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Dinophysoids are a group of thecate dinoflagellates with a very distinctive thecal plate arrangement involving a sagittal suture: the so-called dinophysoid tabulation pattern. Although the number and layout of the thecal plates is highly conserved, the morphological diversity within the group is outstandingly high for dinoflagellates. Previous hypotheses about character evolution within dinophysoids based on comparative morphology alone are currently being evaluated by molecular phylogenetic studies. Sinophysis is especially significant within the context of these hypotheses because several features within this genus approximate the inferred ancestral states for dinophysoids as a whole, such as a (benthic) sand-dwelling lifestyle, a relatively streamlined theca and a heterotrophic mode of nutrition. We generated and analysed small subunit (SSU) rDNA sequences for five different species of Sinophysis, including the type species (S. ebriola, S. stenosoma, S. grandis, S. verruculosa and S. microcephala). We also generated SSU rDNA sequences from the planktonic dinophysoid Oxyphysis (O. oxytoxoides). Temperate and tropical species as well as the complete spectrum of thecal ornamentation within Sinophysis was addressed in our study. The sequences from the Sinophysis species formed a robust monophyletic group that was the sister to a robust clade consisting of all other dinophysoid genera, including Oxyphysis, in some analyses. Although the sister relationship received weak statistical support, this tree topology was consistent with inferences based on comparative morphology.

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