Abstract
An improved genetic linkage map of diploid (2n=2x=16) alfalfa has been developed using more than 800 genetic markers and 137 F2 plant individuals. The F2 segregation population derived from the self pollinated F1/1 hybrid individual of the cross Medicago sativa ssp. quasifalcata x Medicago sativa ssp. coerulea. Some markers on linkage groups 6, 7, and 8 displayed extreme distorted segregation which miss-linked genetic regions. Systematic investigation of the genetic linkage of markers displaying distorted segregation (characterized by the overwhelming number of heterozygous individuals) demonstrated that the excess number of heterozygous genotypes exert a positive influence on the linkage values. The more individuals carrying heterozygotic marker pairs were present in the population the larger was the linkage value. To overcome this disadvantageous influence, the heterozygotic marker pairs were ignored in the subsequent two-point linkage analysis by which, genuine and false linkages could be distinguished. The fidelity of this method was also demonstrated by displaying the genotypes as a colormap. The improved genetic map of alfalfa in its present form contains 868 markers (four morphological, 12 isozyme, 27 seed protein, 213 RFLP, 608 RAPD and four specific PCR markers) on eight linkage groups. The genetic map covers 523 centimorgan (cM) with an average marker density of 0.6 cM. The correlation between the physical and genetic distance is about 1500 kilobase pairs per one cM. Among the 868 mapped markers 80 were known genes including two previously cytologically localized genes, the rDNA and the β-tubulin loci.
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