Neanderthal DNA exists in humans, but a piece is mysteriously missing: ScienceAlert

Neanderthals, modern humans' closest cousins, lived in parts of Europe and Asia until their extinction around 30,000 years ago.

Genetic studies are revealing more and more about the connections between modern humans and these long-extinct relatives – most recently, a wave of interbreeding between our species occurred in a relatively short period of time, around 47 years ago. 000 years. But a mystery remains.

THE Homo sapiens The current genome contains some Neanderthal DNA. These genetic traces come from almost every part of the Neanderthal genome – except for 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 due to mating patterns or inferior function. However, the answer may lie in a centuries-old theory about the health of interspecific hybrids.

Neanderthal sex, genes and chromosomes

Neanderthals and modern humans split between 550,000 and 765,000 years ago in Africa, when Neanderthals wandered into Europe but our ancestors stayed behind.

They won't see each other again until H. sapiens migrated to Europe and Asia between 40,000 and 50,000 years ago.

Scientists have recovered copies of the complete genomes of Neanderthals, using DNA from well-preserved bones and teeth of Neanderthals in Europe and Asia.

Not surprisingly, the Neanderthal genome was very similar to ours, containing around 20,000 genes grouped into 23 chromosomes.

Like us, they had two copies of 22 of these chromosomes (one from each parent), as well as a pair of sex chromosomes. Women had two X chromosomes, while men had one X and one Y.

Y chromosomes are difficult to sequence because they contain a lot of repetitive “junk” DNA, so the Neanderthal Y genome has only been partially sequenced. However, the large portion that has been sequenced contains versions of many of the same genes found in the modern human Y chromosome.

In modern humans, a gene on the Y chromosome called SRY triggers the process of an XY embryo developing into a male. The SRY gene plays this role in all apes, so we assume that this was also the case for Neanderthals – although we did not find the Neanderthal SRY gene itself.

Interspecies mating left us with Neanderthal genes

There are many small clues that indicate that a DNA sequence came from a Neanderthal or a H. sapiens. We can therefore search for fragments of Neanderthal DNA sequence in the genome of modern humans.

The genomes of all human lineages originating in Europe contain approximately 2% Neanderthal DNA sequences. Lineages from Asia and India contain even more, while lineages limited to Africa contain none.

Some old Homo sapiens the genomes contained even more – around 6% – so it appears that Neanderthal genes were gradually disappearing.

Most of this Neanderthal DNA arrived over a 7,000 year period, approximately 47,000 years ago, after modern humans left Africa for Europe, and before the Neanderthals became extinct there approximately 30,000 years ago. At that time, there must have been many pairings between Neanderthals and humans.

At least half of the total Neanderthal genome can be reconstructed from fragments found in the genomes of different contemporary humans. We owe our Neanderthal ancestors characteristics such as red hair, arthritis and resistance to certain diseases.

There is one glaring exception. No contemporary humans have been discovered to harbor part of the Neanderthal Y chromosome.

What happened to the Neanderthal Y chromosome?

Was it bad luck that the Neanderthal Y chromosome was lost? Wasn't he very good at his job of creating males? Did Neanderthal women, but not men, engage in interspecific mating? Or was there something toxic in the Neanderthal Y that wouldn't work with human genes?

The AY chromosome comes at the end of the lineage if its carriers have no sons, so it may simply have been lost over thousands of generations.

Or maybe the Neanderthal Y was never present during interspecific mating. Maybe it was still modern humans who fell in love with (or traded, captured, or raped) Neanderthal women? The sons born to these women would all have H. sapiens form of the Y chromosome.

However, it is difficult to reconcile this idea with the observation that there is no trace of Neanderthal mitochondrial DNA (which is limited to the female lineage) in modern humans.

Or maybe the Neanderthal Y chromosome just wasn't as good at its job as its counterpart. H. sapiens rival. Neanderthal populations have always been small, so harmful mutations would have been more likely to accumulate.

We know that Y chromosomes with a particularly useful gene (for example for more, better or faster sperm) quickly replace other Y chromosomes in a population (called the hitchhiker effect).

We also know that the Y chromosome deteriorates overall in humans. It's even possible that SRY was lost to the Neanderthal Y and that Neanderthals were evolving a new sex-determining gene, as some rodents did.

Was the Neanderthal Y chromosome toxic in hybrid boys?

Another possibility is that the Neanderthal Y chromosome would not work with genes from other chromosomes in modern humans.

The disappearance of Neanderthal Y can then be explained by the “Haldane rule”. In the 1920s, British biologist JBS Haldane noted that in cross-species hybrids, if one sex is sterile, rare, or unhealthy, it is always the one with different sex chromosomes.

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

Many crosses between different mouse species exhibit this pattern, as do feline crosses. For example, in lion-tiger crossbreeds (ligers and tigons), the females are fertile but the males are sterile.

We still lack a good explanation of Haldane's rule. This is one of the enduring mysteries of classical genetics.

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

We know that Y genes evolve much more quickly than genes on other chromosomes and that several of them play a role in sperm production, which may explain the infertility of male hybrids.

This could therefore explain why the Neanderthal Y was lost. It also raises the possibility that it was the fault of the Y chromosome, imposing a reproductive barrier, that Neanderthals and humans became separate species in the first place.

Jenny Graves, Emeritus Professor of Genetics and Vice-Chancellor, La Trobe University

This article is republished from The Conversation under a Creative Commons license. Read the original article.