Biogeography of Sinorhizobium meliloti nodulating alfalfa in different Croatian regions

Sinorhizobium meliloti is a nitrogen-fixing rhizobium symbiont of legumes, widespread in many temperate environments the high genetic diversity of which enables it to thrive as a symbiont of host legumes and free-living in soil. Soil type, together with geographic differences and host plant genotype, seem to be prominent factors in shaping rhizobial genetic diversity. While a large body of research supports the idea that the genetic structure of free-living microbial taxa exhibits a clear biogeographic pattern, few investigations have been performed on the biogeographic pattern of S. meliloti genotypes in a restricted geographic range. In the present study, a collection of 128 S. meliloti isolates from three different regions in Croatia was investigated to analyze the relationship between genetic diversity, geographic distribution, soil features and isolate phenotypes by using amplified fragment length polymorphism (AFLP) as a genome-wide scanning method. Results obtained led to the conclusion that the genotypes of isolates cluster according to the region of origin and that the differentiation of S. meliloti populations can be mainly ascribed to geographic isolation following an isolation-by-distance model, with a strong distance-decay relationship of genetic similarity with distance, in which local soil conditions are not the major component influencing the isolate phenotypes or their genomic differentiation.