Selective pressures of climate change in drought-sensitive relict forestsGenetic responses in Abies pinsapo and Cedrus atlantica to a shifting environment

Résumé

Climate change provides new challenges for understanding species adaptive capacity. Trees are especially concerning given their lower evolutionary rates and ecological importance. Thus, the knowledge of their intraspecific genetic diversity is crucial to predict the evolutionary consequences of global climate change and its long-term effects on biodiversity. Since drought stress is among the first limitations for plants facing a changing climate, understanding the underlying mechanisms involved in drought stress resilience in trees constitutes one of the most valuable research objectives in evolutionary ecology. Relict species are reliable models given their reduced geographic range size, isolation and usually fragmented distribution, which subject them to many shifting genetic processes. Here, we attempted to test the effect of these processes in the genetic diversity and genetic structure of trees, and to determine their evolutionary potential under a climate change scenario. We used as experimental models two relict and endangered conifers: Abies pinsapo, endemic to Southern Spain, and Cedrus atlantica, endemic to Northern Morocco. We attempted to place the conservation insights of population genetic structure in a climate change context, in A. pinsapo. Nuclear nSSR, ISSR and chloroplast cpSSR markers were analysed to investigate the extent of ongoing genetic erosion, isolation and divergent genetic diversity, among populations and cohorts young, adult and old trees , at the landscape scale of this species. A. pinsapo showed a relatively high neutral genetic variation. Evidences for bottlenecks and genetic drift were found in all the studied populations, together with a low among-populations genetic differentiation. No evidences of demographic genetic erosion among cohorts were found. De novo transcriptome sequencing was also carried out for both model species. These transcriptomes constitute their first genomic resources. Both species were used to perform experimental drought treatments and transcriptomic analyses to identify contrasting gene expression over immediate, extended and post-drought stages. Two responses were observed after the post-drought treatment in both species: certain individuals survived drought resilient , while others died non-resilient . Single nucleotide polymorphisms SNPs were also investigated in A. pinsapo. The results showed significant differences in the gene expression through the different drought intensities, and between drought resilient and non-resilient individuals, in both species. These differences were mostly found in genes related to drought resilience in A. pinsapo and to drought sensitiveness in C. atlantica; showing two different strategies in response to the same environmental stress. We also identified SNPs related to drought resilience in A. pinsapo. Thus, candidate genes mechanistically involved in drought adaptive capacity were obtained for both species. We also provide a framework for validating candidate genes of drought resilience in natural populations of A. pinsapo by using landscape genomics. Finally, we detected several climate change-related selection signatures in drought-related mortality in A. pinsapo by using landscape genomics analyses. Dead trees were compared to alive individuals belonging to different ages through an altitudinal ecotone. We investigated the genes GORK and PIP1, previously related to drought response in plant model species. The results supported a significant genetic differentiation for the GORK gene related to survival in one of the populations, showing preliminary evidences on the potential role of GORK in providing drought resilience in A. pinsapo. The results of this thesis offer valuable information in evolutionary ecology, since they provide a better understanding on rapid adaptive capacity in trees, as well as key information on the design of suitable conservation and management strategies for these two endangered species.