Inferring the Evolutionary Reduction of Corm Lobation inIsoëtesusing Bayesian Model-Averaged Ancestral State Reconstruction

Abstract

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 to test hypothesis of character evolution within the genus. One such hypothesis is that extant Isoëtes 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 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. From our results, we can infer that corm lobation, like many other traits in Isoëtes, shows a degree of homoplasy, yet also shows ongoing evolutionary reduction.

Distribution, status, and habitat characteristics of Columbia Quillwort (<i>Isoetes minima</i>, Isoetaceae) in Canada

“…Gross form and leaves-The pale green plants emerge as tufts of 6-12, ~0.5-0.75-mm-wide leaves from a corm-like rootstock (Figure 1), which has been described as either three-lobed (Eaton 1898) or twolobed (Pfeiffer 1922; our study). Freund et al (2018) considered corm lobation to be of significant value in classifying some Isoetes taxa into clades, but their study did not include I. minima.…”