HORIZONTAL GENE TRANSFER & EVOLUTIONARY ORIGIN OF SAXITOXINS

Dinoflagellates make a highly diverse group of prostists, with 2000 known species, many of which live in the marine environment. Saxitoxin (STX) is a group of neurotoxins, which are among the various toxins produced by dinoflagellates. STXs are known for causing paralytic shellfish poisoning (PSP) when filter feeders such as mussel, clams and oysters that have accumulated these toxins, are consumed. The genera of dinoflagellates, Alexandrium, Gymnodinium and Pyrodinium and some cyanobacteria synthesize STX, with lot of similarity in the process of synthesis. The biosynthetic pathway and genes needed for STX synthesis have already been characterized in several cyanobacterial species. The details of the pathway and genes included in it, given in the introduction of this paper, highlight the complex biochemistry of STX and, numerous genes involved in its biosynthesis. It has been proposed that horizontal gene transfer (HGT) has played a key role in cyanobacteria acquiring these genes from other bacteria. In few species of Alexandrium and Gymnodinium, genes for STX synthesis have also been characterized. There are some suggestions about the evolutionary origin of STX based on the data of sxtA gene that initiates STX synthesis. However, stxA is just a single gene in the pathway that may have up to 26 proteins. This study investigated the second core gene sxtG of the STX synthesizing pathway of PSP dinoflagellates.

Dinoflagellate strains (whose identity was determined by using 18S rDNA gene) were cultured. Genomic DNA and total RNA were isolated from cultured cells. Putative sxtG homologs were identified based on previously published dinoflagellate cDNA libraries and primers were designed for PCR of sxtG. PCRs were performed and analyzed. Phylogenetic analyses were also performed. The structure of sxtG dinoflagellate gene was determined by molecular methods.

The findings of the study showed that putative sxtG expressed sequence tags (ESTs) have a typical dinoflagellate structure and they most probably represent dinoflagellate homologs of the cyanobacterial sxtG gene. It was revealed that transcriptomic and genomic structure of sxtG gene have many dinoflagellate-specific features such as monosteric transcripts rather than polysteric transcripts – which is a feature of bacteria. These results suggest that sxtG gene has undergone numerous modifications after its introduction in the dinoflagellates from ancestral bacteria. One such modification would be multiple independent losses and acquisitions of introns of sxtG gene.

Another interesting finding of this study was that the presence and activity of sxtG gene was not restricted only to species of PSP dinoflagellates that produce STX. The gene was present and also got transcribed in the PSP dinoflagellates (all Alexandrium species) that do not have sxtA gene and do not produce STX. Two possible explanations for this finding include necessity of sxtG gene for other biochemical pathways or recent loss of capacity of STX production, in those dinoflagellate species that do not produce STX but still have active sxtG gene. However, some discrepancies were also observed and, sxtG was not detected in non-PSP dinoflagellate genera.

Based on the phylogenetic tree of sxtG, it seems likely that the common ancestor of this gene would be a proteobacterium, as previously suggested for cyanobacteria. It has been suggested that the origin of STX synthesis and genes included in it might have occurred via a HGT event between an ancestral STX producing bacterium and the common ancestor of Alexandrium and Pyrodinium. Few descendant species might have secondarily lost STX synthesis pathway. Species of Gymnodinium may have independently acquired STX from a later dinoflagellate-dinoflagellate transfer. Results of this study support these hypotheses of evolutionary origin of STX.

In summary, no firm conclusions were made on the evolutionary origin of saxitoxins in dinoflagellates, as it is a very complicated phenomenon and needs further studies. However, authors characterized sxtG gene in detail which is a next gene in STX synthesis after the sxtA gene that initiates STX synthesis. This paper also highlights significance of horizontal gene transfer between and/or among bacteria and protists and, consequent evolutionary loss and/or gain of genes and related functions (e.g. STX synthesis) in microbial species (species of bacteria and also protists). Now few questions that came in my mind include, how did HGT events occur among these bacteria and protists? Is there any role of viruses and/or phages in transferring these genes among species of bacteria and protists? How would bacterium-protist HGT occur? Or STX producing bacteria were ingested by a common ancestor of Alexandrium and Pyrodinium and in that way somehow acquired genes for STX?

Orr, R. J. S., Stüken, A., Murray, S. A., & Jakobsen, K. S. (2013). Evolutionary acquisition and loss of saxitoxin biosynthesis in dinoflagellates: the second “core” gene, sxtG. Applied and environmental microbiology, 79(7), 2128-2136.