Abstract
SUMMARY Lateral gene transfer can precipitate rapid evolutionary change. In 2010 the global pandemic of kiwifruit canker disease caused by Pseudomonas syringae pv. actinidiae (Psa) reached New Zealand. At the time of introduction, the single clone responsible for the outbreak was sensitive to copper, however, analysis of a sample of isolates taken in 2015 and 2016 showed that a quarter were copper resistant. Genome sequences of seven strains showed that copper resistance – comprising czc/cusABC and copABCD systems – along with resistance to arsenic and cadmium, was acquired via uptake of integrative conjugative elements (ICEs), but also plasmids. Comparative analysis showed ICEs to have a mosaic structure, with one being a tripartite arrangement of two different ICEs and a plasmid that were isolated in 1921 (USA), 1968 (NZ) and 1988 (Japan), from P. syringae pathogens of millet, wheat and kiwifruit, respectively. Two of the Psa ICEs were nearly identical to two ICEs isolated from kiwifruit leaf colonists prior to the introduction of Psa into NZ. Additionally, we show ICE transfer in vitro and in planta, analyze fitness consequences of ICE carriage, capture the de novo formation of novel recombinant ICEs, and explore ICE host-range.
ORIGINALITY-SIGNIFICANT STATEMENT Lateral gene transfer is a major evolutionary force, but its immediacy is often overlooked. Between 2008 and 2010 a single virulent clone of the kiwifruit pathogen Pseudomonas syringae pv. actinidiae spread to kiwifruit growing regions of the world. After arrival in New Zealand it acquired genetic determinants of copper resistance in the form of integrative conjugative elements and plasmids. Components of these elements are evident in distantly related bacteria from millet (USA, 1921), kiwifruit (Japan, 1988) and wheat (New Zealand, 1968). Additional laboratory experiments capture evidence of the dynamism underpinning the evolution of these elements in real time and further emphasize the potent role that lateral gene transfer plays in microbial evolution.
