Silverback gorillas are known for their impressive, bulging physiques – and their rather modest genitals. Now scientists have discovered a potential genetic link between these apes’ small limbs and infertility problems in male humans.
On average it is only 3 centimeters long, the penis of an adult man gorilla (Gorilla) is the smallest phallus of all the apes. The gorilla’s genital size is associated with other deficiencies in its reproductive capabilities, such as a 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 share these traits. However, this can be explained by the gorillas’ mating system 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 females in his troop. The silverback’s bulky body size means it has no problem finding mates, so its sperm doesn’t have to compete with that of other males and it 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 made researchers wonder whether we can find genes related to spermatogenesis at the genetic level [sperm production] or that we see leads to poor quality sperm,” Bowman told LiveScience. Gorillas and Humans Share the vast majority of the same genes – so if the researchers could pinpoint suspect genes in gorillas, they could next turn their attention to the human genome.
According to figures, about 15% of American couples have difficulty conceiving Yale Medicine, and more than half of these cases involve male infertility. About 30% of infertility cases have a genetic basis Vincent Straub, a doctoral student in public health at the University of Oxford who was not involved in the new research. However, the genes involved in male infertility are poorly understood.
To help unravel these genetics, Bowman and colleagues searched a database of more than 13,000 genes from 261 mammals. This looked at the underlying sequences of genes to see how they changed over time in related animals. The goal was to see whether certain genes in the gorilla branch of the tree of life developed at a dramatically reduced rate, Bowman said.
This can happen when there is no strong pressure to get rid of genetic mutations that could hinder the survival of a population, such as those linked to the low-quality sperm of gorillas. This process, called “relaxed purifying selection,” can cause apparently deleterious mutations to become common in a species.
The data revealed 578 genes in the gorilla lineage that underwent this type of selection. An analysis and existing data suggested that many of these genes are involved in sperm making. However, not all highlighted genes played a 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 turned off each of the genes in male flies to see if they affected the insects’ reproductive ability. 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 people using a genetic database containing data from 2,100 men with infertility, who either had very few sperm or a lack of sperm in their semen. They also looked at data from fertile men, focusing on the genes they noticed in gorillas. They found that in 109 of the relaxed gorilla genes, the infertile men carried more loss-of-function mutations than fertile men; Loss-of-function mutations reduce a gene’s ability to make the protein it codes for.
While it is likely that these genes are involved in human fertility, more research is needed to learn exactly how they work in the body. Straub emphasized that infertility is very complex and not everything has to do with genetics. To fully understand it, scientists must consider how different genes interact with each other and with an organism’s environment and behavior.
The findings from gorillas open the door for future explorations into how these genes, and others 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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