Proteomics and comparative genomic investigations reveal heterogeneity in evolutionary rate of male reproductive proteins in mice (Mus domesticus)

Abstract

Male reproductive fitness is strongly affected by seminal fluid. In addition to interacting with the female environment, seminal fluid mediates important physiological characteristics of sperm, including capacitation and motility. In mammals, the male reproductive tract shows a striking degree of compartmentalization, with at least six distinct tissue types contributing material that is combined with sperm in an ejaculate. Although studies of whole ejaculates have been undertaken in some species, we lack a comprehensive picture of the specific proteins produced by different accessory tissues. Here, we perform proteomic investigations of six regions of the male reproductive tract in mice -- seminal vesicles, anterior prostate, dorsolateral prostate, ventral prostate, bulbourethral gland, and bulbourethral diverticulum. We identify 766 proteins that could be mapped to 506 unique genes and compare them with a high-quality human seminal fluid data set. We find that Gene Ontology functions of seminal proteins are largely conserved between mice and humans. By placing these data in an evolutionary framework, we show that seminal vesicle proteins have experienced a significantly higher rate of nonsynonymous substitution compared with the genome, which could be the result of adaptive evolution. In contrast, proteins from the other five tissues showed significantly lower nonsynonymous substitution, revealing a previously unappreciated level of evolutionary constraint acting on the majority of male reproductive proteins.

FIG. 1.—

Neighbor-Joining tree with midpoint rooting inferred from a protein presence/absence matrix (see Materials and Methods). SV = seminal vesicles, AP = anterior prostate, VP = ventral prostate, DP = dorsolateral prostate, BU = bulbourethral gland, and BD = bulbourethral diverticulum.

FIG. 2.—

Median pairwise d_N/d_S estimated from one-to-one orthologs between mouse and rat. Error bars indicate 95% CIs determined from bootstrap resampling. SV = seminal vesicles, AP = anterior prostate, VP = ventral prostate, DP = dorsolateral prostate, BU = bulbourethral gland, BD = bulbourethral diverticulum. Total = the group of male reproductive proteins identified across all six tissues. Numbers in parentheses indicate the number of genes. SV proteins evolve significantly more rapidly than the genomic average. Proteins from all other tissues evolve significantly more slowly than the genomic average.

FIG. 3.—

A heat map of median d_N/d_S identified across the male reproductive tract. Red indicates relatively rapid evolution, and the seminal vesicles and testis show significantly elevated d_N/d_S compared with the genome median. All other reproductive tissues show significant reduction in d_N/d_S compared with the genome average. Testis-(d_N/d_S = 0.261) and epididymis-selective (d_N/d_S = 0.131) genes were defined according to patterns of gene expression and were taken from Dean et al. (2008). The triangle indicates the median d_N/d_S = 0.128 across the whole genome. Figure modified from Nagy et al. (2003).