Molecular evolution of prolactin in Chiroptera: Accelerated evolution and a large insertion in vespertilionid bats

Pituitary prolactin (PRL) shows an episodic pattern of evolution in mammals, with a slow underlying rate (near stasis) and periods of rapid change in some groups. PRL evolution in bats, the second most speciose mammalian order, has not previously been studied, and is examined here. Slow basal evolution of PRL is seen in some bats, particularly megabats, but in most microbat groups evolution of PRL is more rapid. Accelerated evolution of PRL is particularly notable in the family Vespertilionidae, where analysis of nonsynonymous and synonymous substitutions indicates that it reflects adaptive evolution/positive selection. Remarkably, vespertilionid bats also show a large sequence insertion, of variable length, into exon 4 of PRL, giving a protein sequence 18-60 amino acids longer than normal, with the longest insertions in bats of the genus Myotis. An equivalent insertion has not been reported in PRL of any other vertebrate group. In the 3-dimensional structure of the complex between PRL and the extracellular domain (ecd) of its receptor (PRL:PRLR₂) the inserted sequence is seen to be introduced in the short loop between helices 2 and 3 of PRL; it is far removed from the receptor-binding sites, and may not interfere with binding. The ecd of the receptor also shows variable rates of evolution, with a higher rate in the Vespertilionidae, but this is much less marked than for the hormone. The distribution of substitutions introduced into PRL during vespertilionid evolution appears to be non-random, and this and the evidence for positive selection suggests that the rapid evolution and insert sequence introduction were associated with a significant change in the biological properties of the hormone.