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Keywords:

  • Chloroplast microsatellites;
  • genetic structure;
  • Holocene fragmentation;
  • Iberian Peninsula;
  • mountain isolation;
  • Pinus sylvestris;
  • vertical migration

Abstract

Aim  This work investigates the population genetic effects of periodic altitudinal migrations and interstadial fragmentation episodes in long-term Scots pine (Pinus sylvestris L.) populations at a regional scale.

Location  The study focuses on Scots pine populations in the northern Meseta and peripheral mountain chains, central and north-western Iberian Peninsula. The ample macrofossil record in the area shows that this 60,000-km2 region represent a glacial refugium for Scots pine. The species occupied large areas on the Meseta plains during glacial cold stages, but it has periodically sheltered at high elevation in the surrounding mountain chains during warm episodes, conforming to a fragmented pattern similar to its present-day distribution.

Methods  We perform a fine-scale chloroplast microsatellite (cpSSR) survey to assess the genetic structure of 13 montane Scots pine isolates in the northern Meseta (total N = 322 individuals). Using a hierarchical analysis of molecular variance (amova), we test the hypothesis of genetic isolation among disjunct mountain areas. We use a standard coalescence model to estimate genealogical relationship among populations, investigating the potential role of the regional relief as a factor influencing historic gene exchange among Scots pine populations.

Results  Population haplotypic diversity was high among Scots pine populations (He = 0.978), greater than values reported for other more thermophilic pine species in the Iberian Peninsula. The amova revealed low (but significant) differentiation among populations (ΦST = 0.031, P = 0.010), showed that the disjoint montane distribution could not account for the genetic divergence among areas (ΦCT = 0.012, P = 0.253), and that there was non-trivial subdivision among populations within the same mountain region (ΦSC = 0.021, P = 0.012). The genealogical relationships among populations showed that Scots pine isolates growing on disjoint mountain blocks, but on slopes flowing to the same basin, were genetically closer than populations growing on different slopes of the same mountain chain, flowing to different basins.

Main conclusions  The observed genetic structure for Scots pine is consistent with its population history, inferred from the palaeobotanical record, with vertical migrations throughout climatic pulses and with the drainage basins and large long-term population sizes connecting different mountain blocks during the cooler glacial periods. Overall, the results suggest that, despite periodic interstadial fragmentation episodes, Scots pine biology provides for the long-term maintenance of high within-population and low among-population genetic diversity at neutral genetic markers.