Wednesday October 4, 2023, 5pm CEST, GMT+2

Mariana S. Pandeirada ^{a, b*}, Sandra C. Craveiro ^{a, b}, Niels Daugbjerg ^c, Øjvind Moestrup ^c, António J. Calado ^{a, b} 

Unveiling character evolution in peridinioid dinoflagellates: clarifying phylogeny towards a stable classification

^a Department of Biology, University of Aveiro, P-3810-193 Aveiro, Portugal
^b GeoBioTec Research Unit, University of Aveiro, P-3810-193 Aveiro, Portugal
^c Marine Biological Section, Department of Biology, University of Copenhagen, Universitetsparken 4, DK-2100 Copenhagen Ø, Denmark
^*mpandeirada@ua.pt

Combination of detailed cell ultrastructure with DNA-based phylogenies has led to major changes to dinoflagellate classification over the past two decades. A series of descriptions and redefinitions of genera and families has resulted, both in athecate and thecate forms. In the thecate Peridiniales, or, in a more general sense, peridinioids, the description of new genera and the new family Peridiniopsidaceae has resulted from phylogenetic analyses using the several parts of the ribosomal operon, either concatenated or in isolation, and analyses of ultrastructural features of freshwater species previously placed in the genera Peridinium and Peridiniopsis. Our current understanding of the family Peridiniopsidaceae highlighted the unreliability of features associated with plate arrangements of the theca, even those traditionally viewed as secure markers of close relatedness between species, such as the presence or absence of an apical pore complex (apc), or the number of intercalary plates on the epicone. The family name is based on the generic name Peridiniopsis, which traditionally included species with an apc and 0-1 intercalary plates. Also included in the family are the genera Palatinus and Parvodinium, both with species common in fresh water, and later also the marine Johsia and, most recently, Chiharadinium. The group thus circumscribed now exhibits a surprising array of combinations of these features: apc absent and two intercalary plates (Palatinus); apc present and two intercalary plates (the group of species originally included in Parvodinium sensu stricto and Johsia); and apc present and three intercalary plates (Chiharadinium). In addition, Parvodinium has been shown to include also species with zero or one intercalary plates only.
In view of the inadequacy of traditional tabulation markers, we have looked at the cell organization at the ultrastructural level to try to make sense of the morphological basis of the affinity revealed by DNA sequences of members of the Peridiniopsidaceae.