When a gene affects a trait that has an optimum value that differs between the sexes, its alleles can have opposing effects on fitness in males and females. Such intralocus sexual conflict predicts that the sons of high-fitness females and the daughters of high-fitness males will be of low fitness, undermining the effect of sexual selection. Pischedda and Chippindale directly measured fitness in Drosophila melanogaster to demonstrate the extent of intralocus sexual conflict and showed that it occurs mainly in X-chromosomal genes.
Many genes that affect sexually dimorphic phenotypes have evolved to be imprinted, expressed in a sex-limited manner, or inherited preferentially by the same sex. This reduces the problem of alleles that increase the fitness of one sex interfering with the fitness of the other. However, many loci are not modified in these ways and there is mounting evidence that some are the focus of intralocus sexual conflict.
After screening the D. melanogaster genome for genetic variation in fitness, the authors selected lines of high-fitness and low-fitness males and females, and directly measured the fitness of their male and female offspring. High-fitness females produced fitter daughters but less fit sons than low-fitness females, and high-fitness males produced less fit daughters than low-fitness males. However, the sons of high-fitness males did not inherit the fitness of their fathers, which is consistent with most of the sexually antagonistic loci being on the X chromosome.
These results help explain why strong sexual selection does not eliminate variation for fitness in populations. Moreover, they might explain why the results of sexual selection are more pronounced in birds and butterflies, in which males are the homogametic sex. In these species, sex-linked fitness variation can be passed from fathers to sons, allowing females to select for exaggerated male secondary sexual features more efficiently.
Several field studies indicate that intralocus sexual conflict is taxonomically widespread, but more work must be done to directly measure it outside laboratory populations of D. melanogaster. It can occur in any trait that is differentially selected between the sexes but is genetically constrained from diverging. Research on this topic therefore affects areas such as the evolution of imprinting, the maintenance of polymorphism, the existence of evolutionary constraints and the cost of sex.
ORIGINAL RESEARCH PAPER
Pischedda, A. & Chippindale, A. K. Intralocus sexual conflict diminishes the benefits of sexual selection. PLoS Biol. 4, e356 (2006)
Chenoweth, S. F. & Blows, M. W. Dissecting the complex genetic basis of mate choice. Nature Rev. Genet. 7, 681–692 (2006)