Genomes beyond compare

摘要

Comparisons between related pathogens with different host ranges could reveal potential mechanisms of host interaction and pathogen evolution. This is particularly true for comparisons between generalist (more than one host) and specialist (restricted to one host) pathogens. An excellent example of this has been made possible by publication of the genomes of the vector-borne mammalian pathogens Bartonella quintana and Bartonella henselae. B. quintana, which is the causative agent of trench fever, is transmitted by the human body louse and is restricted to the human host. B. henselae, which is the causative agent of cat-scratch disease in humans, infects felines asymptomatically and is transmitted by the cat flea. Both genomes have evidence of gene loss and decay, but this process is more advanced in B. quintana, the specialist, compared with B. henselae, the generalist, and is reflected in the smaller genome of B. quintana. It is likely that the ancestor of both organisms acquired genes involved in host interaction by horizontal transfer, but that more of these have been lost in B. quintana. Overall, the genome of B. quintana is a subset of the genome of B. henselae, with few unique genes. This supports the hypothesis that host-restricted pathogens evolve from broad-host range ancestors by gene loss. Curiously, comparisons with genomes from the related genus Brucella, which have two chromosomes, identified a region that is present in both of the Bartonella genomes and which contains many Bartonella-unique genes and genes shared with chromosomeⅡ from Brucella. This might indicate that the Bartonella ancestor had two chromomes, and that a section of chromosome Ⅱ was integrated into chromosome Ⅰ before the loss of chromosomeⅡ.