Ontogeny, anatomical structure and function of lobed stems in the evolution of the climbing growth form in Malvaceae (Byttneria Loefl.).

Abstract

Byttneria is one of the few climbing genera in Malvaceae. Some Byttneria are known for their lobed stems. We explore the development of these stems, how they have evolved within the group and their relevance in the evolution of the climbing growth form in Malvaceae. We combine developmental anatomical work with phylogenetic comparative methods. We use Byttneria divaricata and B. filipes as models in the anatomical work, a review of herbarium vouchers, and the most recent phylogeny of Byttneria and allies to elucidate how these stems evolved within the clade under maximum-likelihood and Bayesian approaches. We use Pagel94 tests to analyse the correlated evolution of lobed stems and prickles. Each lobe coincides with one of the five vascular bundles. By augmented activity of the fascicular cambium in the lobes coupled with reduced activity of the interfascicular cambium in the interlobes, secondary growth increases the lobulation already present during primary growth. Within Byttneria and allies, lobed young stems appeared at least three times, once in Ayenia and twice in the paraphyletic Byttneria. Lobed adult stems were conserved in Byttneria s.s., where lobed adult stems in combination with prickles were shown to have evolved as a climbing mechanism within the group; prickles were lost once within Byttneria s.s., in a shrubby subclade. Byttneria Clade 2 comprises climbers with twining cylindrical adult stems and no prickles, which constitutes a different climbing mechanism in the group. We provide evidence of one of the few cambial variants known whose secondary body reflects the primary body vasculature and show that lobed adult stems and prickles in Byttneria could be used in the new delimitation of genera in the group. Lobed stems independently appeared in climbing Grewia, suggesting a convergence favouring the climbing growth form.