Adaptive Evolution of the First Extra Exon in the Murid Rodent Prolactin Gene Family

Abstract

The prolactin gene family in rodents consists of multiple members that coordinate the processes of reproduction and pregnancy. Some members of this family acquired one or two additional exons between exon 2 and exon 3 of the prototypical 5-exon, 4-intron structure, but the evolutionary importance of this insertion is unclear. Here, we focus on those members and survey this question by molecular evolutionary methods. Phylogenetic analysis shows that those members cluster into two distinct groups. Further analysis shows that the two groups of genes originated before the divergence of mouse and rat but after that of rodents from other mammals. We compared the d (N)/d (S) values for each branch of the gene tree but found no evidence to support positive selection for any branch. We found strong evidence, however, that one site (11E) of the 13 sites of the first extra exon underwent positive selection by the site-specific models of the maximum-likelihood method. Combining our molecular evolutionary analysis with other known functional evidence, we believe that the insertion of the extra exon implies some functional adaptation.

Spatial and temporal expression of the 23 murine Prolactin/Placental Lactogen-related genes is not associated with their position in the locus

“…Interestingly, members of the murine PRL/PL family have undergone further diversification by adopting splice variants. Evidence for positive selection within large families of amplified genes, particularly those associated with reproduction, has been accumulating [ 70 - 74 ]. One theory suggests that the gene duplication event itself is positively selected for to allow the amplification of genes that are somewhat pre-adapted to meet a particular environmental challenge or biochemical niche, so that divergence and acquisition of new functionality may follow [ 75 ].…”