The complex interplay between chromosome, climatic niche and morphological traits shapes the diversification of Carex...

Shifts in lineage diversification rates, shaped by speciation and extinction, are influenced by morphological, ecological and genetic changes. In this study, we investigate the drivers of diversification in Carex, considering chromosome number evolution, bioclimatic variables (temperature and precipitation) and morphological traits (culm and lateral spike unit lengths), along with their evolutionary rates.

First, we used Bayesian analysis of macroevolutionary mixtures (BAMM) to estimate diversification and trait evolution rates and probabilistic models for chromosome evolution. Phylogenetic path analyses (PPAs) were then used to assess the relative contribution of the predictors to diversification. We tested three different model sets: (1) a set where diversification is predicted by chromosome number, climate and morphology means for each species; (2) a set where diversification is predicted by the evolutionary rates of these three predictors; and (3) a set combining both mean values and evolutionary rates to predict lineage diversification. Additionally, we used the Quantitative State Speciation and Extinction (QuaSSE) model to test the effects of chromosome evolution rates on speciation and extinction.

Chromosome number and the rate of chromosome evolution have a weak but significant influence on Carex diversification, though it varies across models and clades. Bioclimatic variables and their evolutionary rates also affect diversification, but only indirectly, through their influence on morphology and chromosome evolution.

The subtle but significant influence of chromosome number and its rate of evolution on Carex diversification suggests that the remarkable diversity of the genus cannot be explained by a single driver. Instead, it probably results from a complex interplay among bioclimatic, genomic and morphological traits. Notably, the influence of chromosome number is not consistent across all models and clades, highlighting the context-dependent nature of these relationships. Thus, the associations between traits and lineage diversification resist simple linear explanations and may vary across Carex lineages.