origin of humans; Neanderthal DNA in humans has no Y (Origins)

by David Turell , Monday, June 17, 2024, 17:21 (157 days ago) @ David Turell

Neanderthal sex chromosome Y is missing in humans:

https://www.sciencealert.com/neanderthal-dna-exists-in-humans-but-one-piece-is-mysterio...

"The Homo sapiens genome today contains a little bit of Neanderthal DNA. These genetic traces come from almost every part of the Neanderthal genome – except the Y sex chromosome, which is responsible for making males.

"So what happened to the Neanderthal Y chromosome? It could have been lost by accident, or because of mating patterns or inferior function. However, the answer may lie in a century-old theory about the health of interspecies hybrids.

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"Scientists have recovered copies of the full male and female Neanderthal genomes, thanks to DNA from well-preserved bones and teeth of Neanderthal individuals in Europe and Asia.

"Unsurprisingly, the Neanderthal genome was very similar to ours, containing about 20,000 genes bundled into 23 chromosomes.

"Like us, they had two copies of 22 of those chromosomes (one from each parent), and also a pair of sex chromosomes. Females had two X chromosomes, while males had one X and one Y.

"Y chromosomes are hard to sequence because they contain a lot of repetitive "junk" DNA, so the Neanderthal Y genome has only been partially sequenced. However, the large chunk that has been sequenced contains versions of several of the same genes that are in the modern human Y chromosome.

"In modern humans, a Y chromosome gene called SRY kickstarts the process of an XY embryo developing into a male. The SRY gene plays this role in all apes, so we assume it did for Neanderthals as well – even though we haven't found the Neanderthal SRY gene itself.

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"At least half of the whole Neanderthal genome can be pieced together from fragments found in the genomes of different contemporary humans. We have our Neanderthal ancestors to thank for traits including red hair, arthritis and resistance to some diseases.

"There is one glaring exception. No contemporary humans have been found to harbour any part of the Neanderthal Y chromosome.

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"...maybe the Neanderthal Y was never present in interspecies matings. Perhaps it was always modern human men who fell in love with (or traded, seized or raped) Neanderthal women? Sons born to these women would all have the H. sapiens form of the Y chromosome.

"However, it's hard to reconcile this idea with the finding that there is no trace of Neanderthal mitochondrial DNA (which is limited to the female line) in modern humans.

"Or perhaps the Neanderthal Y chromosome was just not as good at is job as its H. sapiens rival. Neanderthal populations were always small, so harmful mutations would have been more likely to accumulate.

***

"The missing Neanderthal Y may then be explained by "Haldane's rule". In the 1920s, British biologist J.B.S. Haldane noted that, in hybrids between species, if one sex is infertile, rare or unhealthy, it is always the sex with unlike sex chromosomes.

"In mammals and other animals where females have XX chromosomes and males have XY, it is disproportionately male hybrids that are unfit or infertile. In birds, butterflies and other animals where males have ZZ chromosomes and females have ZW, it is the females.

"Many crosses between different species of mice show this pattern, as do feline crosses. For example, in lion–tiger crosses (ligers and tigons), females are fertile but males are sterile.

"We still lack a good explanation of Haldane's rule. It is one of the enduring mysteries of classic genetics.

"But it seems reasonable that the Y chromosome from one species has evolved to work with genes from the other chromosomes of its own species, and might not work with genes from a related species that contain even small changes.

"We know that genes on the Y evolve much faster than genes on other chromosomes, and several have functions in making sperm, which may explain the infertility of male hybrids.

"So this might explain why the Neanderthal Y got lost. It also raises the possibility that it was the fault of the Y chromosome, in imposing a reproductive barrier, that Neanderthals and humans became separate species in the first place."

Comment: it is a mystery looking for an explanation. Presented here as interesting.