Genome analysis and biogeographic distribution of the earliest divergent Frankia clade in the southern hemisphere

Abstract

Coriariaceae are a small family of 14-17 species and subspecies that have a current global, but disjunct, distribution. All species can form root nodules in symbiosis with diazotrophic Frankia cluster-2 strains, which make up the earliest divergent symbiotic clade within this bacterial genus. Studies on Frankia cluster-2 mostly have focused on strains occurring in the northern hemisphere. Except for one strain from Papua New Guinea, namely Candidatus Frankia meridionalis Cppng1, no whole genome of Frankia associated with Coriaria occurring in the southern hemisphere has been published thus far, yet the majority of the plant family’s species occurs here. We present field sampling data of novel Frankia cluster-2 strains, representing two novel species, which are associated with Coriaria arborea and Coriaria sarmentosa in New Zealand, and with Coriaria ruscifolia in Patagonia (Argentina), in addition to identifying Ca. F. meridionalis present in New Zealand. The novel Frankia species were found to be closely related to both Ca. F. meridionalis, and a Frankia species occurring in the Philippines, Taiwan, and Japan. Our data suggest that the different Frankia cluster-2 species diverged early after becoming symbiotic circa 100 million years ago.