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Background  The  NODULATION RECEPTOR KINASE  ( NORK ) gene encodes a Leucine-Rich Repeat (LRR)-containing receptor-like protein and controls the infection by symbiotic rhizobia and endomycorrhizal fungi in Legumes. The occurrence of numerous amino acid changes driven by directional selection has been reported in this gene, using a limited number of messenger RNA sequences, but the functional reason of these changes remains obscure. The  Medicago  genus, where changes in rhizobial associations have been previously examined, is a good model to test whether the evolution of  NORK  is influenced by rhizobial interactions.    Results  We sequenced a region of 3610 nucleotides (encoding a 392 amino acid-long region of the NORK protein) in 32  Medicago  species. We confirm that positive selection in  NORK  has occurred within the  Medicago  genus and find that the amino acid positions targeted by selection occur in sites outside of solvent-exposed regions in LRRs, and other sites in the N-terminal region of the protein. We tested if branches of the  Medicago  phylogeny where changes of rhizobial symbionts occurred displayed accelerated rates of amino acid substitutions. Only one branch out of five tested, leading to  M. noeana , displays such a pattern. Among other branches, the most likely for having undergone positive selection is not associated with documented shift of rhizobial specificity.    Conclusion  Adaptive changes in the sequence of the NORK receptor have involved the LRRs, but targeted different sites than in most previous studies of LRR proteins evolution. The fact that positive selection in  NORK  tends not to be associated to changes in rhizobial specificity indicates that this gene was probably not involved in evolving rhizobial preferences. Other explanations ( e.g . coevolutionary arms race) must be tested to explain the adaptive evolution of  NORK .

Adaptive evolution of the symbiotic gene NORK is not correlated with shifts of rhizobial specificity in the genus Medicago