Correlations among chromosome numbers, nuclear DNA contents and climatic variables of South American Cactaceae in a phylogenetic context

Abstract

Cactaceae are American members of arid and semi-arid regions and are also present in tropical forests and temperate climates. The taxonomy is complex owing to large morphological variability, and polyploidy is a significant mechanism in cactus speciation associated with geographical and environmental range expansions. In South America, Cactaceae have a wide distributional range and provide an informative model in which to examine the correlation between ploidy levels and genome sizes with geographical and bioclimatic variables. We tested whether ploidy levels andDNA contents are related to elevation and the 19 WorldClim variables, and we mapped chromosome numbers and amounts of DNA on a molecular phylogeny to interpret their evolution. We performed Pearson correlation tests between the response variables and each climatic variable and used a multivariate analysis to assess the pattern of variation among variables and chromosome numbers. Our data suggest that polyploidy had independent origins within each subfamily. Chromosome numbers were related to variables associated with temperature, whereas DNA contents were related mostly to precipitation. A negative correlation was observed between the amount of DNA in one chromosome set and the increase in the ploidy level. Based on the results, the small genome can be considered ecologically important as an adaptation to higher temperatures and droughts.