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SummaryAn RFLP linkage map for the nine chromosomes of sugar beet (Beta vulgaris L. ssp. vulgarisvar. altissima Doell) was constructed by using a segregating population from a cross between two plants which were heterozygous for several agronomically interesting characters. One hundred and eleven RFLP loci have been mapped to nine linkage groups using 92 genomic markers. The current RFLP map covers a total length of 540 cM. Evidence for the existence of a major gene for rhizomania resistance ( R r l ) is given, together with its map position on linkage group IV in the interval between loci GS44 and GS28a. The presence of an RFLP fragment at the GS3dlocus is, until now, the best molecular marker for rhizomania-resistant genotypes in segregating populations of sugar beet; GS3d is linked to Rrl with 6.7 cM. The gene MM, controlling the polygerm/monogerm seed type, has been mapped on linkage group IX in a distal position at 4.2 cM from the locus GS7. The gene Rcontrolling the hypocotyl colour maps to linkage group VII and does not recombine with the RFLP locus GS42. The inheritance of a group of RFLP loci revealed the possible presence of a translocation in the population used to establish the map. The data presented are discussed in relation to the possibility of using RFLP markers in sugar beet breeding.

An updated map of sugar beet (Beta vulgaris L. ssp. vulgaris var 'altissima Doell') is presented. In this genetic map we have combined 248 RFLP and 50 RAPD loci. Including the loci for rhizomania resistance Rr1, hypocotyl colour R and the locus controlling the monogerm character M, 301 loci have now been mapped to the nine linkage groups covering 815 cM. In addition, the karyotype of some of the Beta vulgaris chromosomes has been correlated with existing RFLP and RAPD linkage maps.

Several genetic maps for sugar beet {Beta vulgaris L.), from different German research groups, have been published and it is now possible to consider combining them with the aid of the common markers. The computer program JOINMAP (versions 1.3 and 2.0) was used for pairwise combination of three populations. Several problems arose: the genetic background of the populations, different population structures (F, versus F, x F,), different number of polymorphic loci for common probes in the populations to be combined, different estimates of the recombination rates between the same markers and differences between the JOINMAP versions. The maps from two F2 populations could be integrated into a single map, but it was more appropriate to construct separate maps for the F2 populations and the F, x F, population using common markers as reference points only.

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