Using data from genome sequencing, scientists at the University of Illinois show that a significant newly recognized group of mammalian lice, including lice of humans, originated on the common ancestor of Afrotheria. The study found that the first louse to take up residence on a mammalian host likely started as a parasite of birds.
Tens of millions of years ago, a host-jumping event started the lengthy relationship between mammals and lice, paving the way for their coevolution and increasing the likelihood that the lice would spread to additional mammals.
The study compared the genomes and family trees of lice and their mammalian hosts. The effort showed that the two trees share many parallel branches and twigs. In the genomes of the lice that parasitized those animals, those branching points, where one group of mammals began dividing into new forms, were frequently echoed.
Kevin P. Johnson, a principal research scientist and ornithologist at the Illinois Natural History Survey who led the study, said, “After originating on the common ancestor, these lice then went on to colonize other major groups of mammals through the process of host switching.”
“Lice fall into two groups based on their eating habits. Chewing lice munch on skin or secretions while sucking lice pierce the skin to consume the blood of their hosts. Both types feed on mammals, but sucking lice are exclusive to mammals.”
Two kinds of chewing lice that also feed on mammals are closely related to sucking lice, and “each of the significant families within this newly found lineage appears on at least one member of Afrotheria.
According to that research, lice’s first hosts among mammals were thought to be Afrotheria members. Johnson and Jorge Doña, a Marie Curie postdoctoral researcher at the University of Illinois Urbana-Champaign and the University of Granada, Spain- studied the evolutionary histories of the mammals and their lice while broadening the genomic sampling of mammalian louse genomes to include more lice connected to Afrotheria. They specifically had hyrax and elephant shrew lice.
Their analysis indicated that the lice of elephants, hyraxes, and elephant shrews were the most ancient in the group of chewing and sucking lice that fed on mammals.
Johnson said, “This shows that these mammalian lice started in this weird group of African mammals and switched to other mammals after that.”
“Host switching from birds to mammals was infrequent. The team found evidence that this occurred only a few times – to Madagascan lemurs, South American rodents, and some marsupials, for example. But once lice learned how to feed on mammals, they could more easily jump from one mammal species to another and likely had more opportunities. And as certain mammalian groups separated geographically, they diverged, and so did their lice.”
“While more work must be done to track the evolutionary history of lice and their hosts. Lice probably date back 90 million to 100 million years and likely first parasitized dinosaurs or birds.”
“And then, after dinosaurs went extinct roughly 65 million years ago and birds and mammals diversified, the lice also started to jump to new hosts and diversify.”