DNA taxonomy in morphologically plastic taxa: algorithmic species delimitation in the Boodlea complex (Chlorophyta: Cladophorales)
DNA-based taxonomy provides a convenient and reliable tool for species delimitation, especially in organisms in which morphological discrimination is difficult or impossible, such as many algal taxa. A group with a long history of confusing species circumscriptions is the morphologically plastic Boodlea complex, comprising the marine green algal genera Boodlea, Cladophoropsis, Phyllodictyon and Struveopsis. In this study, we elucidate species boundaries in the Boodlea complex by analysing nrDNA internal transcribed spacer sequences from 175 specimens collected from a wide geographical range. Algorithmic methods of sequence-based species delineation were applied, including statistical parsimony network analysis, and a maximum likelihood approach that uses a mixed Yule-coalescent model and detects species boundaries based on differences in branching rates at the level of species and populations. Sequence analyses resulted in the recognition of 13 phylogenetic species, although we failed to detect sharp species boundaries, possibly as a result of incomplete reproductive isolation. We found considerable conflict between traditional and phylogenetic species definitions. Identical morphological forms were distributed in different clades (cryptic diversity), and at the same time most of the phylogenetic species contained a mixture of different morphologies (indicating intraspecific morphological variation). Sampling outside the morphological range of the Boodlea complex revealed that the enigmatic, sponge-associated Cladophoropsis (Spongocladia) vaucheriiformis, also falls within the Boodlea complex. Given the observed evolutionary complexity and nomenclatural problems associated with establishing a Linnaean taxonomy for this group, we propose to discard provisionally the misleading morphospecies and genus names, and refer to clade numbers within a single genus, Boodlea.