Genetic and morphological differentiation in Populus nigra L.: isolation by colonization or isolation by adaptation?

Abstract

Identifying processes underlying the genetic and morphological differences among populations is a central question of evolutionary biology. Forest trees typically contain high levels of neutral genetic variation, and genetic differences are often correlated with geographic distance between populations [isolation by distance (IBD)] or are due to historic vicariance events [isolation by colonization (IBC)]. In contrast, morphological differences are largely due to local adaptation. Here, we examined genetic (microsatellite) and morphological (from a common garden experiment) variation in Populus nigra L., European black poplar, collected from 13 sites across western Europe and grown in a common garden in Belgium. Significant genetic differentiation was observed, with populations from France displaying greater admixture than the distinct Spanish and central European gene pools, consistent with previously described glacial refugia (IBC). Many quantitative traits displayed a bimodal distribution, approximately corresponding to small-leaf and large-leaf ecotypes. Examination of nine climatic variables revealed the sampling locations to have diverse climates, and although the correlation between morphological and climatic differences was significant, the pattern was not consistent with strict local adaptation. Partial Mantel tests based on multivariate summary statistics identified significant residual correlation in comparisons of small-leaf to large-leaf ecotypes, and within the small-leaf samples, but not within large-leaf ecotypes, indicating that variation within the small-leaf morphotype in particular may be adaptive. Some small-leaf populations experience climates very similar to those in large-leaf sites. We conclude that adaptive differentiation and persistent IBC acted in combination to produce the genetic and morphological patterns observed in P. nigra.

Raznolikost i strukturiranost hrvatskih kontinentalnih i alpsko-dinarskih populacija bijele johe (Alnus incana /L./ Moench subsp. incana); geografska i okolišna izolacija kao uzrok fenotipske divergencije

“…These patterns confirm that multiple evolutionary processes influence the morphological diversity and structure of the populations (DeWoody et al 2015). In general, two distinct morphotypes were found: small-leaf in the mountainous Alpine-Dinaric region, and large-leaf in the continental region.…”

“…The isolation by distance assumes that phenotypic differentiation between populations increases with increasing geographic distance because of limited pollen and seed dispersal (Wright 1943(Wright , 1946Orsini et al 2013). On the other hand, gene flow among ecologically divergent habitats is reduced because of reduced success of the establishment of immigrants from different environments, as a result of local genetic adaptation (Nosil and Crespi 2004;Noisl et al 2005Noisl et al , 2008Noisl et al , 2009Orsini et al 2013;DeWoody et al 2015). However, we cannot exclude the possibility that adaptive differentiation and historical migration processes acted in combination to produce the observed pattern of morphological variation in the studied populations (Temunović et al 2012;DeWoody et al 2015).…”

“…On the other hand, gene flow among ecologically divergent habitats is reduced because of reduced success of the establishment of immigrants from different environments, as a result of local genetic adaptation (Nosil and Crespi 2004;Noisl et al 2005Noisl et al , 2008Noisl et al , 2009Orsini et al 2013;DeWoody et al 2015). However, we cannot exclude the possibility that adaptive differentiation and historical migration processes acted in combination to produce the observed pattern of morphological variation in the studied populations (Temunović et al 2012;DeWoody et al 2015). Although the main refugium during the last glacial period was located in Central Europe (Huntley and Birks 1983;Douda et al 2014;Mandák et al 2016), A. incana probably survived in situ in river canyons and protected valleys along the north-western Dinaric Alps, while north-eastern populations from the continental biogeographical region could represent newly colonised populations from the refugium in Central Europe.…”

The challenge of separating signatures of local adaptation from those of isolation by distance and colonization history: The case of two white pines

“…These spatial processes are not mutually exclusive and can act together to decrease gene flow among ecologically divergent populations (DeWoody et al., 2015; Papadopulos et al., 2014). Therefore, disentangling their relative contributions is very challenging, and attributing patterns of genetic variation to a single factor can be an oversimplification of the processes involved.…”

Phenetic Analysis of Populus Nigra, P. Laurifolia and P. × Jrtyschensis in Natural Hybridization Zone

“…Different studies showed that spatial separation of Populus spp. populations is governed by historical processes, mainly by events in the Pleistocene and Holocene (Keller et al, 2010;Macaya-Sanz et al, 2012;Dewoody et al, 2015;Meirmans et al, 2017;Fan et al, 2018). Therefore, the variability of P. laurifolia population structure in the Tom River basin is the subject for a further extended research.…”