Sex-linked hybrid sterility in a butterfly

Recent studies, primarily in Drosophila. have greatly advanced our understa nding of Haldane's rule, the tendency for hybrid sterility or inviability t o affect primarily the heterogametic sex (Haldane 1922). Although dominance theory (Turelli and Orr 1995) has been proposed as a general explanation o f Haldane's rule, this remains to be tested in female-heterogametic taxa, s uch as the Lepidoptera. Here we describe a novel example of Haldane's rule in Heliconius melpomene (Lepidoptera; Nymphalidae). Female F-1 offspring ar e sterile when a male from French Guiana is crossed to a female from Panama , but fertile in the reciprocal cross. Male F(1)s are fertile in both direc tions. Similar female F-1 sterility occurs in crosses between French Guiana and eastern Colombian populations, Backcrosses and linkage analysis show t hat sterility results from an interaction between gene(s) on the Z chromoso me of the Guiana race with autosomal factors in the Panama genome. Large X (or Z) effects are commonly observed in Drosophila, but to our knowledge ha ve not been previously demonstrated for hybrid sterility in Lepidoptera. Di fferences in the abundance of male versus female or Z-linked versus autosom al sterility factors cannot be ruled out in our crosses as causes of Haldan e's rule. Nonetheless, the demonstration that recessive Z-linked loci cause hybrid sterility in a female heterogametic species supports the contention that dominance theory provides a general explanation of Haldane's rule (Tu relli and Orr 2000).