Abstract
The dinoflagellate Karenia mikimotoi forms blooms in the coastal waters of temperate regions and occasionally causes massive fish and invertebrate mortality. This study aimed to elucidate the toxic effect of K. mikimotoi on marine organisms by using the genomics approach; RNA-sequence libraries were constructed, and data were analyzed to identify toxin-related genes. Next-generation sequencing produced 153,406 transcript contigs from the axenic culture of K. mikimotoi. BLASTX analysis against all assembled contigs revealed that 208 contigs were polyketide synthase (PKS) sequences. Thus, K. mikimotoi was thought to have several genes encoding PKS metabolites and to likely produce toxin-like polyketide molecules. Of all the sequences, approximately 30 encoded eight PKS genes, which were remarkably similar to those of Karenia brevis. Our phylogenetic analyses showed that these genes belonged to a new group of PKS type-I genes. Phylogenetic and active domain analyses showed that the amino acid sequence of four among eight Karenia PKS genes was not similar to any of the reported PKS genes. These PKS genes might possibly be associated with the synthesis of polyketide toxins produced by Karenia species. Further, a homology search revealed 10 contigs that were similar to a toxin gene responsible for the synthesis of saxitoxin (sxtA) in the toxic dinoflagellate Alexandrium fundyense. These contigs encoded A1–A3 domains of sxtA genes. Thus, this study identified some transcripts in K. mikimotoi that might be associated with several putative toxin-related genes. The findings of this study might help understand the mechanism of toxicity of K. mikimotoi and other dinoflagellates.
Highlights
Karenia mikimotoi [(Miyake et Kominami ex Oda) G
We identified eight different polyketide synthase (PKS) genes of K. mikimotoi that were remarkably similar to those [29] of K. brevis, which is a relative species of K. mikimotoi by contig assembly and polymerase chain reaction (PCR)-sequencing methods
The biosynthetic mechanisms of polyether ladder polyketides are suggested to be common in dinoflagellates, because their structures are highly similar to each other [26]. These findings suggest that the four PKS orthologs in Karenia species might possibly be related to the biosynthesis of unique structures as polyether ladder components in polyketide, this hypothesis needs to be verified further
Summary
Karenia mikimotoi [(Miyake et Kominami ex Oda) G. Hansen et Moestrup (formerly Gyrodinium aureolum, G. cf aureolum, G. type-’65, G. nagasakiense, and G. mikimotoi) is a harmful dinoflagellate and forms massive blooms in coastal waters of the temperate regions in Europe [1,2,3,4] and East Asia [5,6], especially west Japan [7] Blooms of this species occasionally cause massive mortality of fish and invertebrates, thereby resulting in remarkable damage to fisheries; for example, in 1998, economic losses estimated at US$ 40 million were reported in Hong Kong [8] and, in 2012, economic losses of at least US$ 15 million were reported in Japan [9]. The mechanisms associated with fish and invertebrate mortality caused by K. mikimotoi have been extensively studied; the significant causative agents responsible for the massive mortality of marine organisms have not yet been identified
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