Diversity within the harmful algal species Heterosigma akashiwo

Heterosigma akashiwo is one of the most prevalent fish-killing harmful algal species and is also known for high levels of intra-specific diversity. This study aims to genetically identify different strains of Heterosigma akashiwo by using multiple DNA markers and linking the genetic diversity to growth rates, toxicity and morphology. Twenty-six Heterosigma akashiwo isolates were collected from three blooms from the Delaware Inland Bays. The entire nuclear Internal Transcribed Spacer (ITS) region, an upstream region of the transfer RNA-Leu gene in the chloroplast genome, and the mitochondrial cytochrome c oxidase subunit (COI) gene of these isolates and 18 isolates from other geographical locations were sequenced and compared with 57 other sequences available through GenBank. All of the 25 ITS regions sequenced were 100% identical and very similar (>98% identity) to sequences from previous studies available in GenBank. Both mitochondrial and chloroplast markers classified the sequenced isolates into seven clades. The chloroplast markers included more polymorphic positions than the coding mitochondrial markers. Phylogenetic analysis using the concatenated sequences of the mitochondrial and chloroplast markers classified the isolates into 12 clades and was better supported by boot strap values than using individual genetic markers. Growth rates of genetically different isolates of Heterosigma at different salinities were significantly different across both variables. Preliminary results indicate differences in toxicity and morphology among genetically different strains. Results from this study will enable predictions of the magnitude and toxic levels of Heterosigma blooms at an early stage and provide in-depth knowledge on the evolutionary history of the species.