Inferring the evolutionary reduction of corm lobation in Isoëtes using Bayesian model-averaged ancestral state reconstruction

Premise of the Study: Inferring the evolution of characters in Isoëtes has been problematic, as these plants are morphologically conservative and yet highly variable and homoplasious within that conserved base morphology. However, molecular phylogenies have given us a valuable tool for testing hypotheses of character evolution within the genus, such as the hypothesis of ongoing morphological reductions. Methods: We examined the reduction in lobe number on the underground trunk, or corm, by combining the most recent molecular phylogeny with morphological descriptions gathered from the literature and observations of living specimens. Ancestral character states were inferred using nonstationary evolutionary models, reversible-jump MCMC, and Bayesian model averaging. Key Results: Our results support the hypothesis of a directional reduction in lobe number in Isoëtes, with the best-supported model of character evolution being one of irreversible reduction. Furthermore, the most probable ancestral corm lobe number of extant Isoëtes is three, and a reduction to two lobes has occurred at least six times. Conclusions: From our results, we can infer that corm lobation, like many other traits in Isoëtes, shows a degree of homoplasy, and yet also shows ongoing evolutionary reduction.