Abstract
Nine wild Iberian provenances of Castanea sativa Mill. grouped in two gene pools, North Iberian Peninsula and Mediterranean, were evaluated for several adaptive traits in two provenance–progeny trials with the aim of evaluating the role of natural selection in shaping adaptive variation and increasing our understanding of the genetic structure of this species, as well as reporting complete information on the genetic variation among and within the studied populations. An annual growth rhythm experiment was evaluated during the first 3 years after establishment for phenology, growth, stem form and survival, and a periodic drought-stress experiment was evaluated for dry weight, growth, survival and other related drought traits in both well-watered and drought-stress treatments. The high genetic variability reported in both trials is largely due to the genetic variation among populations. The significant differences reported between quantitative genetic and neutral marker differentiation indicated the local adaptation of these populations through directional selection, mainly for phenology, growth and biomass allocation. A clinal variation among populations was determined through correlations of phenology with latitude and xerothermic index of the provenances, showing that central and southern Mediterranean populations had earlier phenology than northern populations and that drought played a relevant role in this differentiation. The significant correlation between phenological traits and the ancestry values in the Mediterranean gene pool supported the different pattern of behavior between both gene pools and also indicated the existence of two ecotypes: xeric and mesophytic ecotypes, corresponding to Mediterranean and North Iberian gene pools, respectively. The results obtained in the drought-stress experiment confirmed that, in general terms, xeric populations showed a greater adaptability to drought, with more developed root systems and higher survival than northern populations. Moreover, the genetic variability observed within populations indicated the potential response capacity of Iberian C. sativa populations to undergo fast adaptive evolution.
Highlights
Genetic diversity provides the fundamental basis for evolution by natural selection [1] and, its preservation within and among populations of a species is necessary to safeguard its potential to adapt to future environmental changes [2]
In spite of the expected reduction in genetic variation within populations because of the effects of directional selection, which resulted from the extreme environments reported in our annual growth study and the limited gene flow of C. sativa [38], we found significant within population variation for most measured traits
In light of the results obtained in the present study, we can conclude that the high genetic differentiation detected among wild Iberian C. sativa populations responds both to a latitudinal clinal pattern, with drought acting as a main force in central and southern Mediterranean populations, and to an ecotypic variation due to the restricted or absent gene flow among these geographically distant populations
Summary
Genetic diversity provides the fundamental basis for evolution by natural selection [1] and, its preservation within and among populations of a species is necessary to safeguard its potential to adapt to future environmental changes [2]. Two types of genetic diversity should be considered: neutral genetic variation, which includes stochastic processes such as gene flow and genetic drift, and adaptive genetic variation mainly determined by selection [3,4]. The balance of these evolutionary forces determines the level of differentiation among populations, enhanced by divergent selection and genetic drift and constrained by gene flow. In some temperate broadleaved species of scattered distributions and limited gene flow, such as Acer platanoides and Fraxinus excelsior, most genetic variation for traits related to growth rhythm is observed among populations [11,12,13]. Phenology and other fitness-related traits such as tolerance to drought and heat are especially relevant in studies on genetic diversity and tree responses to climatic change [22]
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