Tandem repetitive D domains of the sperm ligand zonadhesin evolve faster in the paralogue than in the orthologue comparison

Evolutionary analysis of genes coding for Cysteine-RIch Secretory Proteins (CRISPs) in mammals

BACKGROUND

Cysteine-RIch Secretory Proteins (CRISP) are expressed in the reproductive tract of mammalian males and are involved in fertilization and related processes. Due to their important role in sperm performance and sperm-egg interaction, these genes are likely to be exposed to strong selective pressures, including postcopulatory sexual selection and/or male-female coevolution. We here perform a comparative evolutionary analysis of Crisp genes in mammals. Currently, the nomenclature of CRISP genes is confusing, as a consequence of discrepancies between assignments of orthologs, particularly due to numbering of CRISP genes. This may generate problems when performing comparative evolutionary analyses of mammalian clades and species. To avoid such problems, we first carried out a study of possible orthologous relationships and putative origins of the known CRISP gene sequences. Furthermore, and with the aim to facilitate analyses, we here propose a different nomenclature for CRISP genes (EVAC1-4, "EVolutionarily-analyzed CRISP") to be used in an evolutionary context.

RESULTS

We found differing selective pressures among Crisp genes. CRISP1/4 (EVAC1) and CRISP2 (EVAC2) orthologs are found across eutherian mammals and seem to be conserved in general, but show signs of positive selection in primate CRISP1/4 (EVAC1). Rodent Crisp1 (Evac3a) seems to evolve under a comparatively more relaxed constraint with positive selection on codon sites. Finally, murine Crisp3 (Evac4), which appears to be specific to the genus Mus, shows signs of possible positive selection. We further provide evidence for sexual selection on the sequence of one of these genes (Crisp1/4) that, unlike others, is thought to be exclusively expressed in male reproductive tissues.

CONCLUSIONS

We found differing selective pressures among CRISP genes and sexual selection as a contributing factor in CRISP1/4 gene sequence evolution. Our evolutionary analysis of this unique set of genes contributes to a better understanding of Crisp function in particular and the influence of sexual selection on reproductive mechanisms in general.

Proteins involved in motility and sperm-egg interaction evolve more rapidly in mouse spermatozoa

Proteomic studies of spermatozoa have identified a large catalog of integral sperm proteins. Rapid evolution of these proteins may underlie adaptive changes of sperm traits involved in different events leading to fertilization, although the selective forces underlying such rapid evolution are not well understood. A variety of selective forces may differentially affect several steps ending in fertilization, thus resulting in a compartmentalized adaptation of sperm proteins. Here we analyzed the evolution of genes associated to various events in the sperm’s life, from sperm formation to sperm-egg interaction. Evolutionary analyses were performed on gene sequences from 17 mouse strains whose genomes have been sequenced. Four of these are derived from wild Mus musculus, M. domesticus, M. castaneus and M. spretus. We found a higher proportion of genes exhibiting a signature of positive selection among those related to sperm motility and sperm-egg interaction. Furthermore, sperm proteins involved in sperm-egg interaction exhibited accelerated evolution in comparison to those involved in other events. Thus, we identified a large set of candidate proteins for future comparative analyses of genotype-phenotype associations in spermatozoa of species subjected to different sexual selection pressures. Adaptive evolution of proteins involved in motility could be driven by sperm competition, since this selective force is known to increase the proportion of motile sperm and their swimming velocity. On the other hand, sperm proteins involved in gamete interaction could be coevolving with their egg partners through episodes of sexual selection or sexual conflict resulting in species-specific sperm-egg interactions and barriers preventing interspecies fertilization.

Positive selection and gene conversion in SPP120, a fertilization-related gene, during the East African cichlid fish radiation

The ability to infer historical natural selection from sequence data aides in finding genes that might be important in adaptation and the formation of new species. As the fastest evolving and largest known vertebrate radiation, the cichlid fish of the African Great Lakes exhibit a wide range of recent morphological diversification. We used DNA databases, mostly of expressed sequence tags, to find candidate orthologous coding sequences from 2 tribes of cichlids and, using an automated procedure, scanned these sequence pairs for high dN/dS, the signal of positive selection and protein adaptation. The results included vertebrate genes commonly found to be under selection (e.g., major histocompatibility complex [MHC] loci) as well as genes known to be important specifically in the cichlid radiation (e.g., long-wave-sensitive opsins). Further investigation focused on a gene encoding a fertilization-related protein, SPP120, which was previously known only from cichlids. Using maximum likelihood analysis on novel SPP120 cDNA sequences from a range of African cichlids, we demonstrate the influence of positive selection in a specific subregion of the protein. We also show that SPP120 is a tandemly arranged, multicopy gene evolving with occasional interlocus gene conversion. A search of the Medaka genome database also revealed a tandem arrangement of multiple SPP120 copies and evolutionary rate differences between Medaka gene subregions mirroring those found for cichlids. Combined, these results suggest that SPP120 has been under repeated diversifying selection for over 100 Myr.

Sequence evolution of the sperm ligand zonadhesin correlates negatively with body weight dimorphism in primates

Sexual selection has repeatedly been shown to be the probable driving force behind the positive Darwinian evolution of genes affecting male reproductive success. Here we compare the sequence evolution of the sperm ligand zonadhesin with body mass dimorphism in primates. In contrast to previous related studies, the present approach takes into account not only catarrhine primates, but also platyrrhines and lemurs. In detail, we analyze the sequence evolution of concatenated zonadhesin fragments (555 bp) of four Lemuroidea, five Platyrrhini, and seven Catarrhini, using the rate ratio of nonsynonymous to synonymous substitutions (dn/ds=omega). Unexpectedly, subsequent regression analyzes between omega estimates for the terminal branches of a primate phylogeny and residual male body mass reveal that sequence evolution of zonadhesin decreases with increasing sexual dimorphism in body weight. Mapping published mating system classifications onto these results illustrates that unimale breeding species show a tendency for rather slow sequence evolution of zonadhesin and comparably pronounced sexual dimorphism in body weight. Female choice and sperm competition can be assumed to drive the evolution of zonadhesin. We speculate that the level of sperm competition is lower in more sexually dimorphic primates because males of these species monopolize access to fertile females more successfully. Thus, variation in sperm competition may be driving the observed negative correlation of sequence evolution and sexual dimorphism in body weight.