Data from: The extent and meaning of hybridization and introgression between Siberian spruce (Picea obovata) and Norway spruce (Picea abies): cryptic refugia as stepping stones to the west?

Boreal species were repeatedly exposed to ice ages and went through cycles
of contraction and expansion while sister species alternated periods of
contact and isolation. The resulting genetic structure is consequently
complex, and demographic inferences are intrinsically challenging. The
range of Norway spruce (Picea abies) and Siberian spruce (Picea obovata)
covers most of northern Eurasia; yet their geographical limits and
histories remain poorly understood. To delineate the hybrid zone between
the two species and reconstruct their joint demographic history, we
analysed variation at nuclear SSR and mitochondrial DNA in 102 and 88
populations, respectively. The dynamics of the hybrid zone was analysed
with approximate Bayesian computation (ABC) followed by posterior
predictive structure plot reconstruction and the presence of barriers
across the range tested with estimated effective migration surfaces. To
estimate the divergence time between the two species, nuclear sequences
from two well-separated populations of each species were analysed with
ABC. Two main barriers divide the range of the two species: one
corresponds to the hybrid zone between them, and the other separates the
southern and northern domains of Norway spruce. The hybrid zone is centred
on the Urals, but the genetic impact of Siberian spruce extends further
west. The joint distribution of mitochondrial and nuclear variation
indicates an introgression of mitochondrial DNA from Norway spruce into
Siberian spruce. Overall, our data reveal a demographic history where the
two species interacted frequently and where migrants originating from the
Urals and the West Siberian Plain recolonized northern Russia and
Scandinavia using scattered refugial populations of Norway spruce as
stepping stones towards the west.