An appraisal of Ulva (Ulvophyceae, Chlorophyta) taxonomy

The green seaweed Ulva is important from ecological and economic perspectives, but the identification of species is often problematic. Here we assessed and discussed different perspectives to establish a stable taxonomic framework for Ulva, which will benefit both ecological and applied research. We evaluated (1) the performance of commonly used DNA-barcode markers (ITS rDNA, rbcL, and tufA) using species delimitation methods (PTP and GMYC), (2) the usage of species names in the literature, and (3) the geographic coverage of genetic data to identify poorly sampled regions. Species delimitation employing the tufA gene was the most consistent across methods. Not surprisingly, DNA-based species delimitation was often in disagreement with traditional morphology-based species definitions. Biological species concepts, where tested, proved to be generally narrower than DNA-based species delimitation. Although the use of molecular markers has greatly improved our view of Ulva diversity, the names associated with DNA sequences in public databases are often unreliable, complicating species identification. Recently, sequencing type materials has considerably reduced the gap between DNA sequence data and Linnaean names, but our knowledge on Ulva diversity remains inadequate, especially in tropical regions. Perspectives for Ulva taxonomy include the consistent use of multiple DNA-barcode markers assisted by species delimitation methods, applications of genomic data, and crossing experiments. To arrive at a stable nomenclature, we outline the benefits and shortcomings of adhering to the rules and practices of the International Code of Nomenclature for algae, fungi, and plants, for example, by sequencing name-bearing types and discuss alternative approaches.

Automated summary

This study assessed different perspectives to establish a stable taxonomic framework for Ulva green seaweed, finding that species delimitation methods using the tufA gene were the most consistent. DNA-based species delimitation often disagreed with traditional morphology-based definitions, and biological species concepts were generally narrower than DNA-based delimitation. Although molecular markers improved our understanding of Ulva diversity, names associated with DNA sequences in public databases are unreliable, highlighting the need for further research in tropical regions.