Date Published: February 12, 2019
Publisher: Public Library of Science
Author(s): B. Míguez-Soto, J. Fernández-Cruz, J. Fernández-López, Alexandre Fournier-Level.
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.
Genetic diversity provides the fundamental basis for evolution by natural selection  and, therefore, its preservation within and among populations of a species is necessary to safeguard its potential to adapt to future environmental changes . 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. On the other hand, the genetic variation within populations has gene flow as a main force maintaining the genetic variability and stabilizing natural selection and genetic drift acting as constraint processes [5, 6].
This study did not involve endangered or protected species. No specific permissions were required for the collection of plant material in the stands described below because they do not belong to protected or restricted areas. The owners of the lands gave their word of mouth to conduct the material collection on these sites. Moreover, all the experimental trials of this study were developed in the Forest Research Center of Lourizán, institution that provided approval for the authors to conduct the experiments and to which the corresponding author belongs. The collection of material and the experimental trials were carried out within the framework of the project “092CastaneaREG (2004–2006)” in compliance with legality.
This study provides a broad view of the genetic variation among and within nine Iberian populations for several important adaptive traits. It reports the first results of a drought-stress experiment in this area, and it is the first study to compare quantitative and molecular data to provide greater information on the role of natural selection on the geographic structure of the studied populations and to relate this structure with the gene pools identified in the Iberian Peninsula.
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. Populations of the Northern Iberian Peninsula gene pool, corresponding to the mesophytic ecotype, showed late flushing and bud set, and had a tendency to a higher growth. On the contrary, populations of the Mediterranean gene pool, corresponding to the xeric ecotype, showed an opposite pattern for these adaptive traits and a greater adaptability for certain drought-related traits. Finally, the high within population genetic variability provided in this study, mainly at loci related to phenology and growth, and the estimates obtained for the related genetic parameters for most measured traits, indicated the potential of this species to adapt to future climate changes.