Genetic variation of isolated and peripheral populations of Pinus sylvestris (L.) from glacial refugia

Abstract

Analysis of variation at 10 polymorphic isoenzyme loci in 51 European populations of Pinus sylvestris both from its main continuous range and from isolated, marginal populations out of the continuous range, revealed differences in genetic structure. The highest genetic richness, measured as mean number of alleles per locus, was recorded for populations from the Iberian Peninsula, followed by those from Scotland and the Balkans. Marginal, isolated populations were characterized by much greater interpopulation variation than populations from the continuous range of the species. The highest mean observed heterozygosity was recorded for the Iberian populations. The highest gene flow was observed among populations in the continuous range of the species. The populations could be classified into five groups based on genetic similarities. Populations from the continuous range formed one group, those from Spain two groups, pines from Scotland one group and those from the Balkans one group. One population from Scotland was found to be most distinct from the other Scottish pines and was found to be grouped with the Balkan populations. The occurrence and frequencies of rare alleles in the populations showed a characteristic pattern, suggesting that populations from the Iberian Peninsula probably did not participate in the colonization of Europe by Pinus sylvestris after the last glaciation. In contrast, the migration of populations from the Balkan refugium into Central and Western Europe had a significant effect on the contemporary gene pools of populations of Pinus sylvestris in its continuous range.

Introduction

Research in the field of population genetics and biogeography in Europe has focused on postglacial migrations of flora and fauna for several reasons. The glacial refugia and migration routes that occurred during the interglacial periods in the Pleistocene, particularly after the last glaciation (about 10,000 years B.P.; Huntley and Birks, 1983) had a crucial effect on the structure of gene pools and the present pattern of genetic variation in most European plant and animal species. Populations from glacial refugia frequently represent old Tertiary gene pools with unique traits. These populations constitute a major part of the genetic variation of the species not affected by the continental ice-sheet (Cheddadi et al., 2006, Dvořákova et al., 2010, Petit et al., 2003, Prus-Głowacki et al., 2003, Soranzo et al., 2000).

With respect to the Scots pine (Pinus sylvestris L.) in Europe it has been postulated that this species survived the last glaciations in four main refugia, namely in the Iberian Peninsula, the north Apennine Peninsula, south of the Carpathian and Sudeten Mountains, and in the Balkans. It also survived in the Russian Plains, wherefrom it spread, after the glaciers had retreated, into the central and western parts of Europe and to Fenoscandia (Bennet et al., 1991, Björkman et al., 2002, Huntley and Birks, 1983, Robledo-Arnuncio et al., 2004, Soranzo et al., 2000). Thus the current shape of the gene pools of European populations of Pinus sylvestris has been determined by the migration of populations from different refugia and by gene exchange. The gene pools of this species have also been affected by isolation, by mutations and selection due to ecological conditions as well as by genetic drift.

Recently the gene pools of P. sylvestris have also been affected to a considerable extent by human activity connected with the management of forest resources and silviculture. Pollution linked to the development of industry has also influenced the genetic structure of populations of forest trees through the (negative) selection of susceptible genotypes (Bergmann and Hozius, 1996, Oleksyn et al., 1994, Prus-Głowacki et al., 1999, Prus-Głowacki et al., 2006). In the future, an increasing level of ozone in the troposphere and predicted global warming are expected to have substantial effects on processes of selection and genetic variation of Scots pine populations (Fowler et al., 1999, Staszak et al., 2004).

The routes of migration of P. sylvestris from glacial refugia have been investigated by several working groups (Table 1) but it remains unclear which migration routes and refugia had a decisive role in the recolonization of Europe during the last interglacial period. Some authors have suggested that refugia in the Pyrenean and Apennine Peninsulas only participated to a limited extent in shaping the current genetic variation of P. sylvestris, owing to the geographic barriers of the Pyrenees and the Alps (Cheddadi et al., 2006, Labra et al., 2006, Prus-Głowacki et al., 2003, Sinclair et al., 1999, Soranzo et al., 2000). However other researchers have assumed that the Pyrenean and Apennine refugia made substantial contributions in the patterns of variation in Scots pine in Europe after the last glaciation (Newton et al., 1999). Naydenov et al. (2007) postulated that the glacial refugia located in Northeastern Europe made a considerable genetic contribution to the Central European populations of P. sylvestris.

Populations from glacial refugia are also interesting subjects of investigation for other reasons: (1) As mentioned above, they represent elements of old gene pools of a given species. (2) Owing due to their peripheral location in relation to the continuous range of the species and their isolation, these populations might show specific pattern of genetic variation. (3) It is interesting to determine the genetic contribution of populations from glacial refugia to the gene pool of a species. As far as the Balkan, Pyrenean, and Scottish refugia of Pinus sylvestris are concerned, the natural, autochthonic populations of this species, which are assumed to represent the Tertiary gene pool, occupy mountain ranges in small, isolated and scattered localities. The genetic structure and variation of these populations have not been investigated sufficiently so that it would be possible to draw general conclusions concerning their contribution to the formation of the current gene pool in the European populations (Bilgen and Kaya, 2007, Cheddadi et al., 2006, Floran et al., 2010, Naydenov et al., 2005, Naydenov et al., 2007).

The analysis of variation in molecular markers including isoenzymes (e.g., Mandák et al., 2011) and chloroplast, mitochondrial and nuclear DNA, facilitates the determination of the precise genetic structure of a population, as well as the directions of gene flows and migrations routes (e.g., Dvořákova et al., 2010). In the investigation of migration routes, analysis of the frequencies of rare alleles is particularly useful (Petit et al., 2002, Prus-Głowacki et al., 2003). The aim of this study was to analyze the level of genetic variation, the genetic structure, genetic similarities, gene flow and genetic diversity among populations of Pinus sylvestris in marginal populations in relation to the populations of this species from its continuous range in Western and Central Europe. This kind of approach was not used in previous studies in such extend. The matter of recolonization routes in Europe, starting from glacial refugia, was also our object. Based on material from 51 widely distributed populations from different locations and of different genetic structure knowledge about the history of this species after the last glaciations is elucidated. The results point at contributions of particular glacial refugia in shaping the gene pools of contemporary Scots pine populations.