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?

Abstract

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.

Genetic variation in Tertiary relics: The case of eastern‐Mediterranean Abies (Pinaceae)

“…The problem of a small number of loci was already mentioned, and tandem‐repeat loci are known to be more prone to homoplasy than the other types of markers. Moreover, we have no reliable information about mutation rates in nSSR loci in the genus Abies and relied on the study performed in another conifer genus (however, the mean mutation rate used by Tsuda et al., also relies on indirect evidence only). Second, the current version of the Diyabc algorithms does not take into account migration or pollen‐mediated gene flow among taxa which counteracts differentiation; this may lead to underestimation of divergence times (van Loo, Hintsteiner, Pötzelsberger, Schüler, & Hasenauer, ; Semerikov, Semerikova, Polezhaeva, Kosintsev, & Lascoux, ).…”

“…Simulations applied the Generalized Stepwise Mutation (GSM) model. As we have no information on the distribution of mutation rates of microsatellite loci in Abies , a mean of 2 × 10 −4 was used in analogy with Picea (Tsuda et al., ). One locus, showing excessively high frequency of single nucleotide insertions/deletions (AB15), was omitted from the analyses.…”

Plant speciation across environmental gradients and the occurrence and nature of hybrid zones

“…A more complex genetic signature of historical introgression is evident between the two forms of Piriqueta caroliniana that form a hybrid zone in Florida (Table ), indicating that the more introgressed P. c. viridis has advanced northwards displacing P. c. caroliniana in southern Florida, pushing the hybrid zone northwards, possibly as a result of climate warming (Martin & Cruzan, ; Cruzan, ). Patterns of asymmetric introgression also indicate hybrid zone movement for some Picea species, e.g., for the northward movement of a hybrid zone between P. engelmannii and P. glauca , possibly as a result of climate warming since the Last Glacial Maximum (De La Torre et al, ), and a postglacial westward movement of the hybrid zone between Picea abies and P. obovata (Tsuda et al, ). However some hybrid zones have remained remarkably stable in the face of climate change as appears to be the case for that between two North American firs, Abies balsamea and A. lasiocarpa .…”

Genomic data provide new insights on the demographic history and the extent of recent material transfers in Norway spruce

“…SSR markers define two well-differentiated groups: southern and northern P. abies, on the one hand, and Siberian spruce, on the other hand (Tsuda et al, 2016). Mitochondrial DNA, in contrast, singles out southern populations of P. abies and grouped northern and P. obovata populations together (Lockwood, Aleksic, Zou, Wang, & Liu, 2013;Tsuda et al, 2016). The latter led Lockwood et al (2013) to propose that southern populations of P. abies be regarded as a different species or, at least, as a subspecies.…”