The phylogenetic tree below shows inferred relationships among five genotypes (M

Question

The phylogenetic tree below shows inferred relationships among five genotypes (Ms, Mw, Mu, N3, and Mr) of the bacterium Staphylococcus aureus. Species of Bacillus (Bh and Bs) and Listeria (Li and Lm) are assumed to be outgroups to S. aureus and were used to root the tree of S. aureus genotypes.

(a) If this tree is true, which S. aureus genotype was the first to diverge?

(b) All of the S. aureus genotypes except Ms show resistance to multiple antibiotics. Describe a hypothesis for the evolution of antibiotic resistance in S. aureus that is consistent with the phylogenetic tree.

Explanation / Answer

Answer:

a. Mr is the first to diverge from the rest of the S. aureus genotypes.

b. It can be hypothesized that the genotypes of Staphylococcus aureus developed antibiotic resistance before Mr diverged from the other genotypes. This got selected in the course of evolution. Ms genotype diverged from Mw genotype through a loss of the antibiotic resistance genes which are otherwise carried by the other strains of Staphylococcus aureus.

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