Phylogenetic Tree & Lyme Disease

Phylogenetic Trees

• A phylogenetic tree shows the evolutionary relationships between different species. Each "node" on the tree represents the emergence of a new species, and if two species can trace their ancestry back to a single node, this node represents the most recent point in time that these two species shared a common ancestor. These trees are constructed based on physical and genetic characteristics, and evolutionary biologists are often forced to change them when new data comes to light.

Borrelia

• Borrelia is actually a genus of bacteria that includes multiple species. The number of Borrelia species has increased multiple times over the past two decades as more strains were identified. Defining species of Borrelia or any other genus of bacteria is actually a very tricky problem. Where plants and animals are concerned, biologists have traditionally defined species based on their inability to breed and produce fertile offspring. A horse and donkey can breed, for example, but the mule that results is sterile, so horses and donkeys are separate species. Bacteria, however, reproduce asexually, so this definition is irrelevant. Consequently, for Borrelia and for many other bacteria, microbiologists define species as distinct based on a technique called whole DNA-DNA hybridization.

Relationships

• Scientists have constructed phylogenetic trees for Borrelia species based on a technique called multilocus sequence analysis (MLSA), which looks at differences between specific "housekeeping" genes present in all the different species. Biologists use this software to build the "most parsimonious tree" -- the phylogenetic tree that requires the smallest number of evolutionary changes to explain the observed differences. Results from this method, however, often need to be refined when new information becomes available.

Relationships

• Currently, B. burgdorferi sensu stricto shares its most recent common ancestor with B. bissettii; these two in turn share a common ancestor with B. lusitaniae. All three species diverged from a common ancestor they share with B. afzelii, B. spielmanii, B. garinii and B. bavariensis. This group of species in turn diverged from a common ancestor shared with B. valaisiana. It's important to note, however, that this phylogenetic tree could very well change in future as new data comes to light. In particular, it's quite possible that the decreasing cost of genome sequencing could affect the way in which phylogenetic trees are developed for bacteria like B. burgdorferi.