Genetic mapping of the non-nodulation phenotype of the mutant MN-1008 in tetraploid alfalfa (Medicago sativa)

Roots of the non-nodulating Medicago sativa mutant MN-1008 neither undergo root-hair curling, cortical cell division nor any of the early molecular events that accompany nodule initiation and development following rhizobial infection or treatment with Nod factor. These observations suggested that the mutation(s) impaired a pivotal function in Nod factor perception or in the signal transduction pathway. In this paper we show that the genetic lesion conditioning the recessive non-nodulation phenotype in the tetraploid alfalfa mutant MN-1008 can be localized to a single region on LG5 of the M. sativa genetic map. This conclusion is based on genetic analyses conducted at the tetraploid level, involving both segregation analysis and genetic mapping of the trait with respect to molecular DNA markers. The genetic mapping of the Nod(-) phenotype was performed in a segregating tetraploid F2 population, taking advantage of the availability of an advanced genetic map for diploid alfalfa. Two tightly linked flanking markers have been identified which will facilitate the physical mapping and cloning of the gene(s) that underlie(s) the non-nodulation phenotype.