The evolution of Mycobacterium bovis and the badger culling controversy
13,000 years ago
The 'Animal' Lineage
The second lineage is thought to be the source of animal tuberculosis. The transition from human to animal hosts probably coincided with the domestication of animals around 13,000 years ago. The major bottleneck that caused the ‘animal’ lineage to split from the ‘human’ lineage involved the deletion of a region of DNA known as RD9. Following deletions led to the loss of subsequent RD regions. This successive loss of DNA is believed to have led to the progressive expansion of this lineage with the RD9-deleted bacteria spreading and adapting to different hosts. Today this lineage of bacteria displays diverse host preferences with M. africanum in humans; M. microti in rodents; M. pinnipedii in seals; M. caprae in goats and of course M. bovis in cattle.
What happened next?
A Dilemma
The MTBC consists of 7 species and subspecies which in terms of their DNA sequences are 99.9% identical and are referred to as a ‘clonal’ group. However these species and subspecies display differing characteristics to one another such as host preferences. What events allowed this great variation in observable characteristics between the MTBC species whilst retaining high genetic similarity?
20,000-40,000 years ago
The Answer
One answer is population bottlenecks. As discussed previously population bottlenecks cause the fixation of a certain trait whilst another is deleted. It is possible that the trait that becomes fixed confers a selective advantage to the bacteria for example the fixed trait may aid the bacteria in overcoming its hosts defences. One such bottleneck is thought to have occurred around 20,000 to 40,000 years ago. This bottleneck is believed to have involved the deletion of a region of DNA known as RD9 and caused the ancestral MTBC lineage to split into two: a ‘human’ lineage that retained the RD9 region and an ‘animal’ lineage that did not.
A reminder of the effects of a population bottleneck:
