A unique, highly variable mitochondrial gene with coding capacity of Heterosigma akashiwo, class Raphidophyceae

Information related to geographical distribution and local strain composition is essential to an understanding of the dynamics of harmful algae in the environment. Previously, we identified a highly variable segment on the mitochondrial genome of Heterosigma akashiwo, a bloom-forming noxious unicellular algal species. Here, we assessed the utility of the mitochondrial hypervariable region for the phylogeographic study of the alga for different distance ranges. The sequences of H. akashiwo strains obtained from different geographic origins were successfully amplified and sequenced. We found differences among the sequences of the strains obtained from high-latitude regions of Northern Pacific/Atlantic; lower latitude regions of North America West Coast; and other regions including Brazil, Japan, Singapore, and North America East Coast. On the other hand, no strong geographic patterns for the sequences among Japanese strains were observed. Therefore, the hypervariable segment may be useful to distinguish H. akashiwo strains originated from distant regions (Atlantic/Pacific, high/low latitudes), rather than regions separated by shorter distances. The sequence contains an open reading frame coding for a protein with unknown function, and the transcription of the gene was confirmed by RNA-seq analysis. Despite the sequence variations observed among H. akashiwo strains originating in different parts of the world, three domains of the protein were highly conserved among all of the strains, suggesting that they may be important to the function of the protein.