Mountain gorilla. Image credit: Greg Gollin (CC0)
The animal kingdom is rich in mating and courtship strategies. Some are obvious, such as elaborate color displays and extravagant physical structures, while others are hidden from view, including the competition between sperm within the female reproductive tract.
Sperm competition is common in polygamous species, where females frequently mate with multiple males. In several primate species, this has driven the evolution of relatively large testes, increased sperm production and greater sperm reserves. In contrast, gorillas live in a largely polygynous social system, in which dominant males have near-exclusive access to females. As a result, gorillas have evolved relatively small testes and produce fewer, lower-quality sperm. However, the molecular basis of this reduced reproductive investment has remained unclear.
Bowman et al. investigated the genetic consequences of the gorilla mating system by examining patterns of natural selection across 13,310 protein-coding genes from 261 mammalian species. They identified 578 genes that evolved under relaxed purifying selection in gorillas, compared with only 96 genes showing evidence of positive selection. Relaxed purifying selection occurs when a trait becomes less important for reproductive success, reducing the evolutionary pressure to eliminate harmful mutations. As a result, deleterious mutations can accumulate over time.
The genes evolving under relaxed selection in gorillas were enriched for expression during male germ cell development and were associated with biological processes involved in testis development, sperm production and sperm function. These findings suggest that as competition shifted from sperm to body size and physical dominance, genes involved in male reproductive function became less critical to reproductive success and were therefore subject to weaker selective constraints.
To further investigate the function of these genes, Bowman et al. used fruit flies to study gorilla relaxed-selection genes that had not previously been linked to reproduction. They identified 41 new genes required for sperm production and fertility and showed that disrupting many of these genes caused sperm defects in fruit flies. The researchers then examined exome and genome sequencing data from infertile men and found that mutations in many of the same genes were associated with human sperm abnormalities and infertility.
Together, these findings provide compelling evidence that reduced sperm competition in gorillas has led to relaxed purifying selection on genes involved in male reproductive function. The resulting accumulation of deleterious mutations likely contributes to the poor sperm quality observed in gorillas and highlights new candidate genes for human male infertility.
The genetic causes of male infertility remain largely unknown, in part because many affected men carry rare or unique mutations that are difficult to identify through traditional genetic studies. By taking advantage of natural evolutionary experiments, studies such as this can reveal previously unrecognized genes involved in fertility and deepen our understanding of how genetic variation shapes reproductive biology. These discoveries may ultimately help identify new diagnostic markers and therapeutic targets for male infertility.
