Positive and negative selection in the DAZ gene family

Because a microdeletion containing the DAZ gene is the most frequently observed deletion in infertile men, the DAZ gene was considered a strong candidate for the azoospermia factor. A recent evolutionary analysis, however, suggested that DAZ was free from functional constraints and consequently played little or no role in human spermatogenesis. The major evidence for this surprising conclusion is that the nonsynonymous substitution rate is similar to the synonymous rate and to the rate in introns. In this study, we reexamined the evolution of the DAZ gene family by using maximum-likelihood methods, which accommodate variable selective pressures among sites or among branches. The results suggest that DAZ is not free from functional constraints. Most amino acids in DAZ are under strong selective constraint, while a few sites are under diversifying selection with nonsynonymous/synonymous rate ratios (d_N/d_S) well above 1. As a result, the average d_N/d_S ratio over sites is not a sensible measure of selective pressure on the protein. Lineage-specific analysis indicated that human members of this gene family were evolving by positive Darwinian selection, although the evidence was not strong.