Abstract
The genetic relationship between the toxic marine dinoflagellates Alexandrium tamarense and Alexandrium catenella, which share similar morphological characteristics and are clustered as a sister group on the phylogenetic tree based on 28S rDNA analysis was examined by DNA–DNA hybridization. This method has contributed to the information on relative comparisons of total sequences in the field of microbiology; however limited DNA–DNA hybridization studies have been carried out on microalgae, including dinoflagellates. The intragroup homology of DNA–DNA hybridization within the A. tamarense and A. catenella groups, that was identified on the basis of morphotypes, was 85.0%–99.8% in both cases, whereas the intergroup homology between these two groups was 62.5%–70.3%, which was lower than the intragroup homology. These percentages revealed that not only the rDNA sequences but also the total sequence reflects slight morphological differences between A. tamarense and A. catenella, indicating the possibility that strains identified as these two species could be differentiated as distinct groups based on the percentage obtained using DNA–DNA hybridization. The homology of A. tamarense with other Alexandrium species was low—37.1%–45.2% with A. tamiyavanichii, 41.0%–45.5% with A. affine, and 24.8%–35.8% with A. ostenfeldii. The relatedness between A. catenella and other Alexandrium strains tested (e.g. between A. catenella and A. tamiyavanichii was 39.0%–51.2%) was also lower than that between A. tamarense and A. catenella. Further, these percentages were consistent with the rDNA phylogenetic analysis, indicating that DNA–DNA hybridization might be a useful tool to understand the relatedness among Alexandrium species.
