Tetsuya Nishikawa

MICROSATELLITE MARKERS REVEAL POPULATION GENETIC STRUCTURE OF THE TOXIC DINOFLAGELLATE ALEXANDRIUM TAMARENSE (DINOPHYCEAE) IN JAPANESE COASTAL WATERS1

This is the first report to explore the fine‐scale diversity, population genetic structure, and biogeography of a typical planktonic microbe in Japanese and Korean coastal waters and also to try to detect the impact of natural and human‐assisted dispersals on the genetic structure and gene flow in a toxic dinoflagellate species. Here we present the genetic analysis of Alexandrium tamarense (Lebour) Balech populations from 10 sites along the Japanese and Korean coasts. We used nine microsatellite loci, which varied widely in number of alleles and gene diversity across populations. The analysis revealed that Neis genetic distance correlated significantly with geographic distance in pair‐wise comparisons, and that there was genetic differentiation in about half of 45 pair‐wise populations. These results clearly indicate genetic isolation among populations according to geographic distance and restricted gene flow via natural dispersal through tidal currents among the populations. On the other hand, high P‐values in Fishers combined test were detected in five pair‐wise populations, suggesting similar genetic structure and a close genetic relationship between the populations. These findings suggest that the genetic structure of Japanese A. tamarense populations has been disturbed, possibly by human‐assisted dispersal, which has resulted in gene flow between geographically separated populations.

DIFFERENCES IN THE PRODUCTION AND EXCRETION KINETICS OF OKADAIC ACID, DINOPHYSISTOXIN-1, AND PECTENOTOXIN-2 BETWEEN CULTURES OF DINOPHYSIS ACUMINATA AND DINOPHYSIS FORTII ISOLATED FROM WESTERN JAPAN(1).

We established clonal cultures of Dinophysis acuminata Clap. et Lachm. and D. fortii Pavill. isolated from western Japan and examined toxin production in them, focusing on intracellular production and extracellular excretion. At the end of incubations, the total amounts of pectenotoxin‐2 (PTX‐2), dinophysistoxin‐1 (DTX‐1), and okadaic acid (OA) in the D. acuminata cultures reached up to 672.7 ± 14.7 (mean ± SD), 88.1 ± 2.8, and 539.3 ± 39.7 ng · mL−1, respectively, and the excreted extracellular amounts were equivalent to 5.1, 79.5, and 79.5% of the total amounts, respectively. Similarly, at the end of incubations, the total amounts of PTX‐2, DTX‐1, and OA in the D. fortii cultures reached up to 526.6 ± 52.6 (mean ±SD), 4.4 ± 0.4, and 135.9 ± 3.9 ng · mL−1, respectively, and the excreted extracellular amounts were equivalent to 1.8, 80.1, and 86.6% of the total amounts, respectively. Further, we tested the availability of cell debris and dissolved organic substances that originated from the ciliate prey Myrionecta rubra for growth and toxin production in D. acuminata. Although no significant growth was observed in D. acuminata in the medium containing the cell debris and organic substances originated from M. rubra, the toxicity was significantly greater than that in the control (P < 0.05–0.001); this finding suggested the availability of organic substances for toxin production. However, toxin productivity was remarkably lower than that of Dinophysis species feeding on living M. rubra.