Waking the dead: morphological and molecular characterization of extant †Posoniella tricarinelloides (Thoracosphaeraceae, Dinophyceae).

The Thoracosphaeraceae are dinophytes that produce calcareous shells during their life history, whose optical crystallography has been the basis for the division into subfamilies. To evaluate the validity of the classification (mainly applied by palaeontologists), living material of phylogenetic key species is necessary albeit frequently difficult to access for contemporary morphological and molecular analyses. We isolated and established five living strains of the rare fossil-taxon †Posoniella tricarinelloides from different sediment samples collected in the South China Sea, Yellow Sea and in the Mediterranean Sea (west coast off Italy). Here, we provide detailed descriptions of its morphology and conducted phylogenetic analyses based on hundreds of accessions and thousands of informative sites on concatenated rRNA datasets. Within the monophyletic Peridiniales, †P. tricarinelloides was reliably nested in the Thoracosphaeraceae and exhibited two distinct morphological types of coccoid cells. The two morphologies of coccoid cells would have been assigned to different taxa at the subfamily level if found separately in fossil samples. Our results thus challenge previous classification concepts within the dinophytes and underline the importance of comparative morphological and molecular studies to better understand the complex biology of unicellular organisms such as †P. tricarinelloides.

An Updated List of Generic Names in the Thoracosphaeraceae.

Calcareous dinophytes produce exoskeletal calcified structures during their life history (a unique character among the alveolates) and are subsumed under the Thoracosphaeraceae as part of the Peridiniales. We provide a brief synopsis about the taxonomic history of the group, from the first descriptions of fossils in the 19th century through to the results of molecular phylogenetics studies undertaken during the past two decades. Delimitation and circumscription of the Thoracosphaeraceae are challenging, as they comprise both phototrophic (presumably including endosymbiotic) as well as heterotrophic (and even parasitic) dinophytes from marine and freshwater environments, respectively. However, calcareous structures are not known from all members of the Thoracosphaeraceae, and the corresponding species and groups are considered to have lost the capacity to calcify. Five years ago, a taxonomic list of 99 generic names assigned to the Thoracosphaeraceae was published, and we update this compendium with 19 additional names based on recent studies.

SAME BUT DIFFERENT: TWO NOVEL BICARINATE SPECIES OF EXTANT CALCAREOUS DINOPHYTES (THORACOSPHAERACEAE, PERIDINIALES) FROM THE MEDITERRANEAN SEA(1).

The diversity of extant calcareous dinophytes (Thoracosphaeraceae, Dinophyceae) is currently not sufficiently recorded. The majority of their coccoid stages are cryptotabulate or entirely atabulate, whereas relatively few forms exhibit at least some degree of tabulation more than the archeopyle. A survey of coastal surface sediment samples from the Mediterranean Sea resulted in the isolation and cultivation of several strains of calcareous dinophytes showing a prominent tabulation. We investigated the morphologies of the thecate and the coccoid cells and conducted phylogenetic analyses using Maximum Likelihood and Bayesian approaches. The coccoid cells showed a distinct reflection of the cingulum (and were thus cingulotabulate), whereas thecal morphology corresponded to the widely distributed and species-rich Scrippsiella. As inferred from molecular sequence data (including 81 new GenBank entries), the strains belonged to the Scrippsiella sensu lato clade of the Thoracosphaeraceae and represented two distinct species. Morphological details likewise indicated two distinct species with previously unknown coccoid cells that we describe here as new, namely S. bicarinata spec. nov. and S. kirschiae spec. nov. Cingulotabulation results from the fusion of processes representing the pre- and postcingular plate series in S. bicarinata, whereas the ridges represent sutures between the cingulum and the pre- and postcingular series in S. kirschiae, respectively. Bicarinate cingulotabulation appears homoplasious among calcareous dinophytes, which is further supported by a comparison to similar, but only distantly related fossil forms.

Delimitation of the Thoracosphaeraceae (Dinophyceae), including the calcareous dinoflagellates, based on large amounts of ribosomal RNA sequence data.

The phylogenetic relationships of the Dinophyceae (Alveolata) are not sufficiently resolved at present. The Thoracosphaeraceae (Peridiniales) are the only group of the Alveolata that include members with calcareous coccoid stages; this trait is considered apomorphic. Although the coccoid stage apparently is not calcareous, Bysmatrum has been assigned to the Thoracosphaeraceae based on thecal morphology. We tested the monophyly of the Thoracosphaeraceae using large sets of ribosomal RNA sequence data of the Alveolata including the Dinophyceae. Phylogenetic analyses were performed using Maximum Likelihood and Bayesian approaches. The Thoracosphaeraceae were monophyletic, but included also a number of non-calcareous dinophytes (such as Pentapharsodinium and Pfiesteria) and even parasites (such as Duboscquodinium and Tintinnophagus). Bysmatrum had an isolated and uncertain phylogenetic position outside the Thoracosphaeraceae. The phylogenetic relationships among calcareous dinophytes appear complex, and the assumption of the single origin of the potential to produce calcareous structures is challenged. The application of concatenated ribosomal RNA sequence data may prove promising for phylogenetic reconstructions of the Dinophyceae in future.