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Haldane’s rule and hybrid male sterility in Drosophila decades ago (Dobzansky, 1936), it was not until recently that this large effect of the hemizygous sex chromosome was documented for birds, using genetic data from natural hybrid zones and domesticated species (Price, 2002; Saetre et al., 2003). In this section, we review empirical and theoretical work that explores these two rules of speciation in birds.

The phenomenon of Haldane’s rule describes patterns of postzygotic incompatibilities in hybrids and is likely caused by negative epistatic interactions between loci derived from divergent parental genomes (Coyne and Orr, 2004). Heterogametic hybrids are more severely affected by these interactions because, unlike the homogametic sex, they fully express recessive sex-linked genes. Interestingly, avian and Lepidopteran F1 hybrid females may suffer from an additional source of negative epistasis between parental genomes, namely that between the maternally derived mitochondria or cytoplasm and the paternally derived Z chromosomes (Presgraves, 2002). There is debate over the extent to which Haldane’s rule is driven by interactions among sometimes rapidly diverging sex chromosomes per se, or whether it is the peculiar dominance patterns exhibited by the hemizygous sex chromosomes that underlie the rule. Support for Haldane’s rule is excellent in birds based on experimental studies of hybrid fitness in ducks (Tubaro and Lijtmaer, 2002), pigeons and doves (Lijtmaer et al., 2003), and many other avian taxa (Price and Bouvier, 2002).

Price and Bouvier (2002) characterized patterns of postzygotic incompatibilities in birds using published data from 254 hybrid crosses and found that the order in which incompatibilities accumulate with increasing species divergence differs between birds and other taxa, a pattern that informs the causes of Haldane’s rule in birds (Fig. 6.3). In Drosophila, male sterility appears at early stages of divergence, followed in turn by male inviability, then female sterility, and finally female inviability. By contrast, avian incompatibilities accumulate in the following order: female sterility, male sterility, female inviability, and male inviability (Price and Bouvier, 2002). Thus, in birds, homogametic (male) sterility evolves at earlier stages than does homogametic (female) sterility in Drosophila (Coyne and Orr, 1989). The appearance of homogametic (male) sterility before heterogametic (female) inviability in birds may reflect a general trend, regardless of sex-chromosome system, of the rapid evolution of male reproductive genes via sexual selection, resulting in high divergence between species at these loci (Wu and Davis, 1993). However, the rapid evolution of male reproductive genes via sexual selection—so-called “faster-male evolution” or the “sexual selection model” of Wu and Davis (1993)—works in opposition to Haldane’s rule in birds because this particular force will negatively affect the homogametic sex (males), not the



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