Head lice have been stigmatized, quickly conjuring images of infested school children and parents combing through their hair. This social stigma reaches many of the estimated 14 million people who are annually infested in the U.S. alone.

However, these blood sucking lice have had a long and complex evolutionary history tied to humans and other mammals. In total, there are 532 species of blood sucking lice and each species parasitizes one or just a few mammal species. In the past, scientists have taken advantage of the close associations of lice to their hosts to approach questions of human evolution. Research is now moving to the bacteria associated with these lice.

It turns out that blood sucking lice are themselves host to symbiotic bacteria. These bacteria, which are passed on maternally, facilitate parasitism by providing essential B-vitamins to the lice.

Now, scientists Bret Boyd et al. have sought to better understand the evolutionary history of bacteria residing within lice. In this study, they see that bacterial evolution is driven by associations with lice, and louse evolution is tied to their mammalian hosts.

Boyd et al. sequenced the genomes of symbiotic bacteria from human lice as well as the closely related chimpanzee, gorilla and red colobus monkey lice. The data provided a new perspective on the evolutionary tree of these symbiotic bacteria. They found a single-trunked, monophyletic tree, suggesting the bacteria have been continually associated with lice for millions of years.

"Our results are a departure from previous findings and supporting monophyly instead of symbiont replacement," said Boyd. The nearest bacterial cousin is a symbiotic bacterial species found in the tsetse fly, another blood feeding parasite.

"We found that the evolutionary tree of the bacteria reflects the phylogenetic patterns seen in the lice," said Boyd. "Within human head and body lice there are distinct clades identified by their mitochondrial haplotypes; potentially a result of modern and extinct hominids exchanging lice. We found that the bacterial tree follows these evolutionary patterns." Boyd also noted that "congruence between the evolutionary trees of lice and symbiotic bacteria can be traced to 20-25 mya when the lice parasitizing monkeys diverged from a common ancestor of hominid lice."

While exploring the genome of a louse symbiont, they encountered an extrachromosomal region called a plasmid. This plasmid encodes genes needed for nutritional synthesis, critical to the symbiosis. Based on the associations with lice, the authors estimate this plasmid arose between 17-25 million years ago. Boyd said, "we suspect that this plasmid arose from a re-organization of the symbiont genome in the common ancestor of human and other hominid lice." They conclude that the results of this study have implications for the progression of genome evolution in symbionts.

"While lice are a highly maligned, they have provided a wealth of scientific information. Because the symbiotic bacteria studied here are tied to a known evolutionary history between lice and primates, this makes an ideal system to study bacterial genome evolution" said Boyd.