2001 Annual Science Report
Marine Biological Laboratory Reporting | JUL 2000 – JUN 2001
Eukaryotic rRNA Evolution: Origins of "Crown Group Taxa"
Origins of Crown Group Taxa (dm)
Previous analyses of small subunit ribosomal RNAs describe the massive evolutionary radiation of eukaryotes that occurred approximately one billion years ago. This event gave rise to plants, animals, fungi, and many other protist groups including stramenopiles, alveotates, and a large number of independent protist lineages. We seek a better understanding of detailed branching pattern for the “crown group” taxa by obtaining additional molecular sequence data and hence greater resolution in our phylogenetic inferences. We are analyzing full length, large subunit rRNA sequences, which have twice as many positions as SSU rRNAs. Our first objective is to study relationships among basal metazoan lineages. After identifying competing hypotheses, we performed maximum likelihood searches for trees that conform to each hypothesis. We employed Kishino-Hasegawa tests to determine if the data (LSU, SSU, and combined) reject any of the competing hypotheses. We also conducted unconstrained tree searches, calculated bootstrap indices, and compared the resulting topologies. We applied Shimodaira-Hasegawa tests to determine if the data reject any of the topologies resulting from the constrained and unconstrained tree searches. LSU, SSU and the combined data strongly contradict two assertions pertaining to sponge phylogeny. Hexactinellid sponges are not likely to be the basal lineage of a monophyletic Porifera or the sister group to all other animals. Instead, Hexactinellida and Demospongia form a well-supported clade of siliceous sponges, Silicea. It remains unclear, based on these data alone, whether the calcarean sponges are more closely related to Silicea or to non-sponge animals. The SSU and combined data reject the hypothesis that Bilateria is more closely related to Ctenophora than it is to Cnidaria. The LSU data alone do not refute either hypothesis, but instead raise the possibility that Bilateria may be sister to Silicea. LSU and SSU data agree in supporting the monophyly of Bilateria, Cnidaria, Ctenophora, and Metazoa. LSU sequence data reveal phylogenetic structure in a data set with limited taxon sampling.
In a related project, we have used similar molecular techniques to explore the phylogeny of the ophistokonths. We have identified the heterotrophic flagellate Ancyromonas sigmoides as a potential common ancestor of fungi, choanoflagellates, and metazoan animals. To re-examine and to substantiate these results, we are reanalyzing the original 18S rRNA dataset and have compiled a comprehensive 28S rRNA dataset, including many new sequences that encompasses all eukaryotic crowngroup lineages that are critical for this analysis. We take multicellularity for granted, but almost the entire tree of life, based on rRNA sequences, is microbial and unicellular; multicellular organisms occupy a few recent branch tips. An over riding question is: What triggered this evolutionary anomaly, multicellularity?
PROJECT MEMBERS:Mitchell Sogin
Linda Amaral Zettler
RELATED OBJECTIVES:Objective 4.0
Expand and interpret the genomic database of a select group of key microorganisms in order to reveal the history and dynamics of evolution.
Describe the sequences of causes and effects associated with the development of Earth's early biosphere and the global environment.
Define how ecophysiological processes structure microbial communities, influence their adaptation and evolution, and affect their detection on other planets.