Although the human genome has ancient fragments of Neanderthal DNA, researchers of a new study have not found evidence of the Neanderthal genes in modern males' Y chromosomes backing up theories that the male offspring of Neanderthals and Homo sapiens may have faced more challenges than their female counterparts.
Earlier studies have sequenced mitochondrial DNA, which is passed to children from their mother, from fossils of Neanderthal women. The new study, which was published in The American Journal of Human Genetics on April 7, is the first to examine Neanderthal Y chromosome, which is passed exclusively from father to son.
The researchers are not sure if the disappearance of the Y chromosome happened by chance or if it occurred by evolutionary circumstances but they think that genes in the Neanderthal's Y chromosome may not have been compatible with human genes.
"The functional nature of the mutations we found suggests to us that Neanderthal Y chromosome sequences may have played a role in barriers to gene flow, but we need to do experiments to demonstrate this and are working to plan these now," said study researcher Carlos Bustamante, Stanford's School of Medicine.
Study researcher Fernando Mendez, also from Stanford, explained that a woman's immune system may attack a male fetus that carries Neanderthal H-Y genes, minor histocompatibility antigens that resemble the HLA antigens that transplant surgeons have to check to ensure that the organ donor and recipient have similar immune profiles.
The absence of the Neanderthal Y chromosomes could be attributed to women consistently miscarrying male babies that carry it. Although this is an unproven idea, modern women's immune system sometimes reacts to male offspring when there is genetic incompatibility.
For instance, a mutation in the KDM5D gene, which plays a role in suppressing cancer, has earlier been linked to greater risk of miscarriage because it can elicit immune response in pregnant women.
"We identify protein-coding differences between Neanderthal and modern human Y chromosomes, including potentially damaging changes to PCDH11Y, TMSB4Y, USP9Y, and KDM5D. Three of these changes are missense mutations in genes that produce male-specific minor histocompatibility (H-Y) antigens," the researchers wrote in their study.
"It is possible that incompatibilities at one or more of these genes played a role in the reproductive isolation of the two groups."
The analysis also allowed researchers to identify when the Neanderthals and Homo sapiens split, which is around 590,000 years ago.
