ANALYSIS OF AUTOSOMAL POLYGENIC VARIATION FOR THE EXPRESSION OF HALDANES RULE IN FLOUR BEETLES

Haldane's rule states that, in interspecific crosses, when hybrid viab ility or fertility is diminished more in one sex of the hybrids than i n the other, the heterogametic sex is more adversely affected. We used quantitative genetic methods to investigate the genetic basis of vari ation for the expression of the viability aspect of Haldane's rule whe n Tribolium castaneum males are crossed to Tribolium freemani females. Using a half-sib design, we found significant genetic variance for th e expression of Haldane's rule, i.e., variation among T. castaneum sir es in the hybrid sex ratios produced by their sons. We also derived 23 independent lineages from the same base population by 8 generations o f brother-sister mating. From the same experiments, we also found heri table variation among surviving hybrid males in the incidence of anten nal 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 dec lined substantially with inbreeding, we infer that hybrid male viabili ty may have a different genetic basis than viability fitness within sp ecies. Deleterious recessive alleles held within species by mutation/s election balance appear not to be a major contributor to hybrid incomp atibility.