In many human infections, hosts and pathogens coexist for years or decades. Important examples include HIV, herpes viruses, tuberculosis, leprosy, and malaria. With the exception of intensively studied viral infections such as HIV͞AIDs, little is known about the extent to which the clonal expansion that occurs during long-term infection by pathogens involves important genetic adaptations. We report here a detailed, whole-genome analysis of one such infection, that of a cystic fibrosis (CF) patient by the opportunistic bacterial pathogen Pseudomonas aeruginosa. The bacteria underwent numerous genetic adaptations during 8 years of infection, as evidenced by a positive-selection signal across the genome and an overwhelming signal in specific genes, several of which are mutated during the course of most CF infections. Of particular interest is our finding that virulence factors that are required for the initiation of acute infections are often selected against during chronic infections. It is apparent that the genotypes of the P. aeruginosa strains present in advanced CF infections differ systematically from those of ''wild-type'' P. aeruginosa and that these differences may offer new opportunities for treatment of this chronic disease.chronic infection ͉ positive selection ͉ virulence ͉ antibiotic resistance M ost cystic fibrosis (CF) patients acquire chronic Pseudomonas aeruginosa infections by their teenage years, if not earlier, and these respiratory infections are responsible for much of the morbidity and mortality caused by CF (1, 2). It has been established that most of these infections are clonal (3), and even among groups of CF patients treated in specific clinics the infections are acquired independently, presumably from diverse environmental reservoirs (4). Previous studies, particularly of the O-antigen biosynthetic locus and the transcriptional regulator mucA, indicate that some P. aeruginosa genes commonly incur loss-of-function mutations as the infections progress (5-7). Mutator phenotypes also arise frequently (8).The overall picture is reminiscent of typical cancers: a clone of cells, albeit in this instance one of exogenous origin, experiences selection for an accumulation of genetic variants that promote long-term survival and clonal expansion. Our data validate this model for P. aeruginosa infections in CF and provide strong evidence for the role of selection in shaping the genotypes of the bacteria that are present during the late, life-threatening phase of the infections. Our data also focus attention on particular aspects of P. aeruginosa metabolism that are premier targets of selection, both in the patient we studied in most detail and in other, independently evolving, P. aeruginosa infections in additional CF patients.

Pathogenicity in Francisella tularensis subspecies

.Sequencing of the non-pathogenic Francisella tularensis sub-species novicida U112, and comparison with two pathogenic sub-species, provides insights into the evolution of pathogenicity in these species.

The reference sequence for each human chromosome provides the framework for understanding genome function, variation and evolution. Here we report the finished sequence and biological annotation of human chromosome 1. Chromosome 1 is gene-dense, with 3,141 genes and 991 pseudogenes, and many coding sequences overlap. Rearrangements and mutations of chromosome 1 are prevalent in cancer and many other diseases. Patterns of sequence variation reveal signals of recent selection in specific genes that may contribute to human fitness, and also in regions where no function is evident. Fine-scale recombination occurs in hotspots of varying intensity along the sequence, and is enriched near genes. These and other studies of human biology and disease encoded within chromosome 1 are made possible with the highly accurate annotated sequence, as part of the completed set of chromosome sequences that comprise the reference human genome.

Francisella tularensis is a bacterial pathogen that causes the zoonotic disease tularemia and is important to biodefense. Currently, the only vaccine known to confer protection against tularemia is a specific live vaccine strain (designated LVS) derived from a virulent isolate of Francisella tularensis subsp. holarctica. The origin and source of attenuation of this strain are not known. To assist with the design of a defined live vaccine strain, we sought to determine the genetic basis of the attenuation of LVS. This analysis relied primarily on the comparison between the genome of LVS and Francisella tularensis holarctica strain FSC200, which differ by only 0.08% of their nucleotide sequences. Under the assumption that the attenuation was due to a loss of function(s), only coding regions were examined in this comparison. To complement this analysis, the coding regions of two slightly more distantly related Francisella tularensis strains were also compared against the LVS coding regions. Thirty-five genes show unique sequence variations predicted to alter the protein sequence in LVS compared to the other Francisella tularensis strains. Due to these polymorphisms, the functions of 15 of these genes are very likely lost or impaired. Seven of these genes were demonstrated to be under stronger selective constraints, suggesting that they are the most probable to be the source of LVS attenuation and useful for a newly defined vaccine.

There has been much speculation as to what role balancing selection has played in evolution. In an attempt to identify regions, such as HLA, at which polymorphism has been maintained in the human population for millions of years, we scanned the human genome for regions of high SNP density. We found 16 regions that, outside of HLA and ABO, are the most highly polymorphic regions yet described; however, evidence for balancing selection at these sites is notably lacking-indeed, whole-genome simulations indicate that our findings are expected under neutrality. We propose that (i) because it is rarely stable, longterm balancing selection is an evolutionary oddity, and (ii) when a balanced polymorphism is ancient in origin, the requirements for detection by means of SNP data alone will rarely be met.