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

Abstract

Inferring the evolution of characters in Isoetes 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 to test hypothesis of character evolution within the genus. One such hypothesis is that extant Isoetes have undergone a morphological reduction from larger arborescent lycophyte ancestors. In this work we examine the reduction in lobe numbers on the underground trunk, or corm, over evolutionary time. Using reversible-jump MCMC and Bayesian inference, our results support the hypothesis of a directional reduction in lobe number in Isoetes, with the best-supported model of character evolution being one of irreversible reduction. Furthermore, the most probable ancestral corm lobe number of extant Isoetes is three, and a reduction to two lobes has occurred at least six times. From our results, we can infer that corm lobation, like many other traits in Isoetes, shows a degree of homoplasy, yet also shows ongoing evolutionary reduction.