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2003 Annual Science Report

Marine Biological Laboratory Reporting  |  JUL 2002 – JUN 2003

Eukaryotic rRNA Evolution: Origins of "Crown Group Taxa"

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Major shifts in Earth’s environment including the increase of oxygen set the stage for evolution of increased biological complexity in the form of multicellular organisms. Analyses of ribosomal ribonucleic acid (rRNA) genes, and more recently other protein families, demonstrate a massive radiation in the eukaryotic line of descent that gave rise to plants, animals, fungi, numerous other independent protist lineages and several complex protist assemblages including Alveolates, (ciliates, dinoflagellates and sporozoans), and Stramenopiles (diverse heterotrophic and phototrophic protists, e.g. diatoms, various chromophytes, slime nets, small flagellates etc.). This sudden radiation obscures the identification of protists that gave rise to animals and the potential common evolutionary history of chlorophytes and other photosynthetic groups such as red algae. We seek to understand relative branching orders for major groups that emerged from this massive radiation and to infer what kind of environmental changes on a planetary scale might have stimulated this massive diversification. To address this issue we have focused our attention on the analyses of both small and large rRNA genes from key taxa. Earlier studies from our laboratory demonstrated that the animals share a common and unique evolutionary history with fungi. Together with a several other protist groups, the animals and fungi now comprise a new complex assemblage that we describe as opisthokont protists.

The origins of multi-cellularity require evolutionary transitions towards increased levels of complexity. We have generated new sequence data from the nuclear large subunit ribosomal deoxyribonucleic acid (DNA) gene (LSU rDNA) and the small subunit (SSU) rDNA gene of several unicellular opisthokont protists – a nucleariid amoeba (Nuclearia simplex), and four choanoflagellates (Codosiga gracilis, Choanoeca perplexa, Proterospongia choanojuncta and Stephanoeca diplocostata). These data provided a basis for re-examining relationships among several unicellular lineages and their multicellular relatives (animals and fungi). Our data indicate that 1) choanoflagellates are monophyletic rather than a paraphyletic assemblage that independently gave rise to animals and fungi as suggested by some authors, and 2) the nucleariid filose amoebae are the likely sister group to Fungi.

We have also been exploring evolutionary relationships within two other crown-groups. The first is a complex assemblage called the Gymnamoebia. Some investigators have suggested that amoeboid protists within the Gymnamoebia are candidates for early diverging lineages. Our phylogenetic analysis clearly demonstrates they are not monophyletic but instead represent distinct clades within a complex assemblage of amoeboid protists that diverged nearly concurrently with plants, animals, fungi, alveolates and stramenopiles.