The evolution of Mycobacterium bovis and the badger culling controversy
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:
Deletion of TbD1
The 'Human' Lineage
One lineage, consisting of M. tuberculosis, spread amongst humans and co-migrated with this host around the globe. By analysing differences between 100 different representative strains of the MTBC scientists found evidence for a bottleneck that occurred only in this lineage. This bottleneck led to the deletion of a region in the DNA known as TbD1. M. tuberculosis can be split into two groups: ‘old’ strains that retain the TbD1 region and ‘modern’ strains that do not. The ‘modern’ strains are the causes of recent major epidemics with the deletion of the TbD1 region thought to have happened before the 18th century. Furthermore urbanisation and industrialisation by human populations appears to be coupled with the explosion of the ‘modern’ human tuberculosis strains.
A diagram depicting the evolutionary relationships of the M. tuberculosis strains
