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Role of Lateral Gene Transfer in Protozoan Genome Plasticity

The adaptation of pathogenic protozoans to specific parasitic niches required the emergence, or "invention," of novel metabolic pathways and extracellular proteins. In turn, adaptation to the host environment affords the opportunity to eliminate select pathways rendered obsolete via metabolic scavenging from the host, thereby creating a highly evolved "streamlined" parasite proteosome. Acquisition of new metabolic pathways might have occurred following endosymbiotic events and transfer of select genes to the nuclear genome, as evidenced by the many examples of bacterial-like genes in protozoan genomes [20, 24, 25] and the shuttling of organelle-encoded genes to the nuclear genome, as is known to have occurred extensively in mitochondrial, chloroplast, and apicoplast genomes [20, 26]. For example, the bacterial component of apicomplexans might be largely attributable to the cyanobacterial endosymbiont component [27-31]. In addition, parasites have probably captured discrete genes via multiple lateral transfer events, perhaps by "sampling" of foreign DNA within their intracellular parasitic niche [32]. Although bacteria and archaea were relatively liberal in exchanging genetic information, in the eukaryotic lineages the extent of isolated lateral gene transfer as against gene acquisition via endosymbiotic events is unknown. Evidence suggests that within extracellular proteins the apicomplexans have acquired animal- and bacterial-like domains via lateral transfer [20], and to a much greater extent than the kinetoplastids and Giardia (Fig. 19.2) - a hypothesis that can be tested via whole-genome annotations and phylogenetic analyses. As might be anticipated, the most highly evolving functional protein class is composed of extracellular receptors mediating adhesion, recognition, and response to host cells and tissues, and protection from the environment [20]. The catalogs of surface proteins have been largely invented in a lineage-specific fashion; that is, it is unlikely that a kinetoplastid such as Trypanosoma will possess more than just a few surface proteins having a common vertical ancestor relationship with the apicom-plexan Plasmodium, and likewise Plasmodium and Cryptosporidium share few surface proteins (Fig. 19.1; see Section 19.4). It is hoped that the role of isolated lateral transfer events, as opposed to bulk acquisition of new genes via endosymbio-tic events, will be better understood through whole-genome sequence annotations and comparative genomics. Regardless of the origin of captured genes, via these mechanisms parasites probably acquire new metabolic capacities conferring specific adaptations.

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