Mito-nuclear Discordance
Reasons for the mitonuclear discordance include the different inheritance patterns of mitochondrial and nuclear markers. Mitochondrial DNA (mtDNA) markers track the maternal lineage of a species whereas nuclear markers include DNA from both parents and undergo recombination, which adds additional levels of complexity (Rubinoff & Holland, 2005; Zink & Barrowclough, 2008). This discordance raises the question of mtDNA’s power to resolve population level relationships. If mtDNA saturation is at play, which can happen due to the rapid evolutionary rate, mtDNA markers could be inaccurately signaling to this deep louse evolutionary history. This seems unlikely, however, because the primary endosymbiont of lice, Candidatus Riesia pediculicola , which is maternally inherited, shows similar distribution patterns to louse mitochondrial DNA (Hammoud et al., 2022) .
On the other hand, it might also be possible that the mito-nuclear discordance is resulting from the unusual gene transfer of human head lice where males preferentially eliminate their paternally inherited chromosomes (PGE- paternal genome elimination) (de la Filia et al., 2018). PGE coupled with sex ratio distortion of human head lice (Perotti et al., 2004) could potentially lead to sex-biased gene flow that is reflected through the differences in mitochondrial and nuclear phylogenies.
Nevertheless, the different views from both nuclear and mitochondrial DNA of lice suggest a rich evolutionary history of lice that we’re only beginning to understand. Despite the differences in inheritance, both mtDNA and nuclear markers in human head lice broadly agree on an out of Africa dispersal, and the differences in mitochondrial and nuclear patterns between human and lice suggest that louse genetics may be capturing human demographic events that are not detectable from human genetics. Recent human dispersal across the world through global ease of travel might be contributing to the continental and regional patterns that we see through the nuclear genome of lice.
It has been suggested that within-species population structure of the parasites have either an equal or higher structure than its host (Criscione et al., 2006). If this was the case for humans and lice, what we can infer from the population structure of lice can potentially be applied towards understanding the history of humans. Therefore, we uncovered some patterns of genetic variation distribution across the world that can only be explained by the recent movements and settlements of its host.