The adaptive potential of hybridization demonstrated with bacteriophages.

The success or failure of hybrids and the factors that determine their fitness have ecological, evolutionary, medical, and economic implications. Hybrid fitness is a major determinant of the size of hybrid zones and the maintenance of related species with overlapping ranges. It also influences the evolution of emerging pathogens and the success of economically important crop species experimentally hybridized in search of strains with increased yields or disease resistance. Hybrid fitness may largely be determined by the pervasiveness of epistasis in the genome, as epistasis is known to debilitate hybrids through disrupted inter- and intragenic interactions. We identified two bacteriophages isolated from their natural environment, one the result of a past hybridization event involving an ancestor of the other phage and a third, unknown phage. By performing a reciprocal cross of the affected region of the genome, consisting of a single complete gene, we both approximately recreated and reversed this original hybridization event in two chimeric bacteriophage genomes. Subsequent adaptation of the hybrid phages allowed for the recovery of fitness losses incurred by the hybrid genotypes. Furthermore, adaptation led to the ascension of a substantially higher and previously inaccessible adaptive peak. We show that by allowing genotypes to take large leaps across the adaptive landscape rather than single mutational steps, hybridization can lead to huge long-term fitness gains in spite of short-term costs resulting from disrupted epistatic interactions, demonstrating that the success or failure of hybrids may be determined not by their initial fitness, but rather by their adaptive potential.