Silverback gorillas are known for their impressive, bulging physiques – and their rather modest genitalia. Now, scientists have discovered a potential genetic link between the small members of these monkeys and male infertility problems.
An adult male’s penis is on average 1.1 inches (3 centimeters) long gorilla (A gorilla) is the smallest phallus of all monkeys. The size of the gorilla’s genitalia comes with other disadvantages in its reproductive capacity, such as a low sperm count compared to other primates and sperm with poor motility and a reduced ability to bind to eggs.
Given that these are reproductive issues that can affect humans as well, it may seem surprising that all male gorillas share these traits. However, this can be explained by the mating system of gorillas Jacob Bowmanlead author of the new study and a postdoctoral fellow at the University at Buffalo.
Gorillas operate in a polygynous system in which a dominant male has almost exclusive access to the females in his group. The clumsy physique of the silverback means that he has no problem securing mates, and thus his sperm does not have to compete with that of other males and can produce offspring without many, highly mobile swimmers. The theory is that this lack of sperm competition led to the evolution of gorillas’ small genitalia.
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This led the researchers to wonder whether, at the genetic level, we could find genes related to spermatogenesis [sperm production] or that we see leading to poor sperm quality,” Bauman told Live Science. Gorillas and humans share most of the same genes – so if researchers can find suspect genes in gorillas, they could turn their attention to the human genome.
Approximately 15 percent of American couples have trouble conceiving, according to the Yale Medicineand more than half of these cases involve male infertility. About 30% of infertility cases have a genetic basis Vincent Straub, a PhD student in population health at the University of Oxford, who was not involved in the new study. However, the genes involved in male infertility are poorly understood.
To help unravel these genetics, Bowman and colleagues reviewed a database of more than 13,000 genes in 261 mammals. This involved looking at the underlying gene sequences to see how they changed over time in related animals. The goal was to see if certain genes in the gorilla branch of the tree of life evolved at dramatically reduced rates, Bowman said.
This can happen when there is no strong pressure to get rid of genetic mutations that could prevent the population from surviving – such as those associated with gorillas’ low-quality sperm. This process, called “relaxed purifying selection,” can lead to seemingly harmful mutations that become common in a species.
The data show 578 genes in the gorilla lineage that have undergone this type of selection. Analysis and existing data suggest that many of these genes are involved in sperm production. However, not all tagged genes had a known role in male fertility.
To better understand the functions of these genes, the team turned to the fruit fly (Drosophila melanogaster), a commonly used genetic model in biology. They systematically silenced each of the genes in the male flies to see if they affected the insects’ ability to reproduce. In doing so, they discovered 41 new genes that had not previously been linked to male fertility.
The researchers then connected the dots back to humans using a genetic database with data from 2,100 infertile men who either had very low or no sperm in their semen. They also looked at data from fertile men, focusing on the genes they tagged in the gorillas. They found that in 109 of the relaxed gorilla genes, infertile males carried more loss-of-function mutations than fertile males; loss-of-function mutations reduce the gene’s ability to produce the protein it codes for.
Although these genes are likely to be involved in human male fertility, more research is needed to learn exactly how they work in the body. Straub stressed that infertility is very complex and that it’s not all about genetics. To fully understand it, scientists need to account for how different genes interact with each other and with the organism’s environment and behavior.
The gorilla findings open the door to future research into how these genes and others closely related to them may affect fertility in humans, Straub said. The study was published May 9 in the journal eLife.
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