| Summary: | 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. === Medicine, Faculty of === Medical Genetics, Department of === Graduate
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