Phylogenetics and Evolution
More on the Molecular Evolution of Zonadhesin
ZAN Evolution in Primates
Studying the molecular evolution of the sperm ligand zonadhesin (ZAN) we observed a negative dependence of species-specific rates of sequence evolution (dN/dS in the figure below) on sexual size dimorphism in primates (residual male body weight in below figure). The pattern makes sense considering that many of the investigated species with increased sexual dimorphism are uni-male breeding (e.g., hamadryas baboon P. hamadryas) which implies that males monopolize access to estrous females by physical competition. Consequently, the degree of male competition at the sperm level (sperm competition) should be lowered in uni-male breeding species. We assume that lowered levels of sperm competition result in decreased rates of sequence evolution of sexually selected loci such as ZAN. The opposite extreme represent species with little or even inverse sexual dimorphism such as the gray mouse lemur (M. murinus) where females mate several males around estrous and levels of sperm competition as well as rates of ZAN evolution are increased. Subsequent studies on sperm adhesion molecule 1 (SPAM1; see Research) suggest that a negative dependence of sequence evolution on sexual size dimorphism is a general phenomenon in the evolution of sperm ligands that, however, might be obscured by socio-cultural factors in apes and human.
Zan Evolution in House Mouse
ZAN is a so-called multi-domain protein consisting of different sections (domains) named, amongst others, D domains (see orange below figure). Mouse Zan is distinguished from ZAN of other mammalian species by 20 extra domains (D3p1-20; light yellow in below figure) that are derived from a fragment of domain D3 (ancestral D3 fragment in below figure).
We investigated the sequence evolution among mouse partial D3 domains (paralog comparison in below figure) and between the ancestral D3 fragment of mouse Zan and the respective D3 fragments of other species (ortholog comparison in below figure). Whichever measure we took (dN, dS etc.) sequences differed more in the paralog comparison and thus within mouse than in the ortholog comparison between ten mammalian species. This is particularly remarkable considering that mouse paralogs represent less divergence time than the D3 fragments compared between species. This suggests that partial D3 domains of mouse Zan evolved very fast, possibly in response to increased levels of sperm competition.
The outlined findings and conclusions are embedded in the work of many other authors and we refer to the list of publications for a first survey of the published literature.
Working Group Members
Institute of Anthropology, Mainz