Analysis of autosomal polygenic variation for the expression of Haldane's rule in flour beetles

Haldane's rule states that, in interspecific crosses, when hybrid viability or fertility is diminished more in one sex of the hybrids than in the other, the heterogametic sex is more adversely affected. We used quantitative genetic methods to investigate the genetic basis of variation for the expression of the viability aspect of Haldane's rule when Tribolium castaneum males are crossed to Tribolium freemani females. Using a half-sib design, we found significant genetic variance for the expression of Haldane's rule, i.e., variation among T. castaneum sires in the hybrid sex ratios produced by their sons. We also derived 23 independent lineages from the same base population by 8 generations of brother-sister mating. From the same experiments, we also found heritable variation among surviving hybrid males in the incidence of antennal deformities. Upon inbreeding, the variance of both traits (hybrid sex ratio and proportion deformities) increased substantially but the means changed little. Because fitness within T. castaneum lineages declined substantially with inbreeding, we infer that hybrid male viability may have a different genetic basis than viability fitness within species. Deleterious recessive alleles held within species by mutation/selection balance appear not to be a major contributor to hybrid incompatibility.