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




Adaptive evolution of the symbiotic gene NORK is not correlated with shifts of rhizobial specificity in the genus Medicago - Download this document for free, or read online. Document in PDF available to download.

BMC Evolutionary Biology

, 7:210

First Online: 06 November 2007Received: 26 April 2007Accepted: 06 November 2007

Abstract

BackgroundThe 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.

ResultsWe 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.

ConclusionAdaptive 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.

Electronic supplementary materialThe online version of this article doi:10.1186-1471-2148-7-210 contains supplementary material, which is available to authorized users.

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Author: Stéphane De Mita - Sylvain Santoni - Joëlle Ronfort - Thomas Bataillon

Source: https://link.springer.com/article/10.1186/1471-2148-7-210







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