Horizontal Gene Transfer, Fitness Costs and Mobility shape the spread of antibiotic resistance genes from Acinetobacter baumannii into experimental populations of Acinetobacter baylyi.. Acinetobacter baylyi ADP1

Horizontal Gene Transfer (HGT) is important for microbial evolution, but how evolutionary forces shape the frequencies of horizontally transferred genetic variants in the absence of strong selection is remains an open question. In this study, we evolve laboratory populations of Acinetobacter baylyi (ADP1) with HGT from multidrug resistant Acinetobacter baumanii (AB5075 and A9844). We find that DNA can cross the species barrier, even without strong selection, despite a large DNA sequence divergence between the two species (23.5%). Our results show that HGT treatment populations harbour the most phenotypic diversity, and that HGT from outside the population can facilitate adaptation to some of the antibiotics that we test. We quantify the costs and benefits of horizontally transferred variants, and use whole population sequencing to track the spread of antibiotic resistant genes from HGT donors to antibiotic sensitive recipients. We find that even though most antibiotic genes are taken up by the A. baylyi, the long-term fate of individual antibiotic resistance genes depends on their cost and on the genetic cause of resistance. Interestingly, we also found that a mobile genetic element, but not its host plasmid, is still able to spread in A. baylyi despite its strong deleterious effect. Altogether, our findings show how HGT provides an evolutionary advantage to evolving populations by facilitating the spread of non-selected genetic variation including costly antibiotic resistant alleles.