Leaf morphoanatomy of Araujia and Morrenia (Asclepiadoideae, Apocynaceae): phylogenetic implications and species key

After synonymization of Araujia and Morrenia (Asclepiadoideae, Apocynaceae), there are 12 Araujia spp., with the Chaco region, including Argentina, Bolivia, Brazil and Paraguay, as their centre of diversity. If recognized independently, neither genus is monophyletic, but consensus on this circumscription remains unclear. Therefore, to improve our understanding of the taxonomy of Araujia and Morrenia and their relationships, this research aimed to (1) find new characters based on leaf morphology and anatomy and (2) investigate how these characters are distributed in the most recent phylogenetic tree for the group. Leaves of four Araujia spp. and six Morrenia spp. were obtained from field trips and herbarium material and analysed using scanning electron and light microscopy, herbarium specimens and images. Although leaf morphoanatomical features vary across these genera, results support inclusion of Morrenia in Araujia. This outcome agrees with previous phylogenetic studies in which Araujia and Morrenia emerged as clades with high bootstrap support (100%). Our results also indicate that leaf morphoanatomical features support fewer inclusive clades in the phylogeny of Araujia–Morrenia, a finding that should be useful in future taxonomic studies. Finally, we present a previously unpublished species key.