Genomic analysis of the lectotype specimens of European Ulva rigida and Ulva lacinulata (Ulvaceae, Chlorophyta) reveals the ongoing misapplication of names

ABSTRACT

Species of Ulva Linnaeus are nearly impossible to identify using morpho-anatomy due to their simple thallus structure and phenotypic plasticity. The current solution to this problem is to sequence DNA from field-collected specimens and match these sequences to those available in public DNA databases. However, because type specimens of many species have not been sequenced, the accuracy of these identifications is highly doubtful. Ulva rigida C.Agardh is reported to be one of the most widespread and ecologically important Ulva species, but these records are based on either morpho-anatomy or, more recently, on DNA sequences. High-throughput sequencing (HTS) was performed on the lectotype specimen of U. rigida from Cádiz, Spain to determine the correct application of the name. The analysis yielded its complete plastid genome. rbcL, tufA and ITS sequences from the lectotype specimen differed at the species level from all U. rigida sequences deposited in public databases. Instead, the lectotype sequences of U. rigida were identical or very similar to sequences identified as U. rotundata Bliding (referred to by some as U. pseudorotundata Cormaci, G.Furnari & Alongi) from Ireland and Portugal, but not to the holotype of U. rotundata from Italy, which was identical to U. lactuca L. HTS of the lectotype of U. lacinulata (Kützing) Wittrock from Lesina, Croatia, a species morphologically similar to U. rigida with macroscopic marginal teeth, also yielded a complete plastid genome, with sequences identical or highly similar to GenBank U. armoricana Dion, Reviers & Coat, U. ‘laetevirens’, U. ‘rigida’ and U. scandinavica Bliding. Since U. lacinulata is the oldest validly published name, it is the correct one to apply to the globally distributed species that was previously but incorrectly known as U. rigida. Based on this genetic evidence, U. rigida is restricted to European waters and confirmed by DNA sequences from Ireland, Portugal and Spain. This analysis shows that many barcode species identifications and taxonomic conclusions in the genus Ulva are incorrect.

Highlights

Ulva rigida was misapplied and is restricted to Atlantic Europe.

Ulva lacinulata is the dentate species of Ulva distributed worldwide.

Ulva rotundata is a heterotypic synonym of U. lactuca.

KEYWORDS:

• ITS
• lectotype
• plastid genome
• rbcL
• tufA
• type specimen
• Ulva lacinulata
• Ulva rotundata
• Ulva scandinavica

Acknowledgements

We thank Patrik Frödén from Lund University and Roxali Bijmoer at the Naturalis Biodiversity Center in Leiden for graciously providing loans of the type materials for genetic study. Our sincere thanks to Shoeb Ahmad and the myGenomics team for their continued expertise and high quality professional genomic work.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary information

The following supplementary material is accessible via the Supplementary Content tab on the article’s online page at https://doi.org/10.1080/09670262.2021.1914862

Supplementary table S1. Information for GenBank sequences used in the rbcL phylogenetic analysis of Ulva species.

Supplementary table S2. Genomic and genetic marker comparisons between the lectotype specimens of U. rigida, U. lacinulata and sequences deposited in GenBank.

Supplementary fig. S1. Locally collinear blocks (LCBs) analysis for six complete Ulva plastid genomes. The figure represents linearized alignments identifying conserved gene regions for the lectotypes of U. rigida and U. lacinulata, and GenBank specimens of U. ‘laetevirens’, U. ‘rigida’ and U. ‘rotundata’. The reference taxon is U. gigantea. Each plastid genome is oriented horizontally and homologous blocks are shown as identically coloured regions. Blocks that are inverted relative to U. gigantea are shifted below the centre axis. Sequence similarities within an LCB are proportional to the heights of the interior bars. Large sections of white within blocks, and gaps between blocks, indicate genome specific sequences. The figure was drawn using the Mauve 1.1.1 plugin in Geneious Prime.

Supplementary fig. S2. Sequence identity plot comparing five complete plastid genomes using mVISTA with U. gigantea designated as the reference using the LAGAN alignment program. Grey arrows above the alignment indicate genes with their orientation and position. A cut-off of 70% identity was used for the plots, and the Y-scale represents the percentage identity ranging from 50% to 100%.

Author contributions

J. Hughey: original concept, DNA extraction and analyses, drafting and editing manuscript; P. Gabrielson: original concept, acquiring type specimens, editing manuscript; C. Maggs: acquiring type specimens, editing manuscript; F. Mineur: DNA extractions, PCR and sequencing.

Additional information

Funding

This work was funded by a private family trust from PWG.