¹Department of Biology, McMaster University, Hamilton, ON, Canada L8S 1J4

Correspondence: DG Torgerson, Department of Molecular Biology and Genetics, Cornell University, 227 Biotechnology Bldg., Ithaca, NY 14850, USA. E-mail: dt224@cornell.edu

Received 25 August 2004; Accepted 25 July 2005; Published online 21 September 2005.

Abstract

Genes on the mammalian X chromosome may be under unique evolutionary pressure due to their hemizygous expression in males. Since any recessive deleterious mutation would immediately be expressed in males and, therefore, efficiently removed from the population, selective constraint could be more pronounced in X-linked genes. Conversely, if a recessive mutation were beneficial, its immediate exposure to selection would be advantageous, and would facilitate adaptive evolution. We tested for positive selection in a total of 86 genes using a maximum likelihood approach, including 40 sperm-expressed and 46 non-sperm, tissue-specific genes. We find evidence to suggest that X-linkage enhances the effects of positive selection in sperm-expressed genes in terms of the number of codons affected, and report a general trend for positively selected genes to reside on the X chromosome rather than on the autosomes. Our data suggest that hemizygous expression in males makes the X chromosome a preferred location for positively selected sperm genes that do not require postmeiotic transcription.