Ghost Lineages Ride Again: the Spit Edition

Guest Blogger: Mareike Janiak

Darcy asked me to write a guest post on “the new spit paper” and it shows that she knows me well. Saliva? Salivary proteins? Functional genetic variation in those proteins? Possible interbreeding with mystery hominins? The microbiome?


The new spit paper” by Duo Xu and her colleagues is titled “Archaic hominin introgression in Africa contributes to functional salivary MUC7 genetic variation” and is going to be published in the journal Molecular Biology and Evolution.

The authors looked at genetic variation in a gene called MUC7, which codes for mucin 7, a protein that is only found in saliva. In an earlier study, they found that the number of times a specific part of the MUC7 gene is repeated varies across different primate species. In gorillas it is repeated only 4-5 times, while vervet monkeys have 11-12 repeats. Humans have 5-6 repeats, but the gene hadn’t been thoroughly investigated in our own species, which is where the current paper comes in.

One known function of mucin 7 is to bind with bacteria in the mouth, so one question the authors asked is whether genetic variation in MUC7 correlates with the type of bacteria found in a person’s mouth. Using data from the Human Microbiome Project the authors found that people that have more similarity in the MUC7 gene also have more similar bacterial profiles (microbiomes) – but only around the mouth. While this is an interesting result, it creates more questions than it answers! Do these different bacterial profiles provide an adaptive benefit? And if so, for what? In what context is it better to have one over the other? Is it dependent on pathogens in the environment or maybe on diet? Lots of great avenues for future research!

But the authors also found something else when they were looking at MUC7 variation across people, something very curious. As expected, they found a number of different patterns of MUC7. These patterns are called haplotypes and they appear as time goes by and (mostly benign) mutations accumulate along the gene. Generally these haplotypes were pretty similar to each other, but (and this is the weird part!) one of them, haplotype E, was totally different.

Most of the MUC7 haplotypes were like these poodles, small differences but all clearly poodles:


And then there’s haplotype E:


Yes, still a poodle, but also kind of…out there and unexpected.

So what’s the deal with haplotype E?

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