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GENE DUPLICATIONS AND HORIZONTAL GENE TRANSFER DURING EARLY EVOLUTION

J. Peter Gogarten, Elena Hilario and Lorraine Olendzenski Department of Molecular and Cell Biology, University of Connecticut,

75 North Eagleville Rd, Storrs, CT 06269-3044, USA, FAX:USA (203) 486-1784, Gogarten@UConnVM.UConn.edu.

The evolutionary history of organisms can be reconstructed using various information sources: the fossil and geological records, the comparative analysis of biochemical pathways, and the reconstruction of molecular phylogenies from macromolecules found in extant organisms. In the absence of a complete archaean microfossil record and unique morphological characters for most microbial groups, molecular markers have been used to unravel the relationships between the major groups ofprokaryotes. The best studied of these molecules is undoubtedly the small subunit ribosomal RNA, but many other macromolecules, especially protein sequences have been useful in studying the relationships among prokaryotic kingdoms and domains. However, at best, the analysis of extant macromolecules can only yield information on tile phylogeny of the molecules under study. Even if molecular phylogenies could be resolved without ambiguity, the step from molecular phylogeny to species phylogeny would remain complicated because genetic information has also been transferred horizontally between independent evolutionary lineages. It is therefore not surprising that comparative analysis of different molecular phylogenies reveals a net-like structure of the species phylogeny (Gogarten, 1995; Hiliario & Gogarten, 1993).

Differences between well resolved molecular phylogenies can be due either to unrecognized gene duplications or to horizontal gene transfer. Two examples that conflict with well documented phylogenetic relationships are the presence of archaeal type H+-ATPases in Thermus and Enterococcus and the close association between Gram positive eubacteria and archaea as supported by many different genes, including heat shock proteins (HSP70), ghitamine synthetases and glutamate dehydrogenases. In both cases the best explanation for these conflicts is horizontal gene transfer. Assuming unrecognized paralogies (duplicated genes) as an explanation would necessitate many instances of convergent evolution; furthermore this assumption would also result in species phylogenies that are at odds with two distinct prokaryotic domains (archaea and eubacteria). The large number of characters that reflect the close association between archaea and eubacteria suggest that a substantial portion of the eubacterial genome participated in this transfer.

Horizontal gene transfer as a possible evolutionary mechanism gives as a result net-like species phylogenies that complicate inferring the properties of the last common ancestor. Even so, the data strongly indicate that the last common ancestor was a cellular organism, with a DNA based genome, and a sophisticated transcription and translation machinery. Furthermore, the analysis of ATPase structure function relationships and the evolution of cytochrome oxidase homologues suggest that the last

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common ancestor already used chemiosmotic coupling to synthesize ATP, and that electron transport chains could be used to energize the plasma membrane.

This presentation will focus on the inference of species phylogeny from sometimes conflicting molecular phylogenies and the properties of the last common ancestor incorporating likely cases of horizontal gene transfer and lineage fusion.

Gogarten, J.P. : 1995, Trends in Ecology and Evolution 10, 147. Hilario, E. and Gogarten, J.P. : 1993, BioSystems 31, 111.

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