Body lice are thought to have diverged from head lice when humans began to wear clothes. However, it is debated whether this was a speciation event, or if head and body lice still interbreed. I used nuclear and mitochondrial genes, and microsatellite markers, to infer the evolutionary history and the population dynamics of the head lice and body lice of humans.
The internal transcribed spacer 2 (ITS2) of ribosomal DNA and the cytochrome oxidase subunit I (COI) gene from mitochondrial DNA are often used as markers for insect populations. However, I found high levels of intragenomic variation in the ITS2 of two head lice and four body lice, and I found that nucleotide sequences from individual lice were not monophyletic. I concluded that this gene was not suitable for phylogenetic or population studies of head lice and body lice. Phylogenetic analysis of COI sequences from 56 lice showed that head lice and body lice were not reciprocally monophyletic. FST statistics from haplotype frequencies for lice from different countries, and for head and body lice, showed that there was more variation explained by differences among lice from different countries than between head and body lice. I interpreted the COI results as evidence that head and body lice are the same species, based on the assumption of a single divergence event for head and body lice. However, the lack of reciprocal monophyly of the COI tree could also be caused by incomplete lineage sorting of the COI gene, if head and body lice had diverged recently.
To examine the population dynamics of head and body lice on a host, 443 lice from 11 double infestations (hosts infested with both head and body lice simultaneously) were typed at five microsatellite loci. The results showed that the head and body lice on a host were genetically distinct. Among hosts that shared sleeping quarters, lice had migrated from heads to heads and bodies to bodies, but not between heads and bodies. This provided strong evidence that head lice and body lice do not interbreed in nature and are therefore separate species.
To infer the evolutionary history of these lice, I generated phylogenetic trees from small subunit ribosomal RNA (SSU rRNA), cytochrome oxidase subunit I (COI) and elongation faction 1 α (EF-1α). The datasets for these trees were a combination of sequences from my laboratory work and from GenBank. Lineages of lice from SSU rRNA and COI trees were tested for selective neutrality (no selection or recent population growth), and lice from Africa, Europe and all countries other than Africa were measured for levels of nucleotide diversity. The results from all genes except EF-1α indicated that: (i) head lice had diverged into at least two lineages which are now interbreeding; (ii) body lice diverged from one lineage of head lice that experienced a recent population expansion; and (iii) the level of nucleotide diversity in lice from Europe was similar to that of lice from sub-Saharan Africa. From these results and those of other studies, I speculated on the most likely scenario for the evolutionary history of head and body lice. I proposed that modem-day head lice have a mixed ancestry from lice that evolved on Neanderthals and lice that evolved on modem humans. I also propose that body lice evolved from just one lineage of head lice that infested modem humans, after modem humans spread out of Africa 80,000-150,000 ya.