Evolution of vegetative morphoanatomical characters in Pabstiella (Pleurothallidinae: Orchidaceae)

Abstract

The subtribe Pleurothallidinae encompasses approximately 48 genera, with Pabstiella standing out due to its 133 species, many of which are found in Brazil's Atlantic Forest. Molecular data strongly support the genus and its ten sections. However, only a few species have been anatomically described, and a comprehensive characterization of the genus for comparative purposes is still needed. Our study aimed to examine the main vegetative morphoanatomical characters of the root, stem, and leaves, and analyze their evolution based on the phylogeny of the genus. We analysed root, stem, and leaf samples from 57 species using light and scanning electron microscopy. We selected 35 characters and mapped them onto the molecular phylogeny. Pabstiella exhibits homogeneous leaf anatomy, with aquiferous tissue on both sides and chlorophyllous parenchyma in the middle, aligned with vascular bundles. The ramicaul anatomy displays a simple epidermis with primary or secondary walls, a simple to stratified hypodermis, a parenchymatous or aerenchymatous cortex, and a sheath of sclerenchyma surrounding the vascular bundles. The roots endovelamen cells are larger than those in the epivelamen, reticulated tilosomes of varying morphotypes, and giant cells in the cortex. Despite the common occurrence of homoplasy among vegetative traits, particularly in the leaves, our analyses revealed several synapomorphies for the genus. These include the presence of giant cells in the root cortex, endovelamen cells larger than those of the epivelamen, a specific distribution pattern of tilosomes, and 2–3 layers of aquifer tissue on the adaxial face of the leaf. Additionally, for the Pabstiella section, a lignified stem epidermis wall is a synapomorphic character state. Our findings demonstrate that vegetative morpho-anatomy is a useful diagnostic character for generic and infrageneric categories within the genus. When analysed from a phylogenetic perspective, it contributes to our comparative understanding of the genus.