MALE-DRIVEN EVOLUTION OF DNA-SEQUENCES IN BIRDS

Assuming that new mutations arise mainly during DNA replication, seque nce evolution in mammals has been seen as 'male driven' (ref. 1) becau se of the many more cell divisions in spermatogenesis than in oogenesi s. Molecular support for this idea has been obtained from the observat ion of higher substitution rates in genes on the Y than on the X chrom osome of primates and rodents(2-4), which are species with male hetero gamety, but has not been confirmed by the reciprocal analysis of organ isms with female heterogamety. The recent suggestion that an intrinsic reduction in the X-chromosome mutation rate may be confounded with ma le effects in previous comparisons(5), and the paradoxical finding of low levels of polymorphism on the primate Y chromosome(6-8) indicate t hat the idea of male-biased mutation rate needs to be re-examined. We have analysed the molecular evolution of the gene CHD, which is presen t on the Z and W sex chromosomes of birds. The substitution rate at sy nonymous positions, as well as in intron DNA, was considerably higher on the Z chromosome than on the female-specific W chromosome, with an estimated male-to-female bias in mutation rate (alpha(m)) of 3.9-6.5. Thus, evolution appears to be male driven in birds-a situation that su pports a neutral model of molecular evolution.