Paving the way to the systematic understanding of a challenging clade of lepidote African Croton (Euphorbiaceae)

Croton is a predominantly woody pantropical genus that exhibits the so-called ‘Odd Man Out’ pattern, with Africa having the lowest species diversity across the tropics. Previous phylogenetic analyses have shown that all the Old World species of Croton sampled so far form a single clade nested within a New World backbone. Here we focus on a taxonomically challenging group of African Croton characterized by the presence of lepidote indument. Species delimitation within this group has proven difficult due to the lack of strong morphological differentiation and the incompleteness of numerous herbarium specimens from tropical Africa. We sequenced two universal genetic regions (ITS, trnL-F) using multiple accessions per species/morphospecies based on herbarium material to provide a preliminary phylogenetic framework to guide the lineage sampling for a broader phylogenomic study targeting this group and the rest of all African Croton. We also used these molecular data, along with information on geographic occurrences, to aid in the identification of poor, immature or infertile specimens and to detect possible instances of taxonomic misidentification in herbarium collections. Our results show that African lepidote species belong to several separate clades, one of them containing the majority of the lepidote species. Phylogenetic resolution within this largest lepidote clade is low, but several highly supported subclades were recovered. The sampling of these subclades should be considered in any future study aimed at reconstructing a phylogenetic tree of African Croton using phylogenomics.