Phylogenetics and morphological character evolution in the achyranthoid clade (Amaranthaceae): Evidence to re-circumscribe the genera Achyranthes and Cyathula and to resurrect a third species of the former genus Sericocomopsis in East Africa

Abstract

The achyranthoid clade constitutes the second most species-rich lineage of the Amaranthaceae after the gomphrenoid clade. The Achyranthoids are mostly African, in contrast to the largely Neotropical Gomphrenoids, and comprise a large number of genera, many of which were revealed as non-monophyletic. Here were focus on subclade II of the Achyranthoids with the so far accepted genera Achyranthes, Achyropsis, Centrostachys, Cyathula, Nelsia, Nototrichium, Pandiaka, Sericocomopsis, and Sericostachys. Building upon an earlier dataset, we extended taxon and character sampling, both for molecular and morphological characters. Our plastid and nuclear trees converge on several highly supported clades, one comprising most species of Cyathula, Nelsia, Pandiaka and two highly divergent lineages of plants hitherto identified as Sericocomopsis hildebrandtii. Morphologically, individuals within these lineages are similar, with one of them matching the type of Sericocomopsis meruensis. This name was formerly considered a synonym and is resurrected at species level. Cymous partial florescences are ancestral in this clade, which corresponds to the genus Cyathula as re-circumscribed here. To the contrary, sterile flowers and bracteoles or tepals modified to hooks serving dispersal by animals appear multiple times within and outside this clade, indicating how their use as diagnostic characters for genera led to non-monophyletic entities. For the phylogenetically isolated and morphologically distinct Cyathula orthacantha the new genus Sebsebea is described, and for Sericocomopsis pallida the new genus Evelynastra. The Achyranthes clade is characterized by solitary, fertile flowers including the genera Achyropsis and Nototrichium both of which are merged to make Achyranthes monophyletic. In addition to a treatment to establish all required nomenclatural changes and typifications, we also provide a taxonomic backbone with full synonymy for the achyranthoid subclade II.