Parasite and Host Discordance
Our results found that much of the genetic structure in lice was similar
to that of their human hosts, a few notable differences deserve further
exploration. The hierarchical structure analysis (AMOVA) found high
amounts of variation (>20%) across multiple spatial
levels. This differs significantly from human genetic diversity where
~13% of variation is among continents,
~1% is among regions within continents and
~85% is within-population among regions (Jorde et al.
2000). These differences point to lice maintaining higher isolation than
their hosts even with the ability of horizontal transfer compared to the
strict vertical transfer in the host. Additional research is needed to
understand the evolutionary forces acting upon local louse populations,
but not on the host, to achieve these levels of louse variation and
continental population genetic structure (High FSTvalues between continents).
The host-mediated geographical structure of nuclear diversity in lice
contrasts with the documented distribution of mitochondrial clades in
lice. Three deeply divergent mitochondrial clades (A, B and C) that are
not geographically well defined have been hypothesized to be associated
with lice on now-extinct species of hominids (e.g., Neanderthals) that
dispersed onto modern humans during periods of contemporaneity and
survived to present day (Reed et al., 2004). In contrast, the more
geographically structured nuclear clusters mirror the human host
distribution following modern human dispersal out of Africa. This is
consistent with Ascunce et al. (2013) where they found no relationship
between the mitochondrial clade a given louse was assigned to and the
nuclear genetic clusters in which the individual was found.