Abstract
Of the hitherto over 60 taxonomically identified species in the genus of Pseudo-nitzschia, 26 have been confirmed to be toxigenic. Nevertheless, the acquisition and evolution of the toxin biosynthesis (dab) genes by this extensive group of Pseudo-nitzschia species remains unclear. Through constructing chromosome-level genomes of three Pseudo-nitzschia species and draft genomes of ten additional Pseudo-nitzschia species, putative genomic integration sites for the dab genes in Pseudo-nitzschia species were explored. A putative breakpoint was observed in syntenic regions in the dab gene cluster-lacking Pseudo-nitzschia species, suggesting potential independent losses of dab genes. The breakpoints between this pair of conserved genes were also identified in some dab genes-possessing Pseudo-nitzschia species, suggesting that the dab gene clusters transposed to other loci after the initial integration. A "single acquisition, multiple independent losses (SAMIL)" model is proposed to explain the acquisition and evolution of the dab gene cluster in Pseudo-nitzschia species.