In the haplodiploid Hymenoptera, haploid males arise from unfertilized eggs, receiving a single set of maternal chromosomes while diploid females arise from fertilized eggs and receive both maternal and paternal chromosomes. Under single-locus complementary sex determination (s/-CSD), sex is determined by multiple alleles at a single locus. Sex locus heterozygotes develop as females, while hemizygous and homozygous eggs develop as haploid and diploid males, respectively. Diploid males, which are inviable or sterile in almost all cases studied, are therefore produced in high frequency under inbreeding or in populations with low sex allele diversity. CSD is considered to be the ancestral form of sex determination within the Hymenoptera because members of the most basal taxa have CSD while some of the more derived groups have other mechanisms of sex determination that produce the haplo-diploid pattern without penalizing inbreeding. In this study, we investigated sex determination in Heterospilus prosopidis Viereck, a parasitoid from a relatively primitive subfamily of the Braconidae, a hymenopteran family having species with and without CSD. By comparing sex ratio and mortality patterns produced by inbred and outbred females, we were able to rule out s/-CSD as a sex determination mechanism in this species. The absence of s/-CSD in H. prosopidis was unexpected given its basal phylogenetic position in the Braconidae. This and other recent studies suggest that sex determination systems in the Hymenoptera may be evolutionary labile.
Bibliographical noteFunding Information:
We thank Frank J Messina for supplying beetle hosts, and M Shimada and A Kobayashi for their Arizona strain of H. prosopidis. We also thank Jeff Berg, Seth Bomgren, Randy Hedlund and Markeeta Keyes for rearing assistance, Jetske De Boer and two anonymous reviewers for advice on the manuscript, J Cook for discussing his unpublished work on H. prosopidis with us, and K Fujii and W Steiner for sharing unpublished information. This study was supported by United States Department of Agriculture National Research Initiative Grant 0002995 to KRH, GEH, PJO and RWF, and by the University of Minnesota Agricultural Experiment Station.
- Complementary sex determination
- Sex determination