Price et al. [(1999). J. Bacteriol. 181:2358–2362] conducted a genetic study of the toxin transport protein (PA) of Bacillus anthracis, the bacterium that causes anthrax in humans. Within the 2294-nucleotide gene in 26 strains they identified five point mutations—two missense and three synonyms—among different isolates. Necropsy samples from an anthrax outbreak in 1979 revealed a novel missense mutation and five unique nucleotide changes among ten victims. The authors concluded that these data indicate little or no horizontal transfer between different B. anthracis strains.
On what basis did the authors conclude that evidence of horizontal transfer is absent from their data?

Verified step by step guidance

1. Understand the concept of horizontal gene transfer (HGT), which involves the movement of genetic material between organisms other than by vertical transmission (from parent to offspring). In bacteria, HGT can introduce new genetic variations rapidly across strains.
2. Recognize that the authors analyzed the gene encoding the toxin transport protein (PA) across 26 strains and found only a few point mutations, including two missense (amino acid changing) and three synonymous (silent) mutations, indicating limited genetic diversity in this gene region.
3. Note that the necropsy samples from the 1979 outbreak showed a novel missense mutation and five unique nucleotide changes, which were specific to those samples and not shared broadly among other strains.
4. Consider that if horizontal transfer had occurred, one would expect to see shared mutations or genetic sequences across different strains that are not explained by common ancestry, reflecting gene flow between strains.
5. Since the mutations observed were unique to specific strains or outbreak samples and there was no evidence of shared novel mutations across unrelated strains, the authors concluded that there was little or no horizontal transfer, implying that the genetic differences arose mainly through vertical inheritance and mutation.

Key Concepts

Horizontal Gene Transfer

Horizontal gene transfer (HGT) is the movement of genetic material between organisms other than by descent from parent to offspring. It allows bacteria to acquire new traits rapidly, such as antibiotic resistance. Detecting HGT involves identifying genetic sequences that differ significantly from the typical lineage or show mosaic patterns.

Point Mutations and Their Types

Point mutations are single nucleotide changes in DNA. Missense mutations alter the amino acid sequence of a protein, potentially affecting function, while synonymous mutations do not change the amino acid and are often neutral. The pattern and distribution of these mutations can reveal evolutionary relationships and gene flow.

Genetic Variation and Strain Differentiation

Genetic variation among bacterial strains arises from mutations and gene transfer events. Low variation and unique mutations confined to specific strains suggest limited gene exchange. By comparing mutation patterns across strains, researchers can infer whether horizontal transfer has occurred.