“Nothing in biology makes sense except in the light of evolution.”
That oft-cited title* from Theodosius Dobzhansky’s famous essay has been running through my mind this week since reading the title of two articles in the September issue in mBio, placed sequentially in the table of contents. One covers the major lineages of the fungal pathogen Cryptococcus gattii across the globe, and the other covers population dynamics of Pseudomonas aeruginosa in a year-long study within a single patient. Side-by-side, you probably see the similarity too: evolutionary changes and selection over time in a set environment
In the first study mentioned, Rhys Farrer, Dr. Christina Cuomo and a large group of collaborative scientists sequenced the genomes of 16 C. gattii isolates and compared these to population-level data available for 37 additional isolates. C. gattii is a primary pathogen that infects and causes disease in otherwise healthy individuals, while other Cryptococcus species are normally opportunistic pathogens. By sequencing a number of lineages, including one highly virulent lineage, the authors hoped to determine what genetic changes conferred C. gattii pathogenicity.
Each lineage was found to carry a unique subset of virulence genes, which likely contribute to disease progression differences among lineages. Loss or gain of gene clusters was largely traceable to single events The most virulent lineage, while missing 146 genes found in other lineages, contained expanded copies of several genes, including HSP70, a stress-related transcriptional coactivator associated with fungal virulence. Each lineage had different patterns of gene family loss, however, suggesting fluctuations in selection pressures as the lineages diverged.
In the second study, Julio Diaz Caballero, Dr. David Guttman, and another collaborative research team examined the sputum samples of a cystic fibrosis (CF) patient over the period of a year. The scientists showed that two different clades of P. aeruginosa alternated dominance over this period. Even within these two clades, allelic selection varied, which was demonstrated by tracking the distribution of pbpB alleles within each clade. The pbpB gene encodes a penicillin-binding protein that confers antibiotic resistance; as the patient underwent many rounds of antibiotic therapy during the year, the selective environment was constantly changing. The fitness conferred by these different genotypes varied based on the changing environment.
Most biological experiments account for selective pressure in discussing results, but these articles directly test selective effects on population evolution. The juxtaposition of these two articles highlights the ability of whole-genome sequencing to test the detailed effects of selective pressure in different conditions: one group directly observed population changes over time, while the other inferred historical changes based on a (relatively) static picture of geographically diverse populations. Both studies demonstrated natural selection in action, highlighting evolution again as the biology’s illuminating focus. But you don’t have to take my word for it: check out both articles in the current issue of mBio.
*Interesting fact: That essay was first published in the journal American Biology Teacher, which goes to show the importance of pedagogical journals to a field. Speaking of pedagogical journals, check out JMBE for more words of wisdom on teaching biology and microbiology. But if you haven’t already, you should really read this essay first.
-- Julie Wolf