Inferring the accumulation of morphological disparity in epiphyllous liverworts

Phylogenetic studies of lineages growing in extreme environments have frequently recovered evidence not only of high level of homoplasy but also of discordance of morphological disparity and species diversity. It has been suggested that this divergence may be caused by developmental constraints and/or natural selection. Here we explored these hypotheses by inferring the phenotypic evolution of the derived liverwort genus Cololejeunea. These liverworts occur preferentially on the surface of leaves or other aerial parts of vascular plants growing in wet forests. The evolution of 12 morphological characters was studied using a phylogenetic framework comprising 70 species of Cololejeunea. The phylogeny was reconstructed using DNA sequences of one nuclear and two plastid regions and enabled the inference of the evolution of the studied morphological characters by determining the frequency of homoplasy. Mantel tests were used to test for correlations of morphological disparity × species diversity and morphological disparity × epiphytism. The phylogenetic informativeness of each binary character was estimated by the D metric of the Fritz and Purvis test, and the relationship between each character and epiphytism was inferred by Pearson’s coefficient. We evaluated the morphospace occupation using principal coordinate analyses. Our results not only recovered high levels of homoplasy but also weak correlations of morphological disparity and species diversity. Morphological disparity was not linked to epiphytism, although positive or negative relationships between some characters and epiphytism were found. The Brownian model of character evolution was not rejected for the studied morphological disparity in Cololejeunea with the exception of asexual propagules. The observations support the prediction that iterative evolution in a well-defined morphospace may result in rampant homoplasy and the observed divergence of disparity and diversity.