Genetic diversity and population structure of <i>Karenia selliformis</i> (Dinophyceae): a global approach based on the 28S gene

Karenia selliformis is a toxin-producing athecate dinoflagellate found worldwide. This species can form extensive harmful algal blooms (HABs) that have been associated with the mass mortality of marine fauna. On 24 August 2020, a dinoflagellate bloom was recorded in Bahía Todos Santos (BTS), Baja California, Mexico. During this high-biomass phytoplankton event, in which K. selliformis was detected and confirmed morphologically and by molecular methods, temperature 23.68°C, chlorophyll a > 10 mg m–3, and cell densities of 200–900 cells l–1 were recorded. A total of 44 sequences (620 bp) of the ribosomal large subunit (LSU) and 37 sequences (570 bp) of the internal transcribed spacer (ITS1 + 5.8S + ITS2) region of rDNA, obtained from GenBank and generated in this study, were analysed. In total, 15 sites of the LSU were informative while 605 sites were invariant. Fifteen polymorphisms and eight ribotypes were identified, with a haplotypic diversity (Hd) of 0.647 and a nucleotide diversity (π) of 0.00558. In addition, 21 sites of the ITS were informative while 549 sites were invariant. For these 21 polymorphic sites, five ribotypes were identified, with an Hd value of 0.611 and π value of 0.01330. Our results demonstrate distinct ribotypes within K. selliformis and provide molecular evidence for a cryptic species complex across biogeographic provinces. This study highlights the need to reassess species boundaries within Karenia using integrative approaches.