On the trail of notorious pests: we owe the infestation with lice to a strange group of mammals, according to a genetic study of the developmental history of the parasites and their victims. According to this, millions of years ago, lice jumped from birds to an early representative of the Afrotheria, which includes elephants and curious small animals. After this change of host, the pests then started their successful career among mammals: They created new forms and adapted to specific victims – including humans, the scientists say.
Many people have already had unpleasant experiences with the nasty crawlers: head lice are making the rounds, especially among children, and clothing lice or pubic lice are also among the nuisances of humans. We are just one victim among many: thousands of different types of lice infest a wide range of animal species. The parasites can be divided into two groups based on their feeding habits: chewing lice feed on the skin or secretions, while sucking lice puncture the skin to ingest the blood of their hosts. Both types can occur in mammals, but the bloodthirsty representatives are found exclusively in mammals.
Developmental history reflected in genetics
As part of their study, the scientists working with Kevin Johnson from the University of Illinois at Champaign have now devoted themselves to researching the “career” of the pests: With the help of genetics, they investigated which animals could have made the first victims among mammals, from which the lice could then attack others and differentiate themselves. The results are based on analyzes and comparisons of genetic information from many louse species and their hosts. Certain similarities in the genome allow conclusions to be drawn about family relationships and the times at which certain lines of development emerged. This resulted in family trees for both the lice and their victims.
As the scientists explain, by comparing the two family trees, many parallel branches and branches became apparent: points at which a group of mammals began to evolve into new forms are often also reflected in the genomes of the lice that inhabit these mammals parasitize. In this context, the scientists came across particularly interesting results when classifying the data that originated from elephants, marmot-sized hyraxes and small elephant-shrews. At first glance, one might think that these animals have little in common. But despite their great differences in appearance, they are relatively closely related: they belong to the African-descended mammal group Afrotheria.
“Out of Afrotheria” scenario
“In this work, we used genome sequencing data to show that a large group of mammalian lice, including human lice, descended from an ancestor that attacked victims from the Afrotheria mammalian group,” says Johnson. “This shows that these mammalian lice first lived in this strange group of African mammals and then spread to other mammals,” the scientist concludes. Much of the subsequent diversification of this group of lice can be attributed to an “out-of-Afrotheria” scenario in which Afrotheria lice colonized other placental mammals, the scientists explain.
But where did the Afrotheria lice come from? As can be seen from other traces in the family trees, they probably came from birds. More needs to be done to understand the evolutionary history of lice and their hosts, Johnson says. However, according to previous evidence, the lice are probably 90 to 100 million years old and first parasitized dinosaurs and birds. A host change from birds to mammals is very rare, Johnson emphasizes. But apparently lice eventually managed to gain a foothold among the Afrotheria. Once the lice learned to exploit them, they could then spread from one species to another quite easily, giving rise to new forms. “After the dinosaurs became extinct about 65 million years ago and birds and mammals became highly diversified, aphids also began to seek out new hosts and diversify,” says Johnson.
Source: University of Illinois Urbana-Champaign, professional article: Nature Ecology & Evolution, doi: 10.1038/s41559-022-01803-1