RESUMO
Environmental conditions are strong selective forces, which may influence adaptation and speciation. The wild tomato species Solanum chilense, native to South America, is exposed to a range of abiotic stress factors. To identify signatures of natural selection and local adaptation, we analysed 16 genes involved in the abiotic stress response and compared the results to a set of reference genes in 23 populations across the entire species range. The abiotic stress-responsive genes are characterized by elevated nonsynonymous nucleotide diversity and divergence. We detected signatures of positive selection in several abiotic stress-responsive genes on both the population and species levels. Local adaptation to abiotic stresses is particularly apparent at the boundary of the species distribution in populations from coastal low-altitude and mountainous high-altitude regions.
RESUMO
Molecular adaptation to abiotic stresses in plants is a complex process based mainly on the modifications of gene transcriptional activity and the alteration of protein-protein interactions. We used a combination of population genetic, comparative transcriptomic and plant physiology approaches to investigate the mechanisms of adaptation to low temperatures in Solanum chilense populations distributed along Andean altitudinal gradients. We found that plants from all populations have high chilling tolerance, which does not correlate with temperatures in their native habitats. In contrast, tolerance to freezing shows a significant association with altitude and temperature variables. We also observed the differences in expression patterns of cold-response genes between plants from high- and low-altitude populations. These results suggest that genetic adaptations to low temperatures evolved in high-altitude populations of S. chilense. At the transcriptional level, these adaptations may include high levels of constitutive expression of the genes encoding ICE1, the key transcription factor of the cold signalling pathway, and chloroplast ω-3 fatty acid desaturase FAD7. At the sequence level, a signature of selection associated with the adaptation to high altitudes was detected at the C-terminal part of ICE1 encoding the ACT regulatory domain.
Assuntos
Adaptação Fisiológica/genética , Temperatura Baixa , Genética Populacional , Solanum/genética , Altitude , Chile , Ecossistema , Congelamento , Regulação da Expressão Gênica de Plantas , Genes de Plantas , Peru , Fenótipo , TranscriptomaRESUMO
After colonization population sizes may vary across the species range depending on environmental conditions and following colonizations. An interesting question is whether local adaptation occurs more frequently in large ancestral populations or in small derived populations. A higher number of new mutations and a lower effect of genetic drift should favor selection in large populations, whereas small derived populations may require an initial local adaptation event to facilitate the colonization of new habitats. Wild tomatoes are native to a broad range of different habitats characterized by variable abiotic conditions in South America, and represent an ideal system to study this interplay between demography and natural selection. Population genetic analyses and statistical inference of past demography were conducted on pooled-sequencing data from 30 genes (8,080 single nucleotide polymorphisms) from an extensive sampling of 23 Solanum chilense populations over Chile and Peru. We reveal first a north-south colonization associated with relaxed purifying selection in the south as shown by a decrease of genetic variation and an increasing proportion of nonsynonymous polymorphism from north to south, and population substructure with at least four genetic groups. Second, we uncover a dual picture of adaptation consisting of 1) a decreasing proportion of adaptive amino acid substitutions from north to south suggesting that adaptation is favored in large populations, whereas 2) signatures of local adaptation predominantly occur in the smaller populations from the marginal ranges in the south.