Comparative wood anatomy and origin of woodiness in subfamilies Secamonoideae and Asclepiadoideae (Apocynaceae)

Abstract

The Apocynaceae subfamilies Secamonoideae and Asclepiadoideae have undergone several transitions during their evolution with regard to growth form and degree of woodiness. In this study, we present a wood anatomical overview of both subfamilies that complements previous work on the remaining Apocynaceae. Detailed microscopic wood descriptions using light and scanning electron microscopy were performed on 60 species that cover most Secamonoideae genera and all major woody Asclepiadoideae lineages. Our observations are in line with subfamilial and (sometimes sub)tribal delimitations. Furthermore, we present for the first time an overview of the estimated number of shifts from herbaceousness to (phylogenetically) derived woodiness in Asclepiadoideae, along with a derived woody species list with distribution and habitat information. In total, at least 168 derived woody species resulting from at least 28 independent woodiness transitions were found, with drought possibly being one of the main drivers of most of these transitions.