New research into six primate species suggests that the Y chromosome in primates — including humans — evolves much faster than the X chromosome.
For example, humans and chimpanzees share more 98% of their DNA throughout the genome, but only 14% to 27% of the DNA sequences on the human Y chromosome are shared with our closest living relatives.
The finding surprised the scientist, given that humans and chimpanzees diverged just 7 million years ago – a leap in evolutionary terms.
“I expect my genome to be very different from that of bacteria or insects because a lot of time has passed, evolutionarily speaking,” study co-author Brandon Pickett, a postdoctoral fellow at the National Human Genome Research Institute (NHGRI) at the National Institutes of Health, told Live Science. “But for other primates, I expect it to be quite similar.
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It is not exactly clear why the Y chromosome evolves so quickly. For starters, there is only one copy of the Y chromosome per cell – in primates, females carry two copies of the X chromosome, while males carry both an X and a Y chromosome – the Y chromosome plays a key role in sperm production and fertility. . Having only one copy of the Y chromosome presents a vulnerability – if changes occur, there is no other chromosome to act as a backup.
And changes they are it is likely due to something called mutational bias. The Y chromosome can be so prone to change because it produces many sperm. This requires a lot of DNA replication. And every time DNA is copied, there is a chance for errors to creep in.
Scientists have sequenced before primate genome for all 16 representative families.
In a new study published May 29 in the journal Naturescientists compared the sex chromosomes of five species of great apes – chimpanzees (Mr. Troglodytes), bonobos (Paniscus), western lowland gorillas (gorilla gorilla gorilla) and Bornean and Sumatran orangutans (Pongo pygmaeus and Pongo abelia) — and one more distantly related to humans, the gibon siamang (Symphalangus syndactylus).
The team studied the chromosomes using telomere-to-telomere (T2T) sequencing. T2T can precisely sequence repetitive elements, including the protective telomere “caps” of chromosomes that have proven difficult to read in the past, Pickett said. The researchers used computer software to compare the sequencing results by creating alignments to reveal which parts of the chromosome had changed and which parts had remained the same.
The X and Y chromosomal sequences of each of the six species were also compared to the human X and Y chromosome, which had already been sequenced in earlier studywith the T2T method.
The findings revealed that the Y chromosome evolved rapidly in all species studied. Even species in the same genus have very different Y chromosomes from each other. For example, chimpanzees and bonobos diverged only 1 million to 2 million years ago, yet there is a dramatic difference in the lengths of their Y chromosomes. Christian Roossenior scientist at the Primate Genetics Laboratory, German Primate Center, who was not involved in the study.
In some cases, the difference in length – caused by chromosome losses or duplications that occur as DNA is copied – was as much as half of the observed differences. For example, the Y chromosome from The Sumatran orangutan is twice as long as the Y gibbon chromosome.
In contrast, the study found that the X chromosome was highly conserved across primate species, as would be expected for a structure with a critical role in reproduction.
One reason the Y appears to have thrived despite such a high mutation rate is that, across all species studied, it contains stretches of highly repetitive genetic material, such as palindromic repeats, where the sequence reads the same forward and backward. Genes are housed in these stretches of repetitive DNA. So repetitive DNA can protect important genes from replication errors and thereby preserve essential biological material, the researchers wrote in their paper.
However, the study had limitations; examined only one representative for each primate species and could not say how much the Y chromosome would vary among animals of the same species, Pickett said.