consequences and many plants and animals have genetic, physiological, and behavioral characters that have evolved in the context of reducing the chances of inbreeding. The relatively few notable exceptions to this general pattern of inbreeding avoidance suggest a variety of questions about the behavior, population biology, and genetics of the species involved. Insects in the order Hymenoptera have broad variation in the amount of inbreeding; some species have regular mating between mothers and sons or between brothers and sisters; other species have recognition systems or dispersal patterns that separate relatives before mating occurs.

Because insects in the order Hymenoptera have a haplo-diploid genetic system, with diploid females arising from fertilized eggs, and normal haploid males developing form unfertilized eggs, harmful recessive alleles will be expressed frequently and selected against in males. Thus, harmful recessive alleles will be tend to be purged from the gene pool relatively quickly and unavailable to cause inbreeding depression if mating among relatives should occur. Perhaps because of these factors, mating systems with inbreeding are a common among haplo-diploid animals.

Even though haplo-diploidy may mitigate some of the costs of inbreeding, many hymenopterans have a complementary sex determination (CSD) system that functions poorly when inbreeding occurs. In these insects, there is a sex determination locus. Diploid individuals that are heterozygous at the sex locus develop as females. Haploid individuals have only one allele at the locus (they are hemizygous) and develop as normal haploid males, but if a diploid individual is homozygous at the sex locus, it develops as a diploid male. When studied, diploid males have been found to be inviable, or unable to mate properly. If they can mate, they produce diploid sperm that results in sterile triploid offspring. In any case, the result seems to be that diploid males are a genetic dead end.

Because of problems with CSD and diploid male production, species with closely inbred populations might be expected to have different sex determination methods, and species with CSD should have traits that promote outbreeding. To a first approximation, this prediction seems to true; many closely inbred chalcid wasps do not have CSD, and many bees and hunting wasps known to have CSD are sexually active only after they have left their natal area where they would be most likely to encounter relatives. We have been investigating a group of solitary eumenine wasps (see natural history of eumenine wasps) in which brothers and sisters often mate at their natal nest before dispersal and who's closest relatives have CSD. To clarify this apparent paradox, we are quantifying the amount of inbreeding in nature using allozymes and microsatellites, and performing laboratory breeding experiments to test for CSD.

Allozymes for the dimeric enzyme phosphoglucose isomerase from twelve females of the wasp Ancistrocerus adiabatus. All are homozygous (mm) except for individuals six (heterozygous, mf) and ten (heterozygous, sm). (thanks to Mark DeMeester)

Microsatellite profile of three loci for an individual of Euodynerus foraminatus. Analysis performed by Julie Stahlhut on a Beckman Coulter CEQ 2000XL. (thanks to Todd Barkman and the WMU Sequencing Facility)