In silico Studies of Early Eukaryotic Evolution

University dissertation from Uppsala : Acta Universitatis Upsaliensis

Abstract: A question of great interest in evolutionary biology is how and why the eukaryotic cell evolved. Several hypotheses have been proposed, ranging from an early emergence of a primitive eukaryotic cell, to various fusion models like the hydrogen hypothesis. Within this context, relevant bacterial gene families and genomes are examined in this thesis.The mitochondrion, the energy producing organelle in the eukaryotic cell, is generally believed to be of α-proteobacterial descent. To learn more about mitochondrial evolution, and therefore eukaryotic evolution, the genomes of the α-proteobacteria Bartonella henselae and Bartonella quintana were sequenced. Software was developed and used in the annotation of these genomes.Several gene products of nuclear-encoded genes are exported to the mitochondrion. Many of these genes are thought to originate from the emerging organelle. An analysis of the more than 400 genes encoding proteins targeted to the yeast mitochondrion indicates that one set of genes originated from the bacterial symbiont, while the eukaryotic host contributed another. Thus, the mitochondrial proteome has a dual origin.The hydrogen hypothesis postulates that the glycolytic genes belong to the group of genes that were transferred from symbiont to host. These genes are thoroughly analysed from a phylogenetic perspective. Contrary to the predictions of the hydrogen hypothesis, the results provide no support for a close relationship between nuclear genes encoding glycolytic enzymes and their α-proteobacterial homologs. In general, it is thought that intensive gene transfers may limit our ability to reconstruct gene and species evolution, especially among microbes. A phylogenetic analysis of a large cohort of genes from the AT-rich genome of the γ-proteobacterium Buchnera aphidicola (Sg) resulted in a high fraction of atypical tree topologies, previously interpreted as horizontal gene transfers. By applying methods that accommodate for asymmetric nucleotide substitutions, it is shown that many well-supported gene topologies are drastically altered, so that they now agree with the rRNA topology. The conclusion is that atypical topologies may not necessarily be evidence for horizontal gene transfers.

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