Mito-nuclear discordance
The population genetic pattern inferred from our mitogenome data is
discordant with the nuclear SNP dataset, although it is not inconsistent
with a scenario of two colonization events to KNP. Phylogenetic analyses
revealed two divergent mitochondrial lineages within mountain
treeshrews; both lineages are found on both mountains, but haplogroups 1
and 2 are equally represented on MK while haplogroup 1 is more frequent
on MT (Figures 3 & 4). As mentioned above, MT provided montane habitat
earlier than MK. If KNP were colonized a second time by mountain
treeshrews from another part of their range, such as the southern
portion of the Crocker Range, this would explain the presence of two
sympatric divergent lineages within Kinabalu Park. The greater frequency
of haplogroup 2 on MK could be explained by the closer geographic
proximity of MK to the Crocker Range (Figure 7) combined with
male-biased dispersal limiting the movement of haplogroup 2 from MK to
MT. Lack of recombination in the mitochondrial genome would have
retained the divergence between these the two lineages whereas
recombination in nuclear SNPs would result in genetic admixture between
the two groups. However, as noted above, the cluster on high elevation
MT has maintained a moderate level of differentiation.
As an alternative explanation, this pattern could have been caused by a
single colonization event of two sympatric lineages that diverged
elsewhere in Borneo, for example, due to isolation in interglacial
refugia and mixing during glacial maxima when montane forest was at its
maximum extent (Cannon, Morley, & Bush, 2009; den Tex et al, 2010).
However, this scenario implies that the colonization of KNP by mountain
treeshrews would have occurred after the divergence between the two
lineages ca. 450,000 ybp, which is relatively recent compared to the age
of MT (at least 7 million years) and the age of the species (ca. 4
million years). Multiple colonization events to MK have been inferred in
other taxa, including plants in the genus Rhododendron (Merckx et
al., 2015).
Gawin et al. (2014) documented a similar pattern in mountain blackeyes
(Chlorocharis emiliae ) in Borneo; they found two divergent
mitochondrial haplogroups on MK, with one lineage sister to a lineage
found on Mt. Trus Madi, a mountain south of MK within the Crocker Range
(Figure 7). The pattern inferred from SNP data in a subsequent study was
not concordant, with a single lineage found on MK (Manthey et al.,
2017). This similar pattern may indicate a common colonization history
between mountain blackeyes and mountain treeeshrews. Future studies
should include broader geographic sampling of mountain treeshrews,
including individuals from across the Crocker Range, to test the
hypothesis of multiple colonization events and to determine the
phylogeographic history of this species in Borneo.