Abstract
BackgroundAlfalfa (Medicago sativa) is a major forage crop. The genetic progress is slow in this legume species because of its autotetraploidy and allogamy. The genetic structure of this species makes the construction of genetic maps difficult. To reach this objective, and to be able to detect QTLs in segregating populations, we used the available codominant microsatellite markers (SSRs), most of them identified in the model legume Medicago truncatula from EST database. A genetic map was constructed with AFLP and SSR markers using specific mapping procedures for autotetraploids. The tetrasomic inheritance was analysed in an alfalfa mapping population.ResultsWe have demonstrated that 80% of primer pairs defined on each side of SSR motifs in M. truncatula EST database amplify with the alfalfa DNA. Using a F1 mapping population of 168 individuals produced from the cross of 2 heterozygous parental plants from Magali and Mercedes cultivars, we obtained 599 AFLP markers and 107 SSR loci. All but 3 SSR loci showed a clear tetrasomic inheritance. For most of the SSR loci, the double-reduction was not significant. For the other loci no specific genotypes were produced, so the significant double-reduction could arise from segregation distortion. For each parent, the genetic map contained 8 groups of four homologous chromosomes. The lengths of the maps were 2649 and 3045 cM, with an average distance of 7.6 and 9.0 cM between markers, for Magali and Mercedes parents, respectively. Using only the SSR markers, we built a composite map covering 709 cM.ConclusionsCompared to diploid alfalfa genetic maps, our maps cover about 88–100% of the genome and are close to saturation. The inheritance of the codominant markers (SSR) and the pattern of linkage repulsions between markers within each homology group are consistent with the hypothesis of a tetrasomic meiosis in alfalfa. Except for 2 out of 107 SSR markers, we found a similar order of markers on the chromosomes between the tetraploid alfalfa and M. truncatula genomes indicating a high level of colinearity between these two species. These maps will be a valuable tool for alfalfa breeding and are being used to locate QTLs.
Highlights
Alfalfa (Medicago sativa) is a major forage crop
Our mapping population is a set of 168 F1 individuals from the cross between Mercedes4.11 used as female parent and Magali2 used as male parent
AFLP markers Using 27 AFLP primer pairs, a total of 599 polymorphic markers were scored on the mapping population (Table 1)
Summary
Alfalfa (Medicago sativa) is a major forage crop. The genetic progress is slow in this legume species because of its autotetraploidy and allogamy. The development of a genetic linkage map would be a significant breakthrough, either to map candidate genes, to search for QTL of agronomical traits, or to make marker-assisted selection Such analysis requires working on cultivated material, either in terms of genetic background or ploidy level. The parental plants originated from CADL plants (Cultivated Alfalfa at the Diploid Level, diploid plants produced from cultivated tetraploid alfalfa) [6] or from natural populations, the crosses being made at the subspecific level [5,7] All these maps comprised from 89 to 153 RFLP, RAPD or isozymes markers, and spanned from 467.5 cM [7] to 659 cM [6]. The number of polymorphic RFLP markers is clearly a limiting factor to the development of a saturated map at the tetraploid level in alfalfa
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