Do tropical plants have smaller genomes? Correlation between genome size and climatic variables in the Caesalpinia Group (Caesalpinioideae, Leguminosae)

While a number of studies have suggested that temperate plants typically have larger genome sizes (GS) than tropical ones, recent analyses have not convincingly confirmed this. We have selected the widely distributed Caesalpinia Group (Leguminosae) to test this hypothesis. We used flow cytometry to estimate the amount of DNA in the haploid genome (C-values) of 40 species of the Caesalpinia Group, aiming to explore the relationship between GS and latitude as well as a range of climatic variables. These comparisons were made using Phylogenetic Comparative Methods (PCM) in a spatio-temporal context. 2C-values varied 7.73-fold, ranging from 0.92 pg (in Cenostigma bracteosum) to 7.11 pg (in Pomaria lactea). By analyzing the data within a statistical framework that took into account phylogenetic relationships, we observed a positive correlation between GS and latitude as well as additional correlations with various temperature variables. While the genetic mechanism(s) underpinning the increase or decrease of genome size in a latitudinal gradient is unclear, we hypothesize that after the origin of the group (c. 55.9 Mya), fragmentation of the Succulent Biome led to the formation of small populations (island-like) which were subject to genetic drift. This in turn led to drastic and rapid changes in the repetitive DNA fractions of the genome correlated with temperature variables. Thus, we suggest that the environment has played a role in contributing to the diversity of genome sizes reported here for the Caesalpinia Group.