Transcriptomic comparison unveils saxitoxin biosynthesis genes in the marine dinoflagellate Gymnodinium catenatum

Abstract

The marine dinoflagellate Gymnodinium catenatum is known to produce saxitoxins (STXs) that are responsible for paralytic shellfish poisoning (PSP); however, the genes involved in STX synthesis are insufficiently understood. In the present study, we determined transcriptome sequences of toxic G. catenatum isolated from Korea (Gc-KR) and compared them with G. catenatum strains reported in other regions such as Spain (Gc-SP) and the United States (Gc-US). Toxin analysis showed that the Korean strain produced the toxins at 6.0 ± 1.9 STXs eq fmol/cell. Comparative transcriptomics of the three strains identified more than 1000 homologs of nearly all STXs biosynthesis genes in dinoflagellates, except sxtB, sxtN, and sxtY. Gene expression analysis revealed similar sxt expression patterns across all strains, with the highest expression levels observed for sxtA and sxtG. Phylogenetic analysis of sxtA, sxtG, sxtI, sxtU, and sxtS revealed distinct evolutionary patterns, with sxtA being more conserved across G. catenatum, Alexandrium spp., and toxic cyanobacteria, particularly at the sxtA4 domain, suggesting its significance in STXs synthesis. Other sxt genes in G. catenatum showed distinct patterns and significant divergence from Alexandrium spp., suggesting independent acquisition in G. catenatum. Moreover, the absence of core genes, such as sxtB, indicates it may not be essential for STXs production in G. catenatum. These findings provide insight into the sxt candidate genes in G. catenatum, enhancing our understanding of STXs biosynthesis in dinoflagellates.