The Pangaean origin of "candidatus liberibacter" species
Résumé
Species of "Candidatus Liberibacter", all vectored by psyllids, are generally recognized as the cause of four serious plant diseases, Huanglongbing, Zebra Chip, Psyllid Yellows and Yellows Decline, which currently threaten and destroy the citrus, potato, tomato and carrot industries, respectively. These are relatively recent diseases in plant crops, thereby inferring a modern evolutionary trajectory, but no overall hypothesis on their origins has been presented. Plate tectonic movements provide an explanation for the disjunction between modern geographic range and phylogeny. Northern species, on Laurasia, represent both modern European and North American ranges. Southern species, on Gondwana, indicate a further speciation event as ancient India, conjoint with Madagascar, separated from Africa, left Madagascar on its own, and produced the single heat tolerant species while drifting across the equator to its modern position. Phylogeny, epidemiology and geography of the modern species point backwards to a single original speciation event from a free-living form, associated opportunistically with angiosperm and psyllid insect ancestors. The original heat-sensitive species lived in the equatorial but cool climate of the Central Pangaean Mountains probably > 300 Ma. The obligately alternating insect/plant host lifestyle developed opportunistically with the evolution and spread of flowering plants and psyllids.