Parallel Evolution of Sperm Hyper-Activation Ca2+ Channels

Abstract

Sperm hyper-activation is a dramatic change in sperm behavior where mature sperm burst into a final sprint in the race to the egg. The mechanism of sperm hyper-activation in many metazoans, including humans, consists of a jolt of Ca2+ into the sperm flagellum via CatSper ion channels. Surprisingly, all nine CatSper genes have been independently lost in several animal lineages. In Drosophila, sperm hyper-activation is performed through the cooption of the polycystic kidney disease 2 (pkd2) Ca2+ channel. The parallels between CatSpers in primates and pkd2 in Drosophila provide a unique opportunity to examine the molecular evolution of the sperm hyper-activation machinery in two independent, nonhomologous calcium channels separated by > 500 million years of divergence. Here, we use a comprehensive phylogenomic approach to investigate the selective pressures on these sperm hyper-activation channels. First, we find that the entire CatSper complex evolves rapidly under recurrent positive selection in primates. Second, we find that pkd2 has parallel patterns of adaptive evolution in Drosophila. Third, we show that this adaptive evolution of pkd2 is driven by its role in sperm hyper-activation. These patterns of selection suggest that the evolution of the sperm hyper-activation machinery is driven by sexual conflict with antagonistic ligands that modulate channel activity. Together, our results add sperm hyper-activation channels to the class of fast evolving reproductive proteins and provide insights into the mechanisms used by the sexes to manipulate sperm behavior.

Keywords: CatSper; evolutionary arms race; parallel evolution; reproductive proteins; sexual conflict; sperm hyper-activation.

Fig. 1.—

The entire CatSper complex evolves adaptively in primates. (A) The CatSper complex evolves under positive selection in almost every lineage in the primate phylogeny. For each CatSper gene, branches where dN/dS >1 are highlighted with a color corresponding to each gene. In cases where multiple genes have a dN/dS >1 for a single branch, the colors are stacked. (B) We detect specific sites under selection in almost every CatSper gene. Extracellular domains are marked as dark segments and intracellular domains as light segments. Orange arrows indicate the sites under selection by the Bayes-Empirical-Bayes test in PAML, with a posterior-probability >0.90 (Yang et al. 2005). Selected sites grouped close together are labeled with a bar specifying the number of sites under selection. The number of extracellular and intracellular sites under selection for each gene are tabulated.

Fig. 2.—

pkd2 evolves adaptively in Drosophila. (A) Fixed nonsynonymous fixed differences between D. melanogaster and D. simulans are marked with purple arrows above the domain structure. Clusters of fixed nonsynonymous changes are labeled with a bar specifying the number of sites. Polymorphic nonsynonymous changes within each species are marked below the domain structure. The gene span of Drosophila pkd2 is annotated as a grey bar with blue rectangles marking the transmembrane domains and a green box marking the coiled-coil domain. Extracellular and intracellular domains are in different shades of grey. (B) McDonald–Kreitman tests show that Drosophila pkd2 evolves under positive selection between D. melanogaster and D. simulans, and this signal is generated by changes in the extracellular domain. The MK table details nonsynonymous fixed (Nf), synonymous fixed (Sf), nonsynonymous polymorphic (Np), and synonymous polymorphic (Sp) sites. We report the Fisher‘s exact (FE) P value, and the alpha-value for each segment of the gene. (C) A polarized McDonald–Kreitman test for the extracellular domain of Drosophila pkd2 demonstrates that this region evolves under positive selection along both lineages. Fixed changes were polarized to the D. melanogaster or D. simulans lineages using D. yakuba as an outgroup species. (D) NSsites model tests using PAML show that Drosophila pkd2 evolves adaptively across many species of Drosophila.

Fig. 3.—

A predicted structural model of Drosophila pkd2 shows that nonsynonymous changes between D. melanogaster and D. simulans reside on the extracellular faces. (A) The homo-tetramer of our predicted Drosophila pkd2 structure. The 2-D gene diagram is shaded for the region that could be successfully modeled. The monomers are alternated in shades of grey for contrast. The extracellular loop of one monomer is colored blue. The diagram to the right describes the orientation of the channel. (B) Sites that diverge between D. melanogaster and D. simulans are on the extracellular region of the channel. The base of the monomer is grey, and the extracellular region is blue. The nonsynonymous changes between the two species shown in yellow.

Fig. 4.—

Both intermale sperm competition and female choice can drive the rapid evolution of the sperm hyper-activation channels. (A) Males that secrete seminal peptides that block the hyper-activation channels of sperm from competing males gain a selective advantage. (B) Females may secrete peptides that inhibit sperm hyper-activation channels to control fertilization rates. Males that can prevent the inhibition of sperm hyper-activation gain a selective advantage.