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Study Sees Parallel Pattern Between Lice, Bacterial Symbionts

NEW YORK (GenomeWeb) – The symbiotic bacteria nestled inside of head and body lice appear to share a long evolutionary history with the blood-sucking parasite pests, new research suggests.

Investigators from the US and Uganda sequenced DNA from human, chimpanzee, gorilla, and monkey lice, producing sequences not only for the parasites themselves, but also for the endosymbiotic bacteria that live within them and provide them with B vitamins. The sequences indicated that the endosymbiotic bacteria phylogenetic patterns overlap with those described in the louse species.

Based on such results, reported online today in Molecular Biology and Evolution, the team speculated that the endosymbiotic bacteria have likely diverged alongside the lice they inhabit starting from a lone lineage, rather than being replaced intermittently with new endosymbionts.

"Our results are a departure from previous findings and support monophyly of human and primate louse endosymbionts instead of endosymbiont replacement," first author Bret Boyd, a post-doctoral entomology researcher affiliated with the University of Georgia and the Illinois Natural History Survey, said in a statement, explaining that "louse, and hence endosymbiont evolution, are uniquely tied to primate and human evolution."

Using Illumina HiSeq2000 or 2500 instruments, the researchers sequenced genomic DNA from chimp, gorilla, and human pubic lice, along with lice parasitizing the red colobus monkey. In the process, they got sequences from half a dozen endosymbiont strains, including Candidatus Riesia bacteria in human, chimp, and gorilla lice and Candidatus Puchtella, typical endosymbionts of lice parasitizing red colobus monkeys and macaques.

By comparing sequences from eight louse endosymbionts to those from dozens of other eubacteria, the team managed to tease out a phylogenetic tree representing the louse symbionts, along with symbiotic bacteria from tsetse flies and other organisms.

In general, the louse endosymbionts appeared to fall into a clade that was most closely related to tsetse fly endosymbionts, the researchers reported, forming a monophyletic group that showed divergence patterns broadly matching those described in louse species.

"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," Boyd said. "We found that the bacterial tree follows these evolutionary patterns."

The team was also able to delve into genome sequence details for endosymbionts associated with human head lice compared with human body lice, along with lice strains linked to other species. While the symbiont tucked away in the monkey lice that they profiled was capable of producing thiamin (vitamin B1), for example, it found that that pathway was missing in symbionts of human, chimp, and gorilla lice.

On the other hand, a vitamin B5-coding plasmid was shared in the human, chimp, and gorilla louse symbionts, but was not detected in symbionts in monkey lice. Instead, the latter bacteria appear to produce vitamin B5 with sequences encoded in the nuclear genome.

From these and other findings, the researchers suspect lice and the bacteria may share an evolutionary history, stretching to a point when lice that feed off of monkeys diverged from those plaguing humans, primates, and other hominids.

"[T]he close relationships of [Candidatus Riesia] and [Candidatus Puchtella] suggests there was no endosymbiont replacement in the common ancestor of hominid lice; instead, the phylogenetic pattern of louse endosymbionts reflects the species history of the lice themselves," the authors wrote.