Molecular variation of the testes-specific beta NACtes genes in the Drosophila melanogaster genome

The beta NACtes gene family of the Drosophila melanogaster genome provides a model for investigating the mechanisms of the molecular evolution of recently evolved genes. The beta NACtes genes code for proteins that are homologous to the subunit of the nascent polypeptide-associated complex (NAC), are expressed exclusively in the testis, and are localized on the X chromosome as two-gene clusters and one separate copy. Population polymorphism of the beta NACtes genes was studied using several wild-type D. melanogaster stocks, and beta NACtes paralogs were compared with each other. A heterogeneous pattern was observed for beta NACtes polymorphism: the 3' genes of the two-gene clusters were low polymorphic, whereas, separate, the beta NACtes1 gene was the most variable. The 5' beta NACtes copies of the two-gene tandems were practically identical, whereas the 3' beta NACtes copies were highly diverged. Hence, local gene conversion was assumed to provide for the selective homogenization of the 5' genes. A comparison of the beta NACtes paralogs showed that the majority of amino acid differences were in the N-terminal region, containing the beta NAC domain. The McDonald-Kreitman test was used to analyze the divergence of beta NACtes paralogs and implicated positive selection in the evolution of the beta NACtes gene family.