A new evolutionary scenario for the Mycobacterium tuberculosis complex

Abstract

The distribution of 20 variable regions resulting from insertion-deletion events in the genomes of the tubercle bacilli has been evaluated in a total of 100 strains of Mycobacterium tuberculosis, Mycobacterium africanum, Mycobacterium canettii, Mycobacterium microti, and Mycobacterium bovis. This approach showed that the majority of these polymorphisms did not occur independently in the different strains of the M. tuberculosis complex but, rather, resulted from ancient, irreversible genetic events in common progenitor strains. Based on the presence or absence of an M. tuberculosis specific deletion (TbD1), M. tuberculosis strains can be divided into ancestral and "modern" strains, the latter comprising representatives of major epidemics like the Beijing, Haarlem, and African M. tuberculosis clusters. Furthermore, successive loss of DNA, reflected by region of difference 9 and other subsequent deletions, was identified for an evolutionary lineage represented by M. africanum, M. microti, and M. bovis that diverged from the progenitor of the present M. tuberculosis strains before TbD1 occurred. These findings contradict the often-presented hypothesis that M. tuberculosis, the etiological agent of human tuberculosis evolved from M. bovis, the agent of bovine disease. M. canettii and ancestral M. tuberculosis strains lack none of these deleted regions, and, therefore, seem to be direct descendants of tubercle bacilli that existed before the M. africanum-->M. bovis lineage separated from the M. tuberculosis lineage. This observation suggests that the common ancestor of the tubercle bacilli resembled M. tuberculosis or M. canettii and could well have been a human pathogen already.

Figures

Figure 1

(A) Amplicons obtained from strains that have the indicated genomic region present or deleted. Sizes of amplicons in each group are uniform. Numbers correspond to strain designation used in refs. and . (B) Sequences in the TbD1 region obtained from strains of various geographic regions. Numbers correspond to strain designation used in refs. and . (C) Spoligotypes of selected M. tuberculosis and M. bovis strains. Numbers correspond to strain designation used in refs. and . *, groups based on katGc463/gyrAc95 sequence polymorphism defined by Sreevatsan et al. (2).

Figure 2

Scheme of the proposed evolutionary pathway of the tubercle bacilli illustrating successive loss of DNA in certain lineages (gray boxes). The scheme is based on the presence or absence of conserved deleted regions and on sequence polymorphisms in five selected genes. Note that the distances between certain branches may not correspond to actual phylogenetic differences calculated by other methods. Blue arrows indicate that strains are characterized by katG463. CTG (Leu), gyrA95 ACC (Thr), typical for group 1 organisms. Green arrows indicate that strains belong to group 2 characterized by katG463 CGG (Arg), gyrA95 ACC (Thr). The red arrow indicates that strains belong to group 3, characterized by katG463 CGG (Arg), gyrA95 AGC (Ser), as defined by Sreevatsan et al. (2).