Whole genome demographic models indicate divergent effective population size histories shape contemporary genetic diversity gradients in a montane bumble bee

Understanding historical range shifts and population size variation
provides important context for interpreting contemporary genetic
diversity. Methods to predict changes in species distributions and model
changes in effective population size (Ne) using whole genomes make it
feasible to examine how temporal dynamics influence diversity across
populations. We investigate Ne variation and climate-associated range
shifts to examine the origins of a previously observed latitudinal
heterozygosity gradient in the bumble bee Bombus vancouverensis Cresson
(Hymenoptera: Apidae: Bombus Latreille) in western North America. We
analyze whole genomes from a latitude-elevation cline using sequentially
Markovian coalescent models of Ne through time to test whether relatively
low diversity in southern high-elevation populations is a result of
long-term differences in Ne. We use Maxent models of the species range
over the last 130,000 years to evaluate range shifts and stability. Ne
fluctuates with climate across populations, but more genetically diverse
northern populations have maintained greater Ne over the late Pleistocene
and experienced larger expansions with climatically favorable time
periods. Northern populations also experienced larger bottlenecks during
the last glacial period which matched the loss of range area near these
sites, however, bottlenecks were not sufficient to erode diversity
maintained during periods of large Ne. A genome sampled from an island
population indicated a severe postglacial bottleneck, indicating that
large recent post-glacial declines are detectable if they have occurred.
Genetic diversity was not related to niche stability or glacial-period
bottleneck size. Instead, spatial expansions and increased connectivity
during favorable climates likely maintain diversity in the north while
restriction to high elevations maintains relatively low diversity despite
greater stability in southern regions. Results suggest genetic diversity
gradients reflect long-term differences in Ne dynamics and also emphasize
the unique effects of isolation on insular habitats for bumble bees.
Patterns are discussed in the context of conservation under climate
change.