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Lateral gene transfer from bacteria to protists de Koning, Audrey Patricia

Abstract

The advent of genome-level sequencing has brought a wealth of genetic information that can be used to trace the evolution of organisms. Phylogenies based on homologous proteins often do not agree on implied species relationships, indicating that the evolutionary path taken by some of these genes differs from the evolution of the organisms. One explanation for this observed pattern is lateral gene transfer, the movement of genes across species boundaries. There are mechanisms to facilitate such movement in prokaryotic organisms. In eukaryotes, it is less clear what mechanisms might facilitate incorporation of foreign genes into a species' genome. Multicellular eukaryotes are presumably more resistant to lateral gene transfer, because the majority of their cells do not have access to genes from other organisms. Moreover, germ and somatic cell lineages are often separate, which greatly reduces the probability that any gene that gains access to a foreign cell will be heritable and subsequently become part of its host species' genome. Phagotrophic protists, however, represent one type of eukaryote where these restrictions do not exist. Such organisms commonly ingest bacteria as food, and chance events may lead to the incorporation and use of bacterial genes. Lateral gene transfer from organelles of symbiotic origin (the chloroplast and mitochondrion) to the nucleus of the host organism represents a specialized case of lateral gene transfer resulting from phagotrophy. Lateral gene transfer in eukaryotes, therefore, is expected to involve bacterial genes incorporated into the genomes of phagocytic species (or those that have evolved from them). Accordingly, genes of bacterial origin were sought among the genetic information available from protists. Over 2000 predicted genes from the genome-sequencing project for the malarial parasite, Plasmodium falciparum, were screened to find those that are highly similar to genes from bacterial species. Phylogenetic reconstruction methods were used to elucidate the evolution of 'bacteria-like' genes. Thirteen laterally transferred genes were identified on the basis of protein phylogeny, of which nine are probably transferred from symbiotic organelles and three appear to involve transfers with bacteria. The conclusion is that genes introduced to the genome through lateral gene transfer are not nearly as prevalent in this eukaryote as they are in bacteria.

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