Nodule bacteria (rhizobia) represent a suitable model to address a range of evolutionary prob-lems including the tradeoff between genetic polymorphism and natural selection. Rhizobia possess the complicated genomes in which symbiotically specialized (sym) genes differ in their natural histories from core genes encoding for housekeeping functions. Diversification of sym genes is responsible for the rhizobia microevolution which depends on the host-induced natural selection. For the rhizobia speciation, diversification of core genes is responsible for which the selective factors are unclear. In this paper we demonstrate that in the goats’ rue rhizobia (Neorhizobium galegae) populations collected at North Caucasus and composed of two host-specific biovars orientalis and officianalis (N2-fixing symbionts of Galega orientalis and G. officinalis, respectively), evolutionary mechanisms are different for core and sym genes. In both N. galegae biovars, core genes are more polymorphic than sym genes. In bv. orientalis, evolution of core genes occurs under the impacts of driving se-lection (dN/dS > 1), while evolution of sym genes is close to neutral (dN/dS ≈ 1). In bv. officinalis, evolution of core genes is neutral, while for sym genes, it is dependent on purifying selection (dN/dS < 1). A marked phylogenetic congruence of core and sym genes revealed using the ANI analysis may be due to a low intensity of gene transfer within and between N. galegae biovars. Polymorphism of both gene groups and the impacts of driving selection on the core gene evolu-tion are more pronounced in bv. orientalis than in bv. officianalis reflecting the diversities of re-spective host plant species. In bv. orientalis, highly significant (P0 < 0,001) positive correlation was revealed between the p-distance and dN/dS values for core genes, while in bv. officinalis this cor-relation is lowly significant (0,05 < P0 < 0,10). For sym genes, correlation between the p-distance and dN/dS values is negative in bv. officinalis but is not revealed in bv. orientalis. These data along with the functional annotation of core genes implemented using the Gene Ontology tools sug-gests that evolution of bv. officinalis is based mostly on adaptation for in planta niches while in bv. orientalis evolution presumable depends on adaptation for soil niches. New insights into the tradeoff between natural selection and genetic diversity are presented suggesting that the gene polymorphism may be extended by driving selection only in the ecologically versatile organ-isms capable to support a broad spectrum of gene alleles in their genepools.

Antimicrobial peptides provide a powerful toolkit for combating multidrug resistance. Combatting eukaryotic pathogens is complicated by the fact, the intracellular drug targets in the eukaryotic pathogen are frequently homologs of cellular structures of vital importance in the host organism. The entomopathogenic bacteria, (symbionts of entomopathogenic nematode species), release a series of non-ribosomal templated antimicrobial peptides, some may be potential drug candidates. The ability of an entomopathogenic nematode /entomopathogenic bacterium symbiotic complex to survive in a given polyxenic milieu is a coevolutionary product, explaining that those gene complexes which are responsible for the biosynthesis of different non-ribosomal templated antimicrobial protective peptides (including those which are potently capable to inactivate the protist mammalian -pathogen Leishmania and the gallinaceous bird pathogen Histomonas) are co-regulated. Our approach is based on comparative antimicrobial bioassays of the culture media of the wild-type and regulatory mutant strains. We concluded that Xenorhabdus budapestensis and X. szentirmaii are excellent sources of non-ribosomal templated antimicrobial peptides, that are efficient antagonists of the mentioned pathogens. Data on selective cytotoxicity of different cell-free culture media encourages us to forecast that the recently discovered “easy-PACId” research strategy is suitable for constructing EPB strains producing and releasing single, harmless, non-ribosomal templated antimicrobial peptides with considerable drug, (probiotic) -candidate potential.