Tuesday, 15 December 2015

Mite Is Right For Evolution

Demodex folliculorum
Global divergence of the human follicle mite Demodex folliculorum: Persistent associations between host ancestry and mite lineages

Evolution is as plain as the mites on your face.

You see, we all have mites living in our hair follicles, especially the hair follicles of our face, such as our eyebrows and eyelashes. We're not unique in this; all mammals have these normally harmless little fellow travellers.

Like all our parasites, because their history is inextricably entangled with ours, they have evolved along with us being naturally selected to survive on the special environment our evolution is providing and changing for them. Just as we can map the genomes of our various lice and those which live on our close relatives, the other African apes, and find they map exactly onto our genomes as we evolved and diverged from the other African apes, so scientists have found that our follicle mites have evolved distinct subtypes which have become specialised to living on the various human ancestral groups.

A team led by Michael F. Palopoli from Department of Biology, Bowdoin College, Brunswick, ME, USA analysed the mitochondrial DNA from 241 mites from 70 human hosts and found a close association between the four specific haplotypes and the ancestry of the hosts.

Mites live in human hair follicles and have been implicated in medically important skin disorders, but we know surprisingly little about these residents of our skin. By analyzing the variation segregating among 241 mite sequences isolated from 70 human hosts, we showed that hosts with different regional ancestries harbor distinct lineages of mites and that these associations can persist despite generations spent in a new geographic region. These results suggest that some mite populations are better able to survive and reproduce on hosts from certain geographic regions. Improving our understanding of human follicle mites promises to shed light on human evolution and to provide important contextual information for their role in human health.

Microscopic mites of the genus Demodex live within the hair follicles of mammals and are ubiquitous symbionts of humans, but little molecular work has been done to understand their genetic diversity or transmission. Here we sampled mite DNA from 70 human hosts of diverse geographic ancestries and analyzed 241 sequences from the mitochondrial genome of the species Demodex folliculorum. Phylogenetic analyses recovered multiple deep lineages including a globally distributed lineage common among hosts of European ancestry and three lineages that primarily include hosts of Asian, African, and Latin American ancestry. To a great extent, the ancestral geography of hosts predicted the lineages of mites found on them; 27% of the total molecular variance segregated according to the regional ancestries of hosts. We found that D. folliculorum populations are stable on an individual over the course of years and that some Asian and African American hosts maintain specific mite lineages over the course of years or generations outside their geographic region of birth or ancestry. D. folliculorum haplotypes were much more likely to be shared within families and between spouses than between unrelated individuals, indicating that transmission requires close contact. Dating analyses indicated that D. folliculorum origins may predate modern humans. Overall, D. folliculorum evolution reflects ancient human population divergences, is consistent with an out-of-Africa dispersal hypothesis, and presents an excellent model system for further understanding the history of human movement.

Michael F. Palopoli, Daniel J. Fergus, Samuel Minot, Dorothy T. Pei, W. Brian Simison, Iria Fernandez-Silva, Megan S. Thoemmes, Robert R. Dunn, and Michelle Trautwein
Global divergence of the human follicle mite Demodex folliculorum: Persistent associations between host ancestry and mite lineages
PNAS 2015 ; published ahead of print December 14, 2015, doi:10.1073/pnas.1512609112

By using known mutation rates, the team estimated that the four haplotypes probably diverged about 3 million years ago, before modern Homo sapiens had evolved. The geographical distribution of the different haplotypes is entirely consistent with an out-of-Africa dispersal hypothesis. So once again, what science currently understands about human origins and human evolution from other strands of evidence, is confirmed by yet another strand.

The team go so far as to suggest that this association is so close it may even be useful in working out some of the details of human group migration patterns from prehistory.

This finding is exactly what the theory of evolution predicts, of course, and once again makes absolutely no sense at all as the creation of an intelligent designer. Since follicle mites for the most part do nothing for us and do us little if any harm under normal circumstances, there is no rational reason why a designer would design these four different haplotypes nor why it would distribute them geographically in such a way as to suggest that they evolved in Africa with us several million years ago and travelled out of Africa with us as we spread out from our species' ancestral homeland.

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