Phylogenomic analysis of pseudocryptic diversity reveals the new genus Deltalsia (Rhodomelaceae, Rhodophyta)

Molecular analyses, in combination with morphological studies, provide invaluable tools for delineating red algal taxa. However, molecular datasets are incomplete and taxonomic revisions are often required once additional species or populations are sequenced. The small red alga Conferva parasitica was described from the British Isles in 1762 and then reported from other parts of Europe. Conferva parasitica was traditionally included in the genus Pterosiphonia (type species P. cloiophylla in Schmitz and Falkenberg 1897), based on its morphological characters, and later transferred to Symphyocladia and finally to Symphyocladiella using molecular data from an Iberian specimen. However, although morphological differences have been observed between specimens of Symphyocladiella parasitica from northern and southern Europe they have yet to be investigated in a phylogenetic context. In this study, we collected specimens from both regions, studied their morphology and analyzed rbcL and cox1 DNA sequences. We determined the phylogenetic position of a British specimen using a phylogenomic approach based on mitochondrial and plastid genomes. Northern and southern European populations attributed to S. parasitica represent different species. Symphyocladiella arecina sp. nov. is proposed for specimens from southern Europe, but British specimens were resolved as a distant sister lineage to the morphologically distinctive Amplisiphonia, so we propose the new genus Deltalsia for this species. Our study highlights the relevance of using materials collected close to the type localities for taxonomic reassessments, and showcases the utility of genome-based phylogenies for resolving classification issues in the red algae.

We thank Gary Saunders, Viviana Peña, Lin Baldock, Francis Bunker, John Huisman, John West and Ignacio Bárbara for providing some samples used in this study, and Jo Wilbraham for providing information on specimens in the BM herbarium. This research was supported by computational facilities of Centro de Supercomputación de Galicia (CESGA). This research was also supported by Xunta de Galicia ‘Axudas de apoio á etapa de formación posdoutoral’ (Grant ED481D/ 2017/011) and ‘Talento Senior’ (Grant 03IN858A2019-1630129) to PD-T. This research was also supported by the Australian Biological Resources Study (Activity ID 4-G046WSD) and field work in Western Australia supported by the Holsworth Foundation.

Peer reviewed