Abstract
The field of ancient genomics has triggered considerable progress in the study of pathogens, including those affecting crops. Herbarium collections have been an important source of dated, identified and preserved DNA, whose use in comparative genomics and phylogeography may shed light into the emergence and evolutionary history of plant pathogens. I will present the reconstruction of 13 historical genomes of the bacterial crop pathogen Xanthomonas citri pv. citri (Xci) from infected citrus herbarium specimens using a shotgun-based deep sequencing strategy. After authentication of the historical genomes based on DNA damage patterns, we compared them to modern genomes to reconstruct their phylogenetic relationships, pathogeny-associated genes content and estimate several evolutionary parameters, using Bayesian tip-dating calibration and phylogeography inferences. Despite a challenging analysis of data, requiring adapted treatment before being compared to modern samples, our results reveal that Xci originated in Southern Asia ~11,500 years ago and diversified during the beginning of the 13th century, after Citrus diversification and before spreading to the rest of the world. This updated scenario links Xci specialization to Neolithic climatic change and the development of agriculture, and its diversification to the human-driven expansion of citriculture through the early East-West trade and later colonization.