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.