The barrier to horizontal transfer of orthologous DHFR proteins is alleviated by experimental evolution.

<p>A) ORF of <i>folA</i> gene encoding DHFR in the <i>E</i>. <i>coli</i> chromosome is replaced with orthologs from 35 other mesophiles, while preserving the endogenous promoter. The strains carrying the orthologous DHFR replacements are evolved for 31 serial passages (~600 generations) under standard conditions. B) Distribution of the growth rates before (immediately upon HGT) and after evolution experiment. Growth rate of the WT strain with <i>E</i>. <i>coli</i> DHFR is marked with a dashed line (see also <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1005612#pgen.1005612.s009" target="_blank">S3 Table</a>). Kolmogorov-Smirnov (KS) test indicates that pre- and post-evolution populations are significantly different with respect to their growth rates (p-value <10⁻¹⁰). While 31 out of the 35 naive strains (88%) have lower growth rates than WT, 30% of the post-evolution strains have higher growth rates than WT. C) Growth rates before and after evolution for each strain as a function of its DHFR’s position in the phylogeny. Color scheme is similar to panel (B). Strains are ordered according to the phylogenetic tree on the left. On the right we show an ID number for each strain (used throughout the text) and the original species carrying that DHFR ortholog. We highlight in orange the ID numbers of strains that experience severe fitness drop (30% and lower) upon DHFR replacement. Error bars represent standard deviation of 4 independent measurements.