Silverback gorillas are famous for their impressive, plump physiques – and their rather modest genitals. Now scientists have uncovered a possible genetic link between these monkeys’ small counterparts and infertility problems in human males.
The average adult male penis is only 1.1 inches (3 centimeters) long gorilla (gorilla) has the smallest phallus of all great apes. The gorilla’s genital size is associated with other deficits in its reproductive capacity, such as low sperm count compared to other primates and sperm with poor motility and a reduced ability to bind to eggs.
Since these are reproductive problems that can also affect humans, it may seem surprising that all male gorillas have these characteristics. However, this can be explained by the gorillas’ mating system, said Jacob BowmanLead author of the new study and postdoctoral researcher at the University at Buffalo.
Gorillas operate in a polygynous system in which a dominant male has almost exclusive access to the females of his group. The silverback’s unwieldy build means he has no problem finding mates. Therefore, his sperm does not have to compete with that of other males and he can produce offspring without many, highly agile swimmers. The theory is that this lack of sperm competition led to the evolution of gorillas’ small genitalia.
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This led researchers to wonder “whether we can find genes related to spermatogenesis at the genetic level [sperm production] or that we see that this leads to poor sperm quality,” Bowman told Live Science. Gorillas and humans share the vast majority of the same genes – So if researchers manage to locate suspect genes in gorillas, they could turn their attention to the human genome next.
About 15% of couples in the US have trouble getting pregnant, Yale Medicineand more than half of these cases involve male infertility. About 30% of infertility cases have a genetic basis, said Vincent Straub, a doctoral 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 decipher these genetics, Bowman and colleagues searched a database of more than 13,000 genes from 261 mammals. This involved studying the underlying gene sequences to see how they changed over time in related animals. The goal was to find out whether certain genes in the gorilla branch of the tree of life evolve at dramatically reduced rates, Bowman said.
This can happen when there is no strong pressure to eliminate genetic mutations that could threaten a population’s survival – such as those linked to the poor sperm quality of gorillas. This process, called “relaxed purifying selection,” can cause seemingly harmful mutations to occur frequently in a species.
The data revealed 578 genes in the gorilla lineage that were subjected to this type of selection. Analysis and existing data suggested that many of these genes are involved in sperm production. However, not all genes marked 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 switched off all of the genes in male flies to see if they affected the insects’ ability to reproduce. In this way, they discovered 41 new genes that had not previously been linked to male fertility.
The researchers then linked the dots back to humans using a genetic database containing data from 2,100 men with infertility who had either very low sperm levels or a lack of sperm in their semen. They also examined data from fertile men, focusing on the genes they had flagged in gorillas. They found that in 109 of the relaxed gorilla genes, the infertile men had more loss-of-function mutations than fertile men; Loss-of-function mutations reduce the ability of a gene to make the protein it encodes.
Although it is likely that these genes are involved in human male fertility, more research is needed to understand exactly how they function in the body. Straub emphasized that infertility is very complex and not everything can be attributed to genetics. To fully understand it, scientists need to explain how different genes interact with each other and with an organism’s environment and behavior.
The findings from gorillas open the possibility for future research into how these and other genes closely linked to them might influence fertility in humans, Straub said. The study was published May 9 in the journal eLife.
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