Comparative transcriptomics of toxin synthesis genes between the non-toxin producing dinoflagellate Cochlodinium polykrikoides and toxigenic Alexandrium pacificum

Toxin production and transcriptomic response to nitrate concentrations in the toxic dinoflagellate Alexandrium tamarense

The bloom-forming dinoflagellate Alexandrium tamarense is one of the most important producers of paralytic shellfish poisoning toxins. Annually recurrent blooms of this dinoflagellate species is associated with the incremental nitrogen influx, especially excessive nitrate input. However, limited studies have been conducted on the toxin production and underlying molecular regulation mechanisms of A. tamarense under various nitrate (N) conditions. Therefore, toxin production and transcriptomic responses of this species were investigated. The toxin profile of A. tamarense was consistently dominated by the C2-toxins, and the cellular toxicity increased with N concentrations peaking at 9.23 ± 0.03 fmol/cell in the 883 μM N-added group. Under lower N conditions, expressions of two STX-core genes, sxtA and sxtG, were significantly down-regulated, suggesting that N regulated sxt expression and triggered responses related to toxin biosynthesis. Results of this study provided valuable insights into the ecophysiology of A. tamarense, enhancing our understanding of the occurrence of toxification events in natural environments.

Environmental Factors Modulate Saxitoxins (STXs) Production in Toxic Dinoflagellate Alexandrium: An Updated Review of STXs and Synthesis Gene Aspects

The marine dinoflagellate Alexandrium is known to form harmful algal blooms (HABs) and produces saxitoxin (STX) and its derivatives (STXs) that cause paralytic shellfish poisoning (PSP) in humans. Cell growth and cellular metabolism are affected by environmental conditions, including nutrients, temperature, light, and the salinity of aquatic systems. Abiotic factors not only engage in photosynthesis, but also modulate the production of toxic secondary metabolites, such as STXs, in dinoflagellates. STXs production is influenced by a variety of abiotic factors; however, the relationship between the regulation of these abiotic variables and STXs accumulation seems not to be consistent, and sometimes it is controversial. Few studies have suggested that abiotic factors may influence toxicity and STXs-biosynthesis gene (sxt) regulation in toxic Alexandrium, particularly in A. catenella, A. minutum, and A. pacificum. Hence, in this review, we focused on STXs production in toxic Alexandrium with respect to the major abiotic factors, such as temperature, salinity, nutrients, and light intensity. This review informs future research on more sxt genes involved in STXs production in relation to the abiotic factors in toxic dinoflagellates.

Polyphyletic origin of saxitoxin biosynthesis genes in the marine dinoflagellate Alexandrium revealed by comparative transcriptomics

The marine dinoflagellate Alexandrium is known to form harmful algal blooms, and at least 14 species within the genus can produce saxitoxins (STXs). STX biosynthesis genes (sxt) are individually revealed in toxic dinoflagellates; however, the evolutionary history remains controversial. Herein, we determined the transcriptome sequences of toxic Alexandrium (A. catenella and A. pacificum) and non-toxic Alexandrium (A. fraterculus and A. fragae) and characterized their sxt by focusing on evolutionary events and STX production. Comparative transcriptome analysis revealed higher homology of the sxt in toxic Alexandrium than in non-toxic species. Notably, non-toxic Alexandrium spp. were found to have lost two sxt core genes, namely sxtA4 and sxtG. Expression levels of 28 transcripts related to eight sxt core genes showed that sxtA, sxtG, and sxtI were relatively high (>1.5) in the toxic group compared to the non-toxic group. In contrast, the non-toxic group showed high expression levels in sxtU (1.9) and sxtD (1.7). Phylogenetic tree comparisons revealed distinct evolutionary patterns between 28S rDNA and sxtA, sxtB, sxtI, sxtD, and sxtU. However, similar topology was observed between 28S rDNA, sxtS, and sxtH/T. In the sxtB and sxtI phylogeny trees, toxic Alexandrium and cyanobacteria were clustered together, separating from non-toxic species. These suggest that Alexandrium may acquire sxt genes independently via horizontal gene transfer from toxic cyanobacteria and other multiple sources, demonstrating monocistronic transcripts of sxt in dinoflagellates.

Nuclear genome of dinoflagellates: Size variation and insights into evolutionary mechanisms

Recent progress in high-throughput sequencing technologies has dramatically increased availability of genome data for prokaryotes and eukaryotes. Dinoflagellates have distinct chromosomes and a huge genome size, which make their genomic analysis complicated. Here, we reviewed the nuclear genomes of core dinoflagellates, focusing on the genome and cell size. Till now, the genome sizes of several dinoflagellates (more than 25) have been measured by certain methods (e.g., flow cytometry), showing a range of 3-250 pg of genomic DNA per cell. In contrast to their relatively small cell size, their genomes are huge (about 1-80 times the human haploid genome). In the present study, we collected the genome and cell size data of dinoflagellates and compared their relationships. We found that dinoflagellate genome size exhibits a positive correlation with cell size. On the other hand, we recognized that the genome size is not correlated with phylogenetic relatedness. These may be caused by genome duplication, increased gene copy number, repetitive non-coding DNA, transposon expansion, horizontal gene transfer, organelle-to-nucleus gene transfer, and/or mRNA reintegration into the genome. Ultimate verification of these factors as potential causative mechanisms would require sequencing of more dinoflagellate genomes in the future.

Development of saxitoxin biosynthesis gene sxtB-targeted qPCR assay for the quantification of toxic dinoflagellates Alexandrium catenella (group I) and A. pacificum (group IV) occurring in the Korean coast

Toxic dinoflagellate Alexandrium can produce saxitoxins (STXs) and cause paralytic shellfish poisoning (PSP), and thus they are monitored for environmental safety management. Microscopic discrimination of dinoflagellates is difficult to distinguish between toxic and non-toxic species due to their similar morphology. Meanwhile, an alternative quantitative PCR (qPCR) assay is sensitive, rapid, and cost-effective for harmful species monitoring. Herein, we developed a novel qPCR assay to detect the STXs biosynthesis gene sxtB of Alexandrium catenella and A. pacificum, the leading cause of PSP outbreaks in Asian coasts and worldwide. The newly designed sxtB TaqMan probes target the species without any positive signal in other relative dinoflagellates. Deming regression analysis revealed that the sxtB copy number of A. catenella and A. pacificum was 3.6 and 4.1 copies per cell, respectively. During the blooming periods (April 13th-14th, 2020), only A. catenella cells were detected through the qPCR assay, ranging from 5.0 × 10 to 2.5 × 104 eq cells L-1. In addition, sxtB qPCR quantified more accurately compared to large subunit (LSU) rRNA targeting qPCR assay that overestimate cell density. Besides, the sensitivity of sxtB was higher compared to the microscope when the species were rarely present (5.0 × 102 cells L-1). These suggest that the sxtB qPCR assay can be applied to toxic Alexandrium monitoring in the Korean coast, even in the early stage of bloomings.

Mechanistic insights into the effects of diuron exposure on Alexandrium pacificum

Diuron (N-(3,4-dichlorophenyl)-N,N‑dimethylurea, DCMU), a ureic herbicide, is extensively used in agriculture to boost crop productivity; however, its extensive application culminates in notable environmental pollution, especially in aquatic habitats. Therefore, the present study investigated the effect of diuron on the dinoflagellate Alexandrium pacificum, which is known to induce harmful algal blooms (HAB), and its potential to biodegrade DCMU. Following a four-day DCMU exposure, our results revealed that A. pacificum proficiently assimilated DCMU at concentrations of 0.05 mg/L and 0.1 mg/L in seawater, attaining a complete reduction (100 % efficiency) after 96 h for both concentrations. Moreover, evaluations of paralytic shellfish toxins content indicated that cells subjected to higher DCMU concentrations (0.1 mg/L) exhibited reductions of 73.4 %, 86.7 %, and 75 % in GTX1, GTX4, and NEO, respectively. Exposure to DCMU led to a notable decrease in A. pacificum's photosynthetic efficacy, accompanied by increased levels of reactive oxygen species (ROS) and suppressed cell growth, with a growth inhibition rate of 41.1 % at 72 h. Proteomic investigations pinpointed the diminished expression levels of specific proteins like SxtV and SxtW, linked to paralytic shellfish toxins (PSTs) synthesis, as well as key proteins associated with Photosystem II, namely PsbA, PsbD, PsbO, and PsbU. Conversely, proteins central to the cysteine biosynthesis pathways exhibited enhanced expression. In summary, our results preliminarily resolved the molecular mechanisms underlying the response of A. pacificum to DCMU and revealed that DCMU affected the synthesis of PSTs. Meanwhile, our data suggested that A. pacificum has great potential in scavenging DCMU.

Identification and genomic analysis of Pseudosulfitobacter koreense sp. nov. isolated from toxin-producing dinoflagellate Alexandrium pacificum

The bacterial strain AP-MA-4^T isolated from the marine dinoflagellate Alexandrium pacificum (KCTC AG60911), was subjected to a taxonomic analysis. Cells of strain AP-MA-4^T were Gram-stain-negative, aerobic, rod-shaped, optimum growth at 20 °C, pH 7.0, in the presence of 5% (w/v) NaCl. Strain AP-MA-4^T shared the highest 16S rRNA gene sequence similarity to Pseudosulfitobacter pseudonitzschiae DSM 26824^T (98.5%), followed by Ascidiaceihabitans donghaensis RSS1-M3^T (96.3%), Pseudoseohaeicola caenipelagi BS-W13^T (95.7%), and Sulfitobacter pontiacus CHLG 10^T (95.3%). Based on 16S rRNA phylogeny, strain AP-MA-4^T is phylogenetically closely related to Pseudosulfitobacter pseudonitzschiae (type species of Pseudosulfitobacter) and could be distinguished from the type species based on their phenotypic properties. The genome length of strain AP-MA-4^T was 3.48 Mbp with a 62.9% G + C content. The average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values between strain AP-MA-4 ^T and its closely related type strains were 72.2-83.3 and 18.2-27.6%, respectively. Summed feature 8 (C_18:1ω7c and/or C_18:1ω6c) was identified the major fatty acids (> 10%). Phosphatidylglycerol (PG), phosphatidylethanolamine (PE), and phospholipid (PL) were demonstrated as the major polar lipids. The major respiratory quinone is ubiquinone-10 (Q-10). Based on genotypic and phenotypic features, strain AP-MA-4^T (= KCTC 92289^T = GDMCC 1.3585^T) represents a new Pseudosulfitobacter species, in which the name Pseudosulfitobacter koreense sp. nov. is proposed.

ROS-dependent cell death of Heterosigma akashiwo induced by algicidal bacterium Hahella sp. KA22

Marine algicidal bacteria and their metabolites are considered to be one of the most effective strategies to mitigate the harmful algal blooms (HABs). The bacterium Hahella sp. KA22 has previously been confirmed to have strong algicidal activity against the HABs causing microalgae, Heterosigma akashiwo. In this study, the molecular mechanism of microalgae cell death was detected. The results showed that the cell growth rate and photosynthetic efficiency were inhibited with addition of algicidal strain KA22, while the accumulation of reactive oxygen species (ROS) and oxidative damage in H. akashiwo cells increased. A total of 2056 unigenes were recognized to be differentially expressed in transcriptome sequences. In particular, the transcriptional levels of light-harvesting pigments and structural proteins in the oxygen-evolving-complex were continuously down-regulated, corresponding to the significant reduction of photosynthetic efficiency and the accumulation of ROS. Furthermore, glutamate dehydrogenase was significantly up-regulated in abundance. Meanwhile, calcium-dependent protein kinases were also detected with significant changes. Collectively, algicidal stress caused the suppressed electron transfer in chloroplast and impaired detoxification of intracellular oxidants by glutathione, which may subsequently result in multiple cell regulation and metabolic responses and ultimately lead to the ROS-dependent cell death of H. akashiwo.

Saxitoxins-producing potential of the marine dinoflagellate Alexandrium affine and its environmental implications revealed by toxins and transcriptome profiling

The marine dinoflagellate Alexandrium occurs widely in coastal waters, and some of them can produce saxitoxins (STXs) that cause paralytic shellfish poisoning (PSP). Alexandrium affine is a harmful algal bloom (HAB)-forming species off the coast of Asia; however, its ability to produce STXs has been controversial. In the present study, we detected STXs in A. affine Alex02 isolated from the southern coast of Korea. The total STXs equivalent (STXs eq) and profiles of Alex02 varied depending on the tested environmental conditions, including the temperature and nitrate concentrations. STXs toxicity levels of A. affine Alex02 (<0.8 STXs eq fmol cell^-1) were significantly lower than those of toxic A. catenella Alex03 and A. pacificum Alex05. On a genetic basis, we identified all the STX biosynthesis sxt genes, except sxtX in A. affine, via large-scale transcriptome analysis. Interestingly, the two proteins, sxtA4 and sxtG, were similar in sequence and domain structure to those of other toxic dinoflagellates and cyanobacteria; however, their transcript levels were extremely low. Our results suggest that A. affine has the potential to produce STXs, while its toxicity is much lower or negligible, which is unlikely to cause PSP incidents in marine environments.

Increased nitrate concentration differentially affects cell growth and expression of nitrate transporter and other nitrogen-related genes in the harmful dinoflagellate Prorocentrum minimum

The molecular mechanisms through which dinoflagellates adapt to nitrate fluctuations in aquatic environments remain poorly understood. Here, we sequenced the full-length cDNA of a nitrate transporter (NRT) gene from the harmful marine dinoflagellate Prorocentrum minimum Schiller. The cDNA length was 2431 bp. It encoded a 529-amino acid protein, which was phylogenetically clustered with proteins from other dinoflagellates. Nitrate supply promoted cell growth up to a certain concentration (∼1.76 mM) but inhibited it at higher concentrations. Interestingly, at the inhibitory concentrations, nitrite levels in the medium were considerably increased. Nitrate concentration affected the expression of PmNRT, nitrite transporter (PmNiRT), nitrate reductase (PmNR), and nitrite reductase (PmNiR). Specifically, PmNRT was upregulated after 24 h, with ∼6-fold change compared with the control level, in both nitrate-depleted and nitrate-repleted cultures. In addition, PmNR transcript levels increased to the maximum of 4-fold at 48 h but decreased thereafter. In contrast, PmNiR levels remained unchanged in both nitrate-repleted and nitrate-depleted cultures. Therefore, P. minimum likely copes with nitrate fluctuations in its environment by regulating a set of genes responsible for nitrate uptake.

Unveiling the genomic structures and evolutionary events of the saxitoxin biosynthetic gene sxtA in the marine toxic dinoflagellate Alexandrium

Marine dinoflagellates Alexandriumare known to produce saxitoxin (STX) and cause paralytic shellfish poisoning (PSP) which can result in mortality in human. SxtA is considered a core gene for the biosynthesis of STX. However, its gene coding structure and evolutionary history have yet to be fully elucidated. Here, we determined the full-length sequences of sxtA cDNA and genomic coding regions from two toxic dinoflagellates, Alexandrium catenella (LIMS-PS-2645 and LIMS-PS-2647) andA. pacificum (LMBE-C4), characterised their domain structures, and resolved evolutionary events. The sxtA gene was encoded on the genome without introns, and was identical in length (4002 bp) between two A. catenella strains, but their sequences differed from A. pacificum (5031 bp). SxtA consists of four domains, sxtA1, sxtA2, sxtA3, and sxtA4; however, A. pacificum has an extra domain TauD near sxtA1. Each domain had >64.4% GC content, with the highest being 71.6% in sxtA3. Molecular divergence was found to be significantly higher in sxtA4 than in the other domains. Phylogenetic trees of sxtA and separate domains showed that bacteria diverged earliest, followed by non-toxic, toxic cyanobacteria, toxic dinoflagellates. While sxtA domains in Alexandrium were similar to the PKS-like structure with the conserved sxtA1, sxtA2, and sxtA3. PKS_KS may be replaced by sxtA4 in toxic Alexandrium. These suggest that sxtA in Alexandrium may have evolved by acquiring specific domains, whose modification and complexity markedly affect toxin biosynthesis.

Temperature influences the content and biosynthesis gene expression of saxitoxins (STXs) in the toxigenic dinoflagellate Alexandrium pacificum

Temperature may affect the production of saxitoxin (STX) and its derivatives (STXs); however, this is still controversial. Further, STX-biosynthesis gene regulation and the relation of its toxicity with temperature are not clearly understood. In the present study, we evaluated the effects of different temperatures (12 °C, 16 °C, and 20 °C) on the growth, toxin profiles, and expression of two core STX-biosynthesis genes, sxtA and sxtG, in the toxic dinoflagellate Alexandrium pacificum Alex05, isolated from Korean coasts. We found that temperature significantly affected cell growth, with maximum growth recorded at 16 °C, followed by 20 °C and 12 °C. HPLC analysis revealed mostly 12 of STXs from the tested cultures. Interestingly, the contents of STXs increased in the cells cultured at 16 °C and exposed to cold stress, compared to the 20 °C culture and heat stress; however, toxin components were much more diverse under heat stress. These toxin profiles generally matched with the sxtA and sxtG expression levels. Incubation at lower temperatures (12 °C and 16 °C) and exposure to cold stress increased sxtA and sxtG expressions in the cells, whereas heat stress showed little change or downregulated the transcription of both genes. Principal component analysis (PCA) showed low correlation between STXs eq and expressional levels of sxtA and sxtG in heat-stressed cells. These results suggest that temperature might be a crucial factor affecting the level and biosynthesis of STXs in marine toxic dinoflagellates.

Salinity Affects Saxitoxins (STXs) Toxicity in the Dinoflagellate Alexandrium pacificum, with Low Transcription of SXT-Biosynthesis Genes sxtA4 and sxtG

Salinity is an important factor for regulating metabolic processes in aquatic organisms; however, its effects on toxicity and STX biosynthesis gene responses in dinoflagellates require further elucidation. Herein, we evaluated the physiological responses, toxin production, and expression levels of two STX synthesis core genes, sxtA4 and sxtG, in the dinoflagellate Alexandrium pacificum Alex05 under different salinities (20, 25, 30, 35, and 40 psu). Optimal growth was observed at 30 psu (0.12 cell division/d), but cell growth significantly decreased at 20 psu and was irregular at 25 and 40 psu. The cell size increased at lower salinities, with the highest size of 31.5 µm at 20 psu. STXs eq was highest (35.8 fmol/cell) in the exponential phase at 30 psu. GTX4 and C2 were predominant at that time but were replaced by GTX1 and NeoSTX in the stationary phase. However, sxtA4 and sxtG mRNAs were induced, and their patterns were similar in all tested conditions. PCA showed that gene transcriptional levels were not correlated with toxin contents and salinity. These results suggest that A. pacificum may produce the highest amount of toxins at optimal salinity, but sxtA4 and sxtG may be only minimally affected by salinity, even under high salinity stress.

Finding Species-Specific Extracellular Surface-Facing Proteomes in Toxic Dinoflagellates

As a sequel to our previous report of the existence of species-specific protein/peptide expression profiles (PEPs) acquired by mass spectrometry in some dinoflagellates, we established, with the help of a plasma-membrane-impermeable labeling agent, a surface amphiesmal protein extraction method (SAPE) to label and capture species-specific surface proteins (SSSPs) as well as saxitoxins-producing-species-specific surface proteins (Stx-SSPs) that face the extracellular space (i.e., SSSPsEf and Stx-SSPsEf). Five selected toxic dinoflagellates, Alexandrium minutum, A. lusitanicum, A. tamarense, Gymnodinium catenatum, and Karenia mikimotoi, were used in this study. Transcriptomic databases of these five species were also constructed. With the aid of liquid chromatography linked-tandem mass spectrometry (LC-MS/MS) and the transcriptomic databases of these species, extracellularly facing membrane proteomes of the five different species were identified. Within these proteomes, 16 extracellular-facing and functionally significant transport proteins were found. Furthermore, 10 SSSPs and 6 Stx-SSPs were identified as amphiesmal proteins but not facing outward to the extracellular environment. We also found SSSPsEf and Stx-SSPsEf in the proteomes. The potential functional correlation of these proteins towards the production of saxitoxins in dinoflagellates and the degree of species specificity were discussed accordingly.

Low Temperature and Cold Stress Significantly Increase Saxitoxins (STXs) and Expression of STX Biosynthesis Genes sxtA4 and sxtG in the Dinoflagellate Alexandrium catenella

Toxic dinoflagellate Alexandrium spp. produce saxitoxins (STXs), whose biosynthesis pathway is affected by temperature. However, the link between the regulation of the relevant genes and STXs' accumulation and temperature is insufficiently understood. In the present study, we evaluated the effects of temperature on cellular STXs and the expression of two core STX biosynthesis genes (sxtA4 and sxtG) in the toxic dinoflagellate Alexandrium catenella Alex03 isolated from Korean waters. We analyzed the growth rate, toxin profiles, and gene responses in cells exposed to different temperatures, including long-term adaptation (12, 16, and 20 °C) and cold and heat stresses. Temperature significantly affected the growth of A. catenella, with optimal growth (0.49 division/day) at 16 °C and the largest cell size (30.5 µm) at 12 °C. High concentration of STXs eq were detected in cells cultured at 16 °C (86.3 fmol/cell) and exposed to cold stress at 20→12 °C (96.6 fmol/cell) compared to those at 20 °C and exposed to heat stress. Quantitative real-time PCR (qRT-PCR) revealed significant gene expression changes of sxtA4 in cells cultured at 16 °C (1.8-fold) and cold shock at 20→16 °C (9.9-fold). In addition, sxtG was significantly induced in cells exposed to cold shocks (20→16 °C; 19.5-fold) and heat stress (12→20 °C; 25.6-fold). Principal component analysis (PCA) revealed that low temperature (12 and 16 °C) and cold stress were positively related with STXs' production and gene expression levels. These results suggest that temperature may affect the toxicity and regulation of STX biosynthesis genes in dinoflagellates.

The dinoflagellate Alexandrium catenella producing only carbamate toxins may account for the seafood poisonings in Qinhuangdao, China

An outbreak of paralytic shellfish poisoning, recorded in April 2016 in Qinhuangdao China, was suspected to be caused by a toxic species in genus Alexandrium. Shortly after the poisoning outbreak, shellfish and net-concentrated phytoplankton samples were collected from the Bohai Sea, and analysed using high performance liquid chromatography coupled with fluorescence detection. Paralytic shellfish toxins (PSTs) were detected in both phytoplankton and shellfish samples, with similar toxin profiles dominated by carbamate toxins. High throughput sequencing data for phytoplankton samples collected previously in the coastal waters of Qinhuangdao were then analysed, and 8 operational taxonomic units (OTUs) were assigned to Alexandrium affine, A. andersonii/A. ostenfeldii, A. catenella, A. fraterculus, A. hiranoi/A. pseudogonyaulax, A. margalefii, A. pacificum and A. pohangense, among which A. catenella, A. pacificum and A. ostenfeldii could be potential producers of PSTs. During a cruise in 2019, three isolates of Alexandrium were established by cyst germination, and identified as A. catenella based on the sequences of the 28S ribosomal RNA gene (28S rDNA) D1-D2 region. Interestingly, all the three strains had the same toxin profile consisting of gonyautoxins 1, 3, 4 (GTX1, 3, 4) and neosaxitoxin (NEO). The toxin profile is similar to those of phytoplankton samples collected previously in the coastal waters of Qinhuangdao, but remarkably different from the general toxin profile of A. catenella dominated by N-sulfocarbamoyl toxins C1-2 in the Bohai Sea and the Yellow Sea. The results suggest that A. catenella is most likely to be the causative species of the poisoning outbreak in Qinhuangdao. As far as we know, this is the first report of A. catenella in the Bohai Sea producing PSTs dominated by high potent gonyautoxins GTX1-4. Occurrence of the highly toxic A. catenella will increase the risk of paralytic shellfish poisoning, which necessitates in-depth mechanism studies and increasing monitoring efforts.

From the sxtA4 Gene to Saxitoxin Production: What Controls the Variability Among Alexandrium minutum and Alexandrium pacificum Strains?

Paralytic shellfish poisoning (PSP) is a human foodborne syndrome caused by the consumption of shellfish that accumulate paralytic shellfish toxins (PSTs, saxitoxin group). In PST-producing dinoflagellates such as Alexandrium spp., toxin synthesis is encoded in the nuclear genome via a gene cluster (sxt). Toxin production is supposedly associated with the presence of a 4th domain in the sxtA gene (sxtA4), one of the core genes of the PST gene cluster. It is postulated that gene expression in dinoflagellates is partially constitutive, with both transcriptional and post-transcriptional processes potentially co-occurring. Therefore, gene structure and expression mode are two important features to explore in order to fully understand toxin production processes in dinoflagellates. In this study, we determined the intracellular toxin contents of twenty European Alexandrium minutum and Alexandrium pacificum strains that we compared with their genome size and sxtA4 gene copy numbers. We observed a significant correlation between the sxtA4 gene copy number and toxin content, as well as a moderate positive correlation between the sxtA4 gene copy number and genome size. The 18 toxic strains had several sxtA4 gene copies (9-187), whereas only one copy was found in the two observed non-toxin producing strains. Exploration of allelic frequencies and expression of sxtA4 mRNA in 11 A. minutum strains showed both a differential expression and specific allelic forms in the non-toxic strains compared with the toxic ones. Also, the toxic strains exhibited a polymorphic sxtA4 mRNA sequence between strains and between gene copies within strains. Finally, our study supported the hypothesis of a genetic determinism of toxin synthesis (i.e., the existence of several genetic isoforms of the sxtA4 gene and their copy numbers), and was also consistent with the hypothesis that constitutive gene expression and moderation by transcriptional and post-transcriptional regulation mechanisms are the cause of the observed variability in the production of toxins by A. minutum.