| Summary: | Diatoms of the genus <i>Pseudo-nitzschia</i> H.Peragallo are known to produce domoic acid (DA), a toxin involved in amnesic shellfish poisoning (ASP). Strains of the same species are often classified as both toxic and nontoxic, and it is largely unknown whether this difference is also genetic. In the Northern Adriatic Sea, there are virtually no cases of ASP, but DA occasionally occurs in shellfish samples. So far, three species—<i>P. delicatissima</i> (Cleve) Heiden<i>, P. multistriata</i> (H. Takano) H. Takano, and <i>P. calliantha</i> Lundholm, Moestrup, & Hasle—have been identified as producers of DA in the Adriatic Sea. By means of enzme-linked immunosorbent assay (ELISA), high-performance liquid chromatography with UV and visible spectrum detection (HPLC-UV/VIS), and liquid chromatography with tandem mass spectrometry (LC-MS/MS), we reconfirmed the presence of DA in <i>P. multistriata</i> and <i>P. delicatissima</i> and detect for the first time in the Adriatic Sea DA in <i>P. galaxiae</i> Lundholm, & Moestrup. Furthermore, we attempted to answer the question of the distribution of DA production among <i>Pseudo-nitzschia</i> species and strains by sequencing the internal transcribed spacer (ITS) phylogenetic marker and the <i>dabA</i> DA biosynthesis gene and coupling this with toxicity data. Results show that all subclades of the <i>Pseudo-nitzschia</i> genus contain toxic species and that toxicity appears to be strain dependent, often with geographic partitioning. Amplification of <i>dabA</i> was successful only in toxic strains of <i>P. multistriata</i> and the presence of the genetic architecture for DA production in non-toxic strains was thus not confirmed.
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