1. ¹Department of Genetics and Genomics, Roslin Institute (Edinburgh), Midlothian, UK
2. ²Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, UK

Correspondence: Dr S Berlin, Department of Evolutionary Biology, EBC, Uppsala University, Norbyvagen 18 D, Uppsala 75236, Sweden. E-mail: sofia.berlin@ebc.uu.se

Received 20 November 2006; Revised 19 April 2007; Accepted 27 April 2007; Published online 6 June 2007.

Abstract

Interference among loci subject to selection (the Hill–Robertson effect) may considerably reduce levels of adaptation and variability in genomic regions that lack recombination. Y- or W chromosomes are particularly vulnerable to such effects, since they represent large, non-recombining blocks of genetic material. In birds, the W chromosome and mitochondrial genomes are both maternally transmitted, and hence fail to recombine with each other, whereas in mammals the Y chromosome is paternally transmitted. We show here that mitochondrial DNA sequence diversity is reduced in non-ratite birds compared with mammals. After considering possible confounding factors, such as differences in generation times, mutation rates and demography, we conclude that Hill–Robertson effects associated with the W chromosome provide the most likely explanation for this difference.