Image of a diverse population of a colony of the bacterium Burkholderia thailandensis growing outwards (to the right). The original bacteria contained two copies of the large region of DNA and were expressing green and red fluorescent proteins, making them look yellowish-green. As the colony grows, the population diversifies. Bacteria lose the second copy of the large region of DNA, which causes them to no longer produce the green fluorescent protein.
Bacterial populations are often diverse, even when originating from a single cell. This diversity helps microbes survive in fluctuating environmental conditions by increasing the odds of population survival. For example, if environmental conditions change such that only a subpopulation with unique abilities survives, the entire population will be saved.
Genomes are naturally dynamic. For example, mobile sections of DNA, called transposable elements, can change their position within a genome. If a transposable element jumps into a gene, it can harm the cell. But if it moves into a different site, it may provide an organism with new features that can help it survive.
Most organisms contain multiple copies of transposable elements in their DNA. For example, a subtype of the soil bacterium Burkholderia thailandensis, strain E264, has two identical transposable elements that book-end a region of DNA that contains 157 genes. Lowrey et al. studied this bacterial strain in different environmental conditions to find out more. The experiments revealed that in growing populations of E264, some bacteria had one copy of the region, while others had two or three. In a rich environment, most bacteria had just one copy of the region. However, when grown in challenging conditions, most bacteria contained two or three copies of the region.
Moreover, bacteria required at least two copies to form dense communities known as biofilms, which are advantageous for bacterial survival in challenging conditions. Bacteria with only one copy, however, were better adapted to a free-swimming lifestyle.
Lowrey et al. further showed that the DNA repair system was required for duplicating the region. Usually, this system finds and recombines identical DNA sequences to repair broken DNA. However, if two identical DNA sequences (a pair of transposable elements) are present, the repair system can recombine them during DNA replication, resulting in the duplication of the DNA between the identical sequences. The same system also reduces the copy number of the region from three or two to just one.
Since the repair system is constantly working and DNA recombination is always occurring at a low level, B. thailandensis E264 maintains a genetically diverse population with bacteria containing different copy numbers of the region. This diversity ensures that the strain survives in fluctuating environmental conditions.
Transposable elements are hotspots of evolution. They are known to interrupt genes and shrink genomes. Lowrey et al. showed that transposable elements also influence evolution by providing DNA sequences that the DNA repair system can use to duplicate DNA. This process of duplicating genes is more frequent than random genetic mutations, expediting adaptation.
