Postzygotic isolation and Haldane's rule in a grasshopper

Abstract

Hybrid dysfunction was examined in reciprocal F1 and backcross hybrids between parapatric subspecies of the meadow grasshopper Chorthippus parallelus. Haldane's rule applies and dysfunction is restricted to the XO males which are sterile. The degree of disruption differs between reciprocal F1S. Males whose mothers were C. p. parallelus have poorer testes, a consequence of either the parallelus X chromosome, which carries a nucleolar organizer region (NOR), or a cytoplasmic factor. There is considerable variation in phenotype amongst F1 families when compared to parental families, which reflects hybrid breakdown. Sterility is probably caused by detrimental epistatic interactions between the single X and the mixed autosomes. Significant differences in testis morphology and survivorship among backcrosses could be attributed to cytoplasmically transmitted factors. An interaction between the cytoplasm and the region of the X chromosome carrying the NOR accounts for variation among backcross individuals. There is no incompatibility between the X and autosomes in backcrosses. This raises questions about the mechanism underlying sterility in the F1. Hybrid dysfunction in this species appears to be polygenically determined. These data emphasize the importance of cytoplasmic influences, as well as the role of the X chromosome, in postzygotic isolation.