Bacterial chromosomes are highly dynamic, continuously changing with respect to gene content and size via a number of processes, including deletions that result in gene loss. How deletions form and at what rates has been the focus of this thesis. In paper II we investigated how chromosomal location affects chromosomal deletion rates in S. typhimurium. Deletion rates varied more than 100-fold between different chromosomal locations and some large deletions significantly increased the exponential growth rate of the cells. Our results suggest that the chromosome is heterogeneous with respect to deletion rates and that deletions may be genetically fixed as a consequence of natural selection rather than by drift or mutational biases. In paper I we examined in a laboratory setting how rapidly reductive evolution, i.e. gene loss, could occur. Using a serial passage approach, we showed that extensive genome reduction potentially could occur on a very short evolutionary time scale. For most deletions we observed little or no homology at the deletion endpoints, indicating that spontaneous deletions often form through a RecA independent process. In paper III we examined further how large spontaneous deletions form and, unexpectedly, showed that 90% of all spontaneous chromosomal deletions required error-prone translesion DNA polymerases for their formation. We propose that the translesion polymerases stimulate deletion formation by allowing extension of misaligned single-strand DNA ends. In paper IV we investigated how the translesion DNA polymerase Pol IV, RpoS and different types of stresses affect mutation rates in bacteria. Derepression of the LexA regulon caused a small to moderate increase in mutation rates that was fully dependent on functional endonucleases but only partly dependent on translesion DNA polymerases. RpoS levels and growth stresses had only minor effects on mutation rates. Thus, mutation rates appear very robust and are only weakly affected by growth conditions and induction of translesion polymerases and RpoS.

Coxiella burnetii est l’agent pathogène responsable de la fièvre Q. Dans le cadre de cette thèse nous nous sommes intéressés à l'étude de souches de C.burnetii responsables d'événements épidémiques. Nous avons séquencé une souche de génotype MST33 (Z3055), proche de la souche responsable de l'épidémie de fièvre Q aux Pays-Bas, et une souche de génotype MST17 (Cb175) clone provoquant l'une des formes les plus virulentes de fièvre Q aiguë jamais décrite auparavant et retrouvée à ce jour uniquement en Guyane Française. Les résultats de ces analyses montrent que le génome de la souche Z3055 était très similaire à celui de la souche de référence Nine Mile I. Les différences observées sont liées à la présence de mutations non synonymes dans le génome de Z3055. Le pourcentage élevé de protéines membranaires mutées pourrait expliquer l’ampleur de cette épidémie en Hollande. En effet, le changement de profil antigénique pourrait être à l’origine de la formation d’un nouveau sérotype capable d'échapper à la réponse immunitaire de l'hôte et de diffuser facilement dans une population au système immunitaire naïf. Nous avons d’ailleurs montré que la souche responsable de la fièvre Q en Guyane (Cb175) présente des différences chromosomiques importantes par rapport à NMI. Ces différences se manifestent principalement par la présence d’une délétion d’une région de 6105pb contenant l’opéron hlyCABD du système de sécrétion de type 1 (T1SS). Ce résultat est cohérent avec ce qui a été observé chez les bactéries épidémiques les plus dangereuses comparées à leurs espèces non-épidémiques plus proches qui ont un génome réduit et contiennent moins de protéines du système de sécrétion. / Coxiella burnetii is a human pathogen that causes the zoonotic disease Q fever. In this work, we focused on the study of strains responsible for epidemic events. Particularly, we sequenced the clone of the strain responsible for Netherlands outbreak having genotype MST33 (Z3055), and strain having MST17 (Cb175) responsible for one of the most severe form of acute Q fever never reported in literature and uniquely described in French Guiana. Our findings showed that the Netherlands outbreak responsible strain (clone Z3055) was highly similar to the reference strain Nine Mile I. Only slight differences were observed, which were related to non-synonymous mutations in Z3055 genome. The high proportion of mutated membrane proteins could explain this large-scale outbreak. Change of antigenic profile may have led to a new serotype, conferring to the novel clone the capacity to escape the host immune response and to disseminate easily in a immunologically naïve population. On the contrary, the type strain responsible for Q fever in Guiana (Cb175) showed an important difference in its chromosome sequence compared to the reference NMI because of the deletion of a sequence of 6105bp containing the Type 1 secretion systems (T1SS) hlyCABD operon. This result appear consistent with previous findings that showed the most dangerous epidemic bacteria compared with their closest non-epidemic species are characterized by reduced genomes accompanied by significant decrease in ORF content and contain less secretion system proteins.

A narrow germplasm base and a complex allotetraploid genome have historically made the discovery of single nucleotide polymorphism (SNP) markers difficult in cotton (Gossypium hirsutum). We conducted a genome reduction experiment to identify SNPs from two accessions of G. hirsutum and two accessions of G. barbadense. Approximately 2 million sequence reads were assembled into contigs with an N50 of 508 bp and analyzed for SNPs. A total of 11,834 and 1,679 SNPs between the accessions G. hirsutum and G. barbadense, respectively, were identified with highly conservative parameters (a minimum read depth of 8x at each SNP and a 100% identity of all reads within an accession at the SNP). Additionally, 4,327 SNPs were identified between accessions of G. hirsutum in and assembly of Expressed Sequence Tags (ESTs). 320 and 252 KASPAR assays were designed for SNP mapping in non-genic and genic regions respectively. 187 markers in total (136 non-genic, 51 genic) were mapped using KBioscience KASPar genotyping assays in a segregating F2 population using the Fluidigm EP1 system. EST The target genome of EST markers was successfully predicted bioinformaticly diploid reference sequences. Examination of nucleotide substitutions and SNP frequencies further confirms validity of new markers. A genetic map was constructed using a large G. hirsutum segregating F2 population. Genetic maps generated by these newly identified markers will be used to locate quantitative, economically important regions within the cotton genome.