COI outperforms in red algae, oldest known multicellular eukaryotes

Red seaweeds, kingdom Rhodophyta, are “weird, wonderful, and extremely ancient” organisms distantly related to plants (Tudge 2000 The Variety of Life). Multicellular red algae arose at least 1.2 billion years ago, predating the earliest multicellular animals by 600 million years. Visual identification is challenging, as “morphology can be highly variable within and between species, and conspicuous features with which they can be readily identified are often lacking. In addition, highly convergent morphology is commonly encountered. …Identification is further compounded by the complexities of red algal life histories, many of which have a heteromorphic alternation of generations. Different life history stages of species have frequently been described as separate species and have only been linked through observations of life histories in culture and use of molecular techniques” (Robba et al 2006 Am J Botany 93:1101). Red seaweeds call out for DNA-based identification.

In a preliminary appraisal, COI barcodes “resolved accurately and unequivocally species identities” in three morphologically confusing species complexes of red algae (Saunders 2005 Phil Trans Royal Soc B 360:1879). In September 2006 Am J Botany, researchers from Natural History Museum, London, and University of Bristol, analyzed COI sequences of 79 specimens from 37 red algae, chosen to span 6 orders and represent species “extremely difficult to distinguish morphologically”. All species could be discriminated and intraspecific variation was generally much lower (0.0-0.7%) than interspecific differences (5.2-27.3%). Large intraspecific distances suggested the presence of cryptic species or incipient speciation in two cases. COI outperformed a commonly used genetic marker, the plastid Rubisco spacer, in revealing cryptic diversity. As is generally observed, “a good taxonomic foundation coupled with extensive sampling of taxa is essential for the development of an effective identification system.”  A holiday wish for red algae identifiers: a well-curated DNA barcode library.