Problematic Biases in the Availability of Molecular Markers in Protists: The Example of the Dinoflagellates
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Dinoflagellates (Alveolata, Dinophyceae) are protists with a truly remarkable diversity in lifestyles (free-living, parasites and mutualistic symbionts), habitats (marine, freshwater, plankton, benthos), and trophic modes (heterotrophic, plastid-containing). Here dinoflagellates are used to evaluate biases in the availability of molecular markers in relation to the variety of functional and ecological characteristics of protists. A large number of dinoflagellate sequences are available in GenBank, at least one for 56% of the 264 described genera. The most common marker is the small ribosomal subunit ribosomal DNA (49%). At the species level, SSU rDNA or the large subunit rDNA are available for 15% of the 2,386 described species. Availability of sequences of the internal transcribed spacers (ITS) and cytochrome oxidase I (COI) show a strong bias towards cultivable species. Relative to trophic mode, while about half of the known dinoflagellates are heterotrophic, only 12% of them have been sequenced compared to 29% of the plastid-containing species. For the COI marker availability is 10 times greater for plastid-containing compared to heterotrophic species. Freshwater species are underrepresented (13%) relative to the marine forms (22%). A high proportion of benthic species have been sequenced (46%) reflecting interest in Symbiodinium and harmful epiphytic taxa. Most of the relatively few described mutualistic species have been sequenced (> 80%). In contrast, only 17% of the described parasitic species have been sequenced, and most of the available sequences were not identified at the species level. In recent years, new species have been described mostly from coastal blooms or cultures. These studies are favored by the availability of abundant material for detailed studies of ultrastructure and multi-gene molecular phylogenies. Many methods are difficult to apply for the scarce specimens available from the open ocean. The requirement of these protocols, easy to apply with cultured species, is an obstacle in our knowledge of the open ocean diversity because it discourages studies based on sparse material. Consequently, in recent years descriptions of new species from the open ocean have declined considerably.
11 - 09 - 2015
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