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Abstract
Past demographic changes and current selection pressures determine the genetic variation displayed by Nothofagus species along rainfall gradients. Based on the diversity trends observed at candidate genes associated to drought stress, we inferred a differential species’ adaptive potential. Clinal genetic variation in natural populations could reflect either recent demographic history or the evolution of adapted genotypes along heterogeneous environments. We describe genetic variation patterns in three Nothofagus species of South American temperate forests, growing along steep rainfall gradients. Our hypothesis is that the selection pressure along this gradient reinforces the genetic structure previously shaped by Pleistocene climate oscillations. We screened variation along gradients at putative adaptive markers: candidate genes involved in response to drought, and EST-SSRs linked to drought stress genes. Genomic SSRs (gSSRs) were used to decouple the incidence of demographic events in the genetic structure. Genetic diversity at SSRs agreed with the putative location of cryptic Pleistocene refugia in Nothofagus. In addition, each species showed different trends for nucleotide diversity at candidate genes. Unbiased heterozygosity significantly correlated with precipitation at EST-SSRs in Nothofagus nervosa. We found evidences of balancing selection and several SNPs departed from neutral expectations. Nothofagus genetic variability shows a strong imprint of demographic changes that reveals refugia location for the species during Pleistocene. This variability is modelled by environmental conditions across natural gradients, which impose selection pressure at genome regions related to stress response, providing clues about inter-specific differences in adaptive potential to water deficit.
Highlights
Climate is currently experiencing significant changes on a global scale, affecting ecosystems and generating shifting environments (Allen et al 2010)
Discriminant analysis (DAPC) and STRUCTURE bring about a similar grouping for both Genomic SSRs (gSSRs) and EST-SSRs, when pre-defined populations were set (Fig. 3)
Our work show evidences supporting that selection pressure caused by varying precipitation regimes could be one of the driving factors promoting different trends at candidate genes
Summary
Climate is currently experiencing significant changes on a global scale, affecting ecosystems and generating shifting environments (Allen et al 2010). The impact on tree species is of concern due to their role as keystone species in the structuring of forests and maintenance of biodiversity (Kremer et al 2012). In this context, species survival is challenging and there are different possible responses: either be sufficiently plastic, migrate to more suitable areas, rely on genetic adaptation, or become extinct (Aitken et al 2008; Pardo-Diaz et al 2015). The need for adaptation to current rapid climate changes probably exceeds what long-lived organisms like trees may be able to cope with (Fady et al 2016; Roschanski et al 2016)
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