Male-biased mutation rates revealed from Z and W chromosome-linked ATP synthase alpha-subunit (ATP5A1) sequences in birds

Whether the mutation rate differs between sexes has been a matter of discus sion for years. Molecular analyses of mammals have indicated that males mut ate more often than females, as manifested by the faster rate of neutral se quence evolution on the Y chromosome than on the X chromosome. However, the se observations can as well be interpreted as specific reduction of the X c hromosome mutation rate, which would be adaptive because of reducing the nu mber of slightly deleterious recessive mutations exposed in hemizygote male s. Recently, data from birds have suggested that vertebrate mutation rates may indeed be male-biased. In birds, females are the heterogametic sex (ZW) , and analyses of the Z-linked CHD1Z gene have shown that it evolves faster than its W-linked and thus female-specific homologue, CHD1W. We have now s tudied the second avian gene known to exist in a copy on the nonrecombining regions of both the Z and the W chromosome, viz., the ATP synthase alpha-s ubunit (ATP5A1). In independent comparisons of three pairs of bird species from divergent lineages, intron sequences of the Z-linked copy (ATP5A1Z) we re consistently found to evolve faster than the W-linked copy (ATP5A1W). Fr om these data we calculated male-to-female mutation rate ratios (alpha) of 1.8, 2.3, and 5.0 in Galliform, Anseriform, and Ciconiiform lineages, respe ctively. Therefore, this study provides independent support for a male-bias ed mutation rate in birds.