Results and discussion Natural variation in symbiosis specificity in M. truncatu It had previously been reported that the M. truncatula plants showed differential nitrogen fixation efficiency when inoculated with different rhizobial strains However, to our knowledge, natural variation in nodula tion specificity (i e, Nod vs. Nod- phenotypes) has not been well-documented. To gain a better understanding of the genetic mechanisms underlying symbiosis specifi city in the M. truncatula-Sinorhizobium interaction, we screened a core collection of 31 M. truncatula genotypes using the S. meliloti strains NGR34, NGR247, and Rm41 These plant genotypes capture a wide range of genetic di versity present in natural populations of M. truncatula 136]. This experiment revealed tremendous variation i nodulation capacity and nitrogen fixation specificity be- tween different genotype-rhizobial combinations (Table 1) In particular, this screen revealed that Rm41 was unable nodulate the plant genotypes F83005.5 and Turkey (Figure 1), while the same plant genotypes nodulated nor- mally with other meliloti strains. Thus, we postulate that there exist host genes that control strain-specific nodula tion in M. truncatula. For genetic analysis of the nodula tion specificity in this system, we chose to focus on F83005.5 because Turkey was not compatible when crossed with several other M. truncatula genotypes