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).