| Abstract |
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, alpha-tubulin and beta-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.
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