Experimental tests of sex determination in Goniozus nephantidis (Hymenoptera: Bethylidae)

Abstract

Single locus complementary sex determination (CSD) occurs in several species of Hymenoptera. Individuals that carry two different alleles (heterozygotes) are female while those with one and two copies of the same allele (hemizygotes and homozygotes) are haploid and diploid males, respectively. A multilocus model, in which diploids are only male if homozygous at all sex loci, has been proposed for species with regular but not exclusive inbreeding. To date, the only explicit test of the multilocus model is for Nasonia vitripennis. The latter is an inbreeding species in the Infraorder Parasitica and was shown not to conform to the multilocus model. In this study, inbreeding experiments are used to refute single and multilocus sex determination in a second inbreeding hymenopteran, Goniozus nephantidis (Infra-order Aculeata). Single locus CSD has been demonstrated in five other aculeates and G. nephantidis is the first aculeate species shown not to have CSD. The absence of CSD in G. nephantidis is probably a derived condition, favoured by the penalty of increased diploid male production under inbreeding.

Sex determination in the Hymenoptera: a review of models and evidence

“…Crozier (1971) argued that diploid male production would remain low as long as occasional outcrosses occurred between inbreeding lines. The multilocus CSD model is more difficult to exclude since there is no a priori specification of the number of loci involved, which determines the number of generations in inbreeding required to generate detectable diploid male production (Cook, 1993). I am aware of only two studies that have explicitly tested the multilocus model, both in inbreeding species.…”

“…First, it has been tested rigorously in very few species and, secondly, results may suggest single-locus CSD because experiments are carried out on lines with little genetic diversity and any multilocus CSD system can collapse to a single-locus system if alleles become fixed at all but one locus (Cook, 1993). The common initiation of experimental stocks with very small numbers of wild-caught individuals increases the importance of this bias.…”

“…If an inbreeding programme is started with a mother/son mating, 10 subsequent generations of inbreeding should be sufficient to detect a system involving at least 15 independent loci (Cook, 1993). More generations would be required if a mother/son cross was not possible.…”

“…For example, broods with 0 per cent and 50 per cent diploid males should be equally common following sibmatings. As another example, the diploid offspring of any mother/son mating should be 50 per cent male (Cook, 1993 Whiting's (1943) single-locus model andSneil's (1935) multiocus, two allele model as special cases of Crozier's (1971) more general formulation. Crozier (1971) envisaged variation between species in the number of sex loci.…”

“…Quantitative tests of multilocus CSD are essentially extensions of the inbreeding tests for single locus CSD for several generations (Cook, 1993).…”