Big Testicles Drive Genetic Evolution: Size Does Matter After All

Large testicles drive genetic evolution, with animals with larger testis having an increased speed of genome evolution, researchers have found.

Published in the advanced online edition of Molecular Biology and Evolution, a team of scientists looked at how testicle size impacted on the evolution of genes.

Among most primates, females will mate with multiple partners, causing competition among males to pass along their genes.

Study author Alex Wong, of Carleton University in Ottawa used data from 55 species of primates to look for a correlation between evolutionary rates across genomes and the weight of the animal's testes.

Wong notes that it is widely believed increased production of sperm is a result of more rounds of cell division. With more cell division, more mutations arise, leading to more genome evolution.

He applied an evolutionary method to detect a relationship between testicle size and substitution rates – or gene mutation rates - in primates. Findings showed there was a positive correlation while accounting for other factors.

"In general, the speed of genome evolution is higher for species in which males have large testes in comparison to species in which males have small testes," Wong said.

"This finding helps us to understand why genomes evolve at different rates in different species, and has implications for our understanding of the relationship between female mate choice and the overall fitness of a population."

The results could support the general prediction that sperm competition should result in higher mutation rate as a result of higher spermatogenic (sperm production) activity among species where females mate with more than one male.

Wong explained: "The current finding of covariance between sperm competition intensity and substitution rates adds to a growing body of knowledge concerning the sources of substitution rate variation.

"The extent to which this covariance is widespread is not yet clear; application of robust comparative methods to large phylogenetic datasets in other taxa, such as birds and insects, will help to establish its generality."