Ultraconserved elements improve resolution of marmot phylogeny and offer insights into biogeographic history

Marmots (Marmota spp.) comprise a lineage of large-bodied ground squirrels that diversified rapidly in the Pleistocene, when the planet quickly transitioned to a drier, colder, and highly seasonal climate-particularly at high latitudes. Fossil evidence indicates the genus spread from North America, across Beringia, and into the European Alps over the course of only a few million years, beginning in the late Pliocene. Marmots are highly adapted to survive long and severely cold winters, and this likely favored their expansion and diversification over this time period. Previous phylogenetic studies have identified two major subgenera of marmots, but the timing of important speciation events and some species relationships have been difficult to resolve. Here we use ultraconserved elements and mitogenomes, with samples from all 15 extant species, to more precisely retrace how and when marmots came to inhabit a vast Holarctic range. Our results indicate marmots arose in North America in the mid Miocene (∼16.3 Mya) and dispersed across the Bering Land Bridge in the late Pliocene (∼3-4 Mya); in addition, our fossil-calibrated timeline is suggestive of the rise and spread of open grasslands as being particularly important to marmot diversification. The woodchuck (M. monax) and the Alaska marmot (M. broweri) are found to be more closely related to the Eurasian species than to the other North American species. Paraphyly is evident in the bobak marmot (M. bobak) and the hoary marmot (M. caligata), and in the case of the latter the data are highly suggestive of a second, cryptic species in the Cascade Mountains of Washington.

A Study of Hybridization Between Marmota baibacina and M. sibirica in Their Secondary Contact Zone in Mongolian Altai

The role of hybridization as one of the factors of speciation in mammals has been underestimated for a long time, but now there is a lot of data on its impact in mammalian evolution. Hybridization of species often occurs in their secondary contact zones, which is a natural model for testing factors that ensure species integrity. Studies of hybrid zones are increasingly revealing the essential role of ecological and behavioral features both in initiating crossbreeding and in maintaining interspecific barriers. We studied the hybridization of two species of marmots Marmota baibacina and M. sibirica in the zone of sympatry in Mongolian Altai Mountains. We used a bioacoustic approach to determine the localization of individuals of different species and their cohabitation sites. Genetic typing with two diploid nuclear markers and one marker each of paternal and maternal lines was used to identify hybrids. Habitat preferences of marmots were studied to understand the conditions for the formation of heterospecific pairs. We found a high proportion of hybrid individuals in boulder screes where conditions for the formation of heterospecific pairs probably exist. Our data indicate the viability and fertility of F1 hybrids and their descendants. We hypothesize that the environmental preferences and behavioral features of both species of marmots are important factors that both create conditions for hybridization and limit hybrid dispersal.

Complex history of isolation and gene flow in hoary, Olympic, and endangered Vancouver Island marmots

Climate change resulting in a reduction of alpine habitat is believed to pose a considerable risk to alpine-dependent species, including many marmots. Hoary marmots (Marmota caligata) range throughout much of the mountainous Pacific Northwest (PNW) and Rocky Mountains while the closely related Olympic and Vancouver Island marmots (M. olympus and M. vancouverensis, respectively) are restricted to small isolated regions of the PNW. The endemic Vancouver Island marmot is currently classified as Critically Endangered and the Olympic marmot has recently experienced dramatic population declines. Previous phylogenetic studies of PNW marmot species have had limited power as they focused on resolving interspecific relationships, implicitly assumed an absence of gene flow among currently recognized species, included relatively few individuals, and relied heavily or entirely on mitochondrial DNA. We sequenced 2 mitochondrial and 4 nuclear markers from 167 hoary, 4 Vancouver Island, and 5 Olympic marmots in order to investigate phylogenetic relationships and historic gene flow among these species. We recovered 2 monophyletic (and predominantly allopatric) mitochondrial clades of hoary marmots that are not sister groups. Instead, Vancouver Island marmots formed a monophyletic mitochondrial sister clade to 1 of the hoary marmot clades. Nuclear loci did not recover the 2 mitochondrial clades of hoary marmots and suggest that Vancouver Island marmots may have experienced mitochondrial introgression from coastal mainland hoary marmots. Additionally, our nuclear results suggest possible gene flow between hoary and Olympic marmots despite different chromosomal formulas. Rather than resolving what has previously been considered a straightforward 3-taxon phylogenetic question, our findings suggest a complicated history of rapid divergence of the 3 species followed by intermittent and possibly ongoing gene flow between hoary marmots and both Olympic and Vancouver Island marmots. These results therefore have significant implications for the conservation of the latter 2 species, both of which are conservation concerns.