1. ¹Department of Biology, Colorado State University, Fort Collins, CO 80523, USA
2. ²Department of Entomology, North Dakota State University, Fargo, ND 58105, USA
3. ³Department of Entomology, University of Minnesota, St Paul, MN 55108, USA
4. ⁴Department of Ecology and Systematics, University of Helsinki, Helsinki, Finland
5. ⁵Department of Entomology, University of Georgia, Athens, GA 30602, USA

Correspondence: MF Antolin, Department of Biology, Colorado State University, Fort Collins, CO 80523-1878, USA. E-mail: antolin@lamar.colostate.edu

Received 10 April 2003.

Abstract

Besides haplo-diploid sex determination, where females develop from fertilized diploid eggs and males from unfertilized haploid eggs, some Hymenoptera have a secondary system called complementary sex determination (CSD). This depends on genotypes of a 'sex locus' with numerous sex-determining alleles. Diploid heterozygotes develop as females, but diploid homozygotes become sterile or nonviable diploid males. Thus, when females share sex-determining alleles with their mates and produce low fitness diploid males, CSD creates a genetic load. The parasitoid wasp Habrobracon hebetor has CSD and displays mating behaviours that lessen CSD load, including mating at aggregations of males and inbreeding avoidance by females. To examine the influence of population structure and the mating system on CSD load, we conducted genetic analyses of an H. hebetor population in Wisconsin. Given the frequency of diploid males, we estimated that the population harboured 10–16 sex-determining alleles. Overall, marker allele frequencies did not differ between subpopulations, but frequencies changed dramatically between years. This reduced estimates of effective size of subpopulations to only N_e20–50, which probably reflected annual fluctuations of abundance of H. hebetor. We also determined that the mating system is effectively monogamous. Models relating sex-determining allele diversity and the mating system to female productivity showed that inbreeding avoidance always decreased CSD loads, but multiple mating only reduced loads in populations with fewer than five sex-determining alleles. Populations with N_e less than 100 should have fewer sex-determining alleles than we found, but high diversity could be maintained by a combination of frequency-dependent selection and gene flow between populations.