Genetic architecture of thermal adaptation in Escherichia coli

Elucidating the genetic basis of adaptation on a genomewide scale has evade d biologists, but complete genome sequences and DNA high-density array tech nology make genomewide surveys more tractable. Six lines of Escherichia col i adapted for 2,000 generations to a stressful high temperature of 41.5 deg reesC were examined on a genomewide scale for duplication/deletion events b y using DNA high-density arrays. A total of five duplication and deletion e vents were detected. These five events occurred in three of the six lines, whereas the remaining three lines contained no detectable events. Three of the duplications were at 2.85 Mb of the E. coli chromosome, providing evide nce for the replicability of the adaptation to high temperature. Four candi date genes previously shown to play roles in stress and starvation survival were identified in the region of common duplication. Expression of the two candidate genes examined is elevated over expression levels in the ancestr al lines or the lines without the duplication. In the two cases where the d uplication at 2.85 Mb has been further characterized, the timing of the gen ome reorganization is coincident with significant increases in relative fit ness. In both of these cases, the model for the origin of the duplication i s a complex recombination event involving insertion sequences and repeat se quences. These results provide additional evidence for the idea that gene d uplication plays an integral role in adaptation, specifically as a means fo r gene amplification.