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Project Progress

We have started a new initiative to study key steps in the evolution of complex life and the potential relationship to aqueous conditions (oxygen, ion concentrations, etc.) at the time of those events. We are studying two “model” systems.

The first are placazoans: Placozoa remains one of the most enigmatic and poorly described groups of the Metazoa (animals). The only officially described species, Trichoplax adhaerans, has been assigned its own phylum based on its unique characters. These mm-sized organisms are found in the littoral zone of tropical seas, where they are part of the biofouling community. Although placozoans were first described more than a century ago they were subsequently thought to be a derived cnidarian larva. Most of what is known about placazoans is based on observations of a single strain (Grell) from the Red Sea. They possess a radial symmetry, with upper and lower epithelium cell layers separated by a fiber layer responsible for changes in shape. They possess no nerve cells, yet immuno reaction to neuropeptide antibodies has been reported. In the laboratory, they feed upon the biofilm and can be readily cultured by feeding with Cryptomonad species. In culture, placazoans readily reproduce by fission and budding but it is not known if the organism possesses a complete sexual life cycle: Oocytes have been observed but cell division has never been observed to pass the first 5 divisions. The phylogenetic position of the Placozoa with respect to the Bilateria and the other basal metazoan phyla (Ctenophora, Cnidaria, Porifera) has been controversial. It was earlier suspected that placazoans might be derived cnidarians, but subsequent information does not support this. We are currently carrying out several parallel investigations on these organisms, including (1) constructing and sequencing a cDNA expressed gene library; (2) using PCR and FISH techniques to identify and describe putative endosymbiotic bacteria that have been reported within the placazoans; and (3) measuring and constructing a mathematical model of the motility of these organisms. We are particularly interested in using Ca and Ca-dependent signaling and activity as a probe to understand the interaction and dynamics of the cells in the multicellular system.

The second system is a non-photosynthetic ciliate (Tetrahymena thermophila) and cyanobacterium (Synechocystis PCC6803). We are studying mechanisms by which feeding by the ciliate on the cyanobacterium might be replaced by a “détente” in which the cyanobacterium avoids digestion and provides oxygen to the ciliate for growth advantage during hypoxic conditions. This is one mechanism by which endosymbiotic relationships — a major step in the evolution of complex eukaryotic life — might have emerged over evolutionary time. Both organisms have had their compete genomes sequenced and have genetic systems already in place, and thus are very tractable. Our preliminary data shows that flow cytometric selection of candidate ciliate-cyano individuals is feasible.

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