Phylogeny, Taxonomy, and Systematic Position of Clethra (Clethraceae, Ericales) with Notes on Biogeography: Evidence from Plastid and Nuclear DNA Sequences

The phylogenetic relationships of Clethra were investigated by cladistic analyses of DNA sequences from the plastid region trnL‐F and the nuclear region ITS. In accordance with the current taxonomy of the genus, section Clethra and section Cuellaria constitute two separate monophyletic groups, but the species Clethra arborea is to be moved to section Clethra from its former position in the monotypic subsection Pseudocuellaria of section Cuellaria. No support for any of the four series within subsection Cuellaria (section Cuellaria) was obtained. Most of the few morphological characters used as a basis for the current classification are homoplasious when optimized on the molecular phylogeny. An extended data set including various representatives of the ericoid clade was analyzed in order to test whether the choice of outgroup would influence the results, but no variation in the general topology of the tree was observed. The same analysis was used to explore the basal branching of the ericoid clade obtained from an earlier published analysis of sequences from the chloroplast and mitochondrial genomes. The combined tree does not contradict the general topology, but whereas our plastid sequence data fully support the earlier proposed phylogenetic relationships within the clade, the basal nodes remain unresolved in the nuclear phylogeny. Our results indicate multiple biogeographic events responsible for Clethra’s eastern North American–eastern Asian distribution.