Juan F. Saldarriaga

We investigate an organism that closely resembles the nonphotosynthetic dinoflagellate “Gymnodinium elongatum” Hope 1954 using EM and molecular methods. Cells are 20–35 μm long, 10 μm wide, biconical, transparent, and have a faint broad girdle. Thecal plates are thin but present (plate formula Po Pi CP 3′ 1–2A 5″ 3C 6S 4‴ 3″″). With the exception of one feature, the presence of three antapical plates, the amphiesmal arrangement of this species is consistent with that of the order Peridiniales, family Podolampaceae; it is not at all consistent with the characteristics of the genus Gymnodinium. On the basis of these ultrastructural findings, we establish a new genus, Lessardia, and a new species, Lessardia elongata Saldarriaga et Taylor. Molecular phylogenetic analyses were performed using the small subunit rRNA genes of L. elongata as well as Roscoffia capitata, a member of a genus of uncertain systematic position that has been postulated to be related to the Podolampaceae. These analyses place Lessardia and Roscoffia as sister lineages within the so‐called GPP complex. Thecal plate arrangements led us to expand the family Podolampaceae to include the genus Lessardia and, in combination with new molecular results, to propose a close relationship between the Podolampaceae and Roscoffia. Within this lineage, Lessardia and Roscoffia appear to have retained a number of ancestral characters: Roscoffia still has a well‐developed cingulum, a feature absent in all members of the Podolampaceae, and Lessardia has more than one antapical plate, a character reminiscent of some members of the family Protoperidiniaceae.

Oxyrrhis marina and Perkinsus marinus are two alveolate species of key taxonomic position with respect to the divergence of apicomplexans and dinoflagellates. New sequences from Oxyrrhis, Perkinsus and a number of dinoflagellates were added to datasets of small-subunit (SSU) rRNA, actin, α-tubulin and β-tubulin sequences, as well as to a combined dataset of all three protein-coding genes, and phylogenetic trees were inferred. The parasitic Perkinsus marinus branches at the base of the dinoflagellate clade with high support in most of the individual gene trees and in the combined analysis, strongly confirming the position originally suggested in previous SSU rRNA and actin phylogenies. The SSU rRNA from Oxyrrhis marina is extremely divergent, and it typically branches with members of the Gonyaulacales, a dinoflagellate order where SSU rRNA sequences are also divergent. Conversely, none of the three protein-coding genes of Oxyrrhis is noticeably divergent and, in trees based on all three proteins individually and in combination, Oxyrrhis branches at the base of the dinoflagellate clade, typically with high bootstrap support. In some trees, Oxyrrhis and Perkinsus are sisters, but most analyses indicate that Perkinsus diverged prior to Oxyrrhis. Morphological characters have previously pointed to Oxyrrhis as an early branch in the dinoflagellate lineage; our data support this suggestion and significantly bolster the molecular data that support a relationship between Perkinsus and dinoflagellates. Together, these two organisms can be instrumental in reconstructing the early evolution of dinoflagellates and apicomplexans by helping to reveal aspects of the ancestors of both groups.

Dinoflagellates are a trophically diverse group of protists with photosynthetic and non-photosynthetic members that appears to incorporate and lose endosymbionts relatively easily. To trace the gain and loss of plastids in dinoflagellates, we have sequenced the nuclear small subunit rRNA gene of 28 photosynthetic and four non-photosynthetic species, and produced phylogenetic trees with a total of 81 dinoflagellate sequences. Patterns of plastid gain, loss, and replacement were plotted onto this phylogeny. With the exception of the apparently early-diverging Syndiniales and Noctilucales, all non-photosynthetic dinoflagellates are very likely to have had photosynthetic ancestors with peridinin-containing plastids. The same is true for all dinoflagellates with plastids other than the peridinin-containing plastid: their ancestors have replaced one type of plastid for another, in some cases most likely through a non-photosynthetic intermediate. Eight independent instances of plastid loss and three of replacement can be inferred from existing data, but as more non-photosynthetic lineages are characterized these numbers will surely grow.