渦鞭毛藻類の分類と遺伝子解析

抄録

Dinoflagellates are a large microalgal group, widely distributed both in marine and freshwater environments. The taxonomy of dinoflagellates has been studied on the basis of their morphology. Since many morphological variations, overlapping with other taxa, have been observed for some dinoflagellate taxa, the classification has been recognized as artificial for a long time. Recent molecular phylogenetic analyses have revealed the phylogeny of dinoflagellates. Even though the phylogeny of higher taxa has not yet been clarified, the presences of several supported clades have at least been verified. Molecular phylogeny showed the pass of evolutionary acquisition of some characteristic morphological features of dinoflagellates, so far unveiled only by morphological observations, e.g., the nematocyst obtained in the clade of Gymnodinium sensu stricto, and several different eyespot types acquired in different phylogenetic clades. Molecular phylogeny has also revealed species related to the type species in several genera, e.g., Amphidinium, Gymnodinium and Gyrodinium. Based on these results, the type and related species have been re-characterized and the diagnoses of the genera emended, while other species not related to the type species have been transferred to newly established genera, e.g., Apicoporus, Testudodinium and Togula from Amphidinium, Akashiwo, Karenia and Takayama from Gymnodinium, and Levanderina and Moestrupia from Gyrodinium. For several genera the classifications were revised based on their molecular phylogeny, however, taxonomic problems related to their phylogeny still remain. For example, in the case of the taxa for which their diagnoses were revised based on their phylogeny, e.g., Amphidinium sensu stricto, new isolates having identical morphological characters and different phylogenetic positions to previously reported Amphidinium species, are difficult to identify to any particular species. Moreover, even discrepancies in phylogenetic positions of some newly isolated thecate dinoflagellates have been demonstrated, making it hard to assign them to genera having identical thecal plate numbers due to their different phylogenetic positions. Importance of the type species and specimens collected from the type locality were therefore recognized as important for molecular and morphological characterization of genera, species and intraspecific populations of dinoflagellates.