Abstract
Repeated evolution of functionally similar phenotypes is observed throughout the tree of life. The extent to which the underlying genetics are conserved remains an area of considerable interest. Previously, we reported the evolution of colony switching in two independent lineages of Pseudomonas fluorescens SBW25. The phenotypic and genotypic bases of colony switching in the first lineage (Line 1) have been described elsewhere. Here, we deconstruct the evolution of colony switching in the second lineage (Line 6). We show that, as for Line 1, Line 6 colony switching results from an increase in the expression of a colanic acid-like polymer (CAP). At the genetic level, nine mutations occur in Line 6. Only one of these—a nonsynonymous point mutation in the housekeeping sigma factor rpoD—is required for colony switching. In contrast, the genetic basis of colony switching in Line 1 is a mutation in the metabolic gene carB. A molecular model has recently been proposed whereby the carB mutation increases capsulation by redressing the intracellular balance of positive (ribosomes) and negative (RsmAE/CsrA) regulators of a positive feedback loop in capsule expression. We show that Line 6 colony switching is consistent with this model; the rpoD mutation generates an increase in ribosomal gene expression, and ultimately an increase in CAP expression.
Highlights
The repeated appearance of similar phenotypes is a striking feature amid the diversity of life
Clonal populations of Escherichia coli adapt to thermal stress via different genetic routes (Riehle et al 2001), whereas pigmentation changes in mice and lizards are both underpinned by mutations in the Mc1r gene (Nachman et al 2003; Rosenblum et al 2004)
6B4 Capsule Expression Is Due to Transcriptional Regulation of wcaJ–wzc To identify the genetic basis of the 6B4 capsule, 6B4 was subjected to transposon mutagenesis
Summary
The repeated appearance of similar phenotypes is a striking feature amid the diversity of life. Many phenotypes have evolved multiple independent times in different lineages (Conway Morris 1999). An intriguing aspect of repeated phenotypic evolution is the extent to which the underlying genetics are conserved. It is commonly thought that the degree of genetic parallelism correlates with the degree to which two organisms are related. Clonal populations of Escherichia coli adapt to thermal stress via different genetic routes (Riehle et al 2001), whereas pigmentation changes in mice and lizards are both underpinned by mutations in the Mc1r gene (Nachman et al 2003; Rosenblum et al 2004). The increasing number of examples of disparity between genetic parallelism and degree of relatedness (reviewed in Arendt and Reznick 2008) hints at the underappreciated and poorly understood complexity of biological systems
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