Résumé
Background: The NODULATION RECEPTOR KINASE (NORK) gene encodes a Leucine-Rich Repeat(LRR)-containing receptor-like protein and controls the infection by symbiotic rhizobia andendomycorrhizal fungi in Legumes. The occurrence of numerous amino acid changes driven bydirectional selection has been reported in this gene, using a limited number of messenger RNAsequences, but the functional reason of these changes remains obscure. The Medicago genus, wherechanges in rhizobial associations have been previously examined, is a good model to test whetherthe evolution of NORK is influenced by rhizobial interactions.Results: We sequenced a region of 3610 nucleotides (encoding a 392 amino acid-long region ofthe NORK protein) in 32 Medicago species. We confirm that positive selection in NORK hasoccurred within the Medicago genus and find that the amino acid positions targeted by selectionoccur in sites outside of solvent-exposed regions in LRRs, and other sites in the N-terminal regionof the protein. We tested if branches of the Medicago phylogeny where changes of rhizobialsymbionts occurred displayed accelerated rates of amino acid substitutions. Only one branch outof five tested, leading to M. noeana, displays such a pattern. Among other branches, the most likelyfor having undergone positive selection is not associated with documented shift of rhizobialspecificity.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 thatpositive selection in NORK tends not to be associated to changes in rhizobial specificity indicatesthat 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