Molecular evolution of the semenogelin 1 and 2 and mating system in gibbons

Semenogelin 1 and 2 (SEMG1 and SEMG2) are known as semen coagulating proteins in primates with a repetitive structure of 60-amino acids. The number of repeats varies among species and is hypothesized to be related to the level of primate sperm competition. Gibbons until recently were thought to be monogamous primates, but it is now known that gibbon social structure is flexible. Thus, hypotheses of the relationship between the SEMGs evolution and mating systems were tested.

The sequences of the exon 2 of the SEMG1 and SEMG2 were obtained from 50 captive gibbons comprising six species belonging to three genera (Hylobates, Symphalangus, and Nomascus). Then we quantified the levels of polymorphism and estimated rates of protein evolution by calculating d _N /d _S ratio.

Several mutations that create a premature stop codon in the SEMG1 and a reduction in the repeats of the SEMG2 in the genus Hylobates were observed and may alter the coding properties for these proteins. We also found different level of nucleotide diversity in each gene and between genera. Strikingly, in Nomascus leucogenys we discovered a high d _N /d _S ratio in the SEMG1 and SEMG2. The Nomascus SEMG2 also showed significantly lower nucleotide diversity than the other two genera.

These results are consistent with the presence of a strong positive selection in the Nomascus lineage even if the exact selective forces acting on these genes are not yet conclusively known. We were not able to demonstrate, among gibbons, unambiguous relationships between the SEMGs evolution and mating systems.