All Lineages Underwent Expansion
All expansion tests supported expansion for all lineages despite Central
and Southwest occurring in unglaciated areas within North America. This
result was somewhat surprising as climatic projections indicated
suitable habitat persisted within these areas during the LGM (Herrera
2021). Refugial populations are expected to harbor the most genetic
diversity versus those on the expansion front, which could help with
identifying refugia for the Southwest and Central lineages. For the
Southwest lineage, repeated haplotypes only occurred in northern Texas
and Kansas, which suggests a possible northward expansion front. Within
the reduced dataset, the Central lineage exhibited higher genetic
diversity in the southern range with shared haplotypes only occurring
within Indiana, Illinois, and Kansas. We caution that the high frequency
of unique haplotypes within these lineages may obscure patterns of
expansion, but both appear to have expanded north as greater diversity
occurs in the southern areas within each lineage. Furthermore, expansion
from southern refugia has been found in multiple lineages within the
southwestern United States and Mexico (e.g., Barton and Wisely 2012,
Myers et al. 2020, McDonough et al. 2022).
While expansion patterns in Central and Southwest are somewhat unclear,
our analysis confirmed strong signatures of expansion within the East
and Midwest lineages. Both lineages occupy areas once covered by
glaciers, so both spatial and demographic expansion is expected
following glacial recession. Furthermore, both lineages are expanding
north into Canada due to climate change (e.g., Fiset et al. 2015,
Garcia-Elfring et al. 2017). Previously glaciated areas like the
northeastern United States and Michigan had the lowest number of
haplotypes compared to more southern populations as expected for
refugial and recolonized populations (Petit et al. 2003). In addition to
northward expansions, the East lineage, in particular, appears to be
expanding west given that haplotypes were found in Wisconsin, Louisiana,
Illinois, and western Tennessee. The East and Midwest lineages are both
known to carry B. burgdorferi , so this westward movement likely
will facilitate the continued expansion of Lyme Disease when
black-legged ticks (Ixodes scapularis ) are present. Indeed,Ixodes scapularis populations are becoming established and
increasing throughout their range including in western areas (Maestas et
al. 2016, Gardner et al. 2020, Pasternak and Palli 2022), and typically,
Lyme Disease reports quickly follow population establishment. Expansion
of East and Midwest lineage white-footed mice may certainly contribute
to Lyme Disease, but many other factors contribute to this spread
including climate, landscape heterogeneity, questing behavior of tick
nymphs, and movement of other hosts (e.g., Eisen et al. 2016, Halsey et
al. 2018, Gardner et al. 2020). It is currently unknown if Central and
Southwest lineages carry B. burgdorferi like East and Midwest,
but Lyme Disease remains rare in warmer climates despite both P.
leucopus and I. scapularis being present (Ginsberg et al. 2021).