Abstract
Understanding the selective constraints of partner specificity in mutually beneficial symbiosis is a significant, yet largely unexplored, prospect of evolutionary biology. These selective constraints can be explored through the study of nucleotide polymorphism at loci controlling specificity. The membrane-anchored receptor NORK (nodulation receptor kinase) of the legume Medicago truncatula controls early steps of root infection by two symbiotic microorganisms: nitrogen-fixing bacteria (rhizobia) and endomycorrhizal fungi (Glomales). We analyzed the diversity of the gene NORK by sequencing 4 kilobases in 28 inbred lines sampled from natural populations. We detected 33 polymorphic sites with only one nonsynonymous change. Analysis based on Tajima’s D and Fay and Wu’s H summary statistics revealed no departure from the neutral model. We analyzed divergence using sequences from the closely related species M. coerulea. The McDonald-Kreitman test indicated a significant excess of nonsynonymous changes contributing to this divergence. Furthermore, maximum-likelihood analysis of a molecular phylogeny of a few legume species indicated that a number of amino acid sites, likely located in the receptor domain of the protein, evolved under the regime of positive selection. Further research should focus on the rate and direction of molecular coevolution between microorganisms’ signaling molecules and legumes’ receptors.
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Acknowledgments
We thank Mikkel Schierup, Leif Schauser, Nicolas Galtier, Deborah Charlesworth, and two anonymous reviewers for useful comments on early versions of the manuscript, Nicolas Galtier for helpful discussion, and Ziheng Yang, Wendy Wong, and Rasmus Nielsen for communicating the manuscript before publication.
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[Reviewing Editor: Dr. Deborah Charlesworth]
Sequence data were deposited in the GenBank database under accession nos. AY676428 to AY676457 and AJ884582.
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De Mita, S., Santoni, S., Hochu, I. et al. Molecular Evolution and Positive Selection of the Symbiotic Gene NORK in Medicago truncatula. J Mol Evol 62, 234–244 (2006). https://doi.org/10.1007/s00239-004-0367-2
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DOI: https://doi.org/10.1007/s00239-004-0367-2