Natural selection under contrasting ecological conditions in the aromatic plant Lippia graveolens (H.B.K., Verbenaceae)

Abstract

Understanding the genetic basis for how organisms adapt to a changing environment is a major topic in plant evolution, particularly in widely distributed species. Mexican oregano (Lippia graveolens) grows naturally in an extensive area with marked climatic and environmental differences. This species is therefore an interesting model that can be used to understand how genetic variation responds to environmental changes along climatic clines. The present study aimed to answer the questions: are there alleles likely to be under natural selection in different environments? And, how is climate related to candidate loci under selection? Genetic structure, genetic diversity, and potential adaptive molecular markers under selection were evaluated in 22 populations under contrasting environmental conditions. To assess genetic diversity, six inter-simple sequence repeat (iSSR) primers were used on a total of 327 individuals. Bioclimatic prediction data from WORLDCLIM v1.4 were used for each population. Population structure revealed three clearly distinct genetic groups (F ST = 0.589, P < 0.0001). Mean within-population genetic diversity was low (H S = 0.104 ± 0.008), populations in the north region showed the lowest diversity. Three different approaches, Bayesian analysis, hierarchical analysis, and a third one using logistic regression, were employed to identify adaptive loci likely to be under selection. The first two approaches together detected 12 loci with substantial evidence of selection, but the logistic regression approach identified only two loci associated with four environmental variables. Results show that this species is responding differently to environmental conditions (temperature and humidity) and suggest ongoing processes of local adaptation.