Y chromosome In Humans and Apes

Comparing spermatogenesis in humans, chimpanzees and gorillas has always been fascinating. Chimpanzees, which weigh only about 100 pounds have enormous 8 centimeter diameter round (not oval) testes with sperm counts of over a billion per ml. Yet gorillas, which weight as much as 600 pounds or more, have tiny testes, very poor spermatogenesis, and in the sparse literature on gorilla testicular histology, in the majority of cases have what appears to be Sertoli cell only. Humans, the closest living relatives to chimpanzees and gorillas, fall somewhere in between.

The generally accepted reason for this massive discrepancy in spermatogenesis between these three closely related species lies in their differing mating patterns. Chimpanzees congregate in troupes of 30 to 40 in an extended family wherein any female who goes into heat is instantly mounted by every single male in the troupe. Therefore, there is an intense “competition” between the sperm of the different males to see which one will fertilize the females’ eggs. It is far more likely that the male with the highest sperm count, and the biggest testicles, will become the father of the male offspring, because of the high degree of “sperm competition” in chimpanzees.

In gorillas, it is the opposite. Any female is permanently attached to just one single silverback alpha male, and if she ever gets pregnant, it will have to be with his sperm only. So in gorillas there is no sperm competition. That results in small testes with very low sperm counts in these otherwise huge, very macho animals. But why is that? The answer lies in the peculiar instability of the non-recombining Y chromosome.

The multiple nucleotide sequence direct and inverted repeats (amplicons and palindromes) are where all the testis specific spermatogenesis genes on the Y are located. These areas are prone to frequent deletions caused by non-homologous, or “illegitimate” homologous recombination with itself, resulting in drop-outs of often huge chunks of DNA, making the concentration of spermatogenesis genes on the Y chromosome have a very fragile existence. So without sperm competition, sperm counts over eons of time are likely to go down.

Most intriguing is to compare the human Y chromosome to the chimpanzee Y chromosome, both of which have been fully and accurately sequenced. Unfortunately, the gorilla Y has not yet been sequenced. Nonetheless some interesting differences are noted between the human Y and the chimpanzee Y. Firstly, the chimpanzee Y has many more amplicons and palindromes than the human, and nonetheless, much fewer ampliconic genes (25 compared to 60). Furthermore, the chimpanzee Y is missing a gene (PRY) that is present on AZFc in the human, but completely absent in the chimpanzee. Furthermore, this gene has also been found to be absent in rare humans who have incredibly high sperm counts, approaching half a billion. Thus, comparing the super fertile chimpanzee Y chromosome to its less fertile human cousin, can help us understand better the genetic control of spermatogenesis in our infertile male patients.