TEMPERATURE EFFECTS AND GENOTYPE-BY-ENVIRONMENT INTERACTIONS IN HYBRIDS: HALDANE'S RULE IN FLOUR BEETLES.

When males of the flour beetle, Tribolium castaneum, are crossed to females of its close relative T. freemani, the sex ratio of the hybrids is female biased, owing in part to hybrid male mortality. Morphological abnormalities are also frequent in the surviving hybrid males, but not in the hybrid females. The finding that the heterogametic sex (male) is more adversely affected in interspecific crosses than the homogametic sex is consistent with Haldane's rule, which predicts that hybrid dysfunction should emerge as an indirect byproduct of divergent adaptation to differing environments. If so, environmental effects and genotype-by-environment interactions (GEI) should characterize the expression of Haldane's rule and interspecific hybrid traits in general. We used two wild-collected populations of T. castaneum (from Infantes, Spain, and Madagascar) to investigate the effects of environmental variation on the expression of Haldane's rule. Males from each population were mated to several T. freemani females and the half-sibling hybrid progenies were reared across a series of temperature regimes. For both populations, we found that hybrids raised at higher temperatures exhibited a more extreme expression of Haldane's rule: The hybrid sex ratios were more biased toward females and hybrid males had a much higher incidence of morphological abnormalities. The average response to temperature, the norm of reaction for Haldane's rule, varied between the two populations, and we found considerable and significant GEI for both hybrid traits within both populations. The evolutionary implications of these findings are discussed in the context of speciation arising as an indirect effect of local adaptation.