A tale of two lineages: how the strains of the earliest divergent symbiotic Frankia clade spread over the world.

It is currently assumed that around 100 million years ago, the common ancestor to the Fabales, Fagales, Rosales and Cucurbitales in Gondwana, developed a root nodule symbiosis with a nitrogen-fixing bacterium. The symbiotic trait evolved first in Frankia cluster-2; thus, strains belonging to this cluster are the best extant representatives of this original symbiont. Most cluster-2 strains could not be cultured to date, except for Frankia coriariae, and therefore many aspects of the symbiosis are still elusive. Based on phylogenetics of cluster-2 metagenome-assembled genomes (MAGs), it has been shown that the genomes of strains originating in Eurasia are highly conserved. These MAGs are more closely related to Frankia cluster-2 in North America than to the single genome available thus far from the southern hemisphere, i.e., from Papua New Guinea.To unravel more biodiversity within Frankia cluster-2 and predict routes of dispersal from Gondwana, we sequenced and analysed the MAGs of Frankia cluster-2 from Coriaria japonica and Coriaria intermedia growing in Japan, Taiwan and the Philippines. Phylogenetic analyses indicate there is a clear split within Frankia cluster-2, separating a continental from an island lineage. Presumably, these lineages already diverged in Gondwana.Based on fossil data on the host plants, we propose that these two lineages dispersed via at least two routes. While the continental lineage reached Eurasia together with their host plants via the Indian subcontinent, the island lineage spread towards Japan with an unknown host plant.

Streptomyces coriariae sp. nov., a novel streptomycete isolated from actinorhizal nodules of Coriaria intermedia.

An actinobacterial strain, CMB-FB, was isolated from surface-sterilized root nodules of a Coriaria intermedia plant growing along Halsema Highway in the province of Benguet (Luzon, Philippines). The 16S rRNA gene sequence of CMB-FB showed high sequence similarity to those of the type strains of Streptomyces rishiriensis (99.4 %), Streptomyces humidus (99.1 %), Streptomyces cacaoi subsp. asoensis (99.0 %), and Streptomyces phaeofaciens (98.6 %). The major menaquinones of CMB-FB were composed of MK-9(H4), MK-9(H6) and MK-9(H8), and there was a minor contribution of MK-9(H10). The polar lipid profile consisted of phosphatidylethanolamine, unidentified aminolipids and phospholipids, a glycophospholipid and four unidentified lipids. The diagnostic diamino acid of the peptidoglycan was meso-diaminopimelic acid. The major fatty acids were iso-C16 : 0, anteiso-C15 : 0 and anteiso-C17 : 0. The results of physiological analysis indicated that CMB-FB was mesophilic. The results of phylogenetic, genome-genome distance calculation and average nucleotide identity analysis indicated that the isolated strain represents the type strain of a novel species. On the basis of these results, strain CMB-FB (=DSM 112754T=LMG 32457T) is proposed as the type strain of the novel species Streptomyces coriariae sp. nov.