<p>Species of the genus <i>Rickettsia</i> are obligate intracellular parasites of the a-proteobacterial subdivision. It has been suggested that obligate intracellular bacteria have evolved from free-living bacteria with much larger genome sizes. Transitions to intracellular growth habitats are normally associated with radical genomic alterations, particularly genome rearrangements and gene losses.</p><p>This thesis presents a comparative study of evolutionary processes such as gene rearrangements, deletions and duplications in a variety of <i>Rickettsia</i> species. The results show that early intrachromosomal recombination events mediated by duplicated genes and short repeats have resulted in deletions as well as rearrangements. For example, an exceptional organization of the elongation factor genes was found in all species examined, suggesting that this rearrangement event occurred at the early stage of the evolution of <i>Rickettsia</i>. Likewise, it was found that a repetitive element, the so-called <i>Rickettsia</i> Palindromic Element (RPE) flourished prior to species divergence in <i>Rickettsia</i>. Finally, a phylogenetic analysis shows that the duplication events that gave rise to the five genes encoding ATP/ADP transporters occurred long before the divergence of the two major groups of <i>Rickettsia</i>. Taken together, this suggests that <i>Rickettsia</i> have been intracellular parasites for an extensive period of time.</p><p>A detailed analysis of the patterns of nucleotide changes in genes and intergenic regions among the different species provides evidence for a gradual accumulation of short deletions. This suggests that different distributions of genes and repeated sequences in modern <i>Rickettsia</i> species reflect species-specific differences in rates of deterioration rather than variation in rates of intra-genomic sequence proliferation. </p>
| Identifer | oai:union.ndltd.org:UPSALLA/oai:DiVA.org:uu-1789 |
| Date | January 2002 |
| Creators | Amiri, Haleh |
| Publisher | Uppsala University, Molecular Evolution, Uppsala : Acta Universitatis Upsaliensis |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Doctoral thesis, comprehensive summary, text |
| Relation | Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, 1104-232X ; 689 |
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