A complete physical map of a wild beet (Beta procumbens) translocation in sugar beet

Two sugar beet lines carry homologous translocations of the wild beet Beta procumbens. Long-range restriction mapping with rare cutting enzymes revealed that both translocations are different in size, however, an overlapping region of about 350 kb could be identified. Both lines are resistant to the beet cyst nematode but only TR520 carries the previously cloned resistance gene Hs1pro-1. Hence, a second gene for nematode resistance (Hs1-1) must be located within this region. A bacterial artificial chromosome (BAC) library was constructed from line TR520. The library was screened with a number of B. procumbens specific probes and 61 BAC clones were identified. Five BAC clones formed a minimal tiling path of 580 kb to cover the overlapping region between both translocations including the translocation breakpoint. The five BACs from the overlapping region and one additional BAC distal from that contig were sequenced. The total sequence length from the five BACs of the overlapping region amounted to 524 kb which is 74.35% of the total insert size of these BACs. The frequency of retrotransposon sequences ranged between 14.7 and 43.3%. A total of 133 ORFs were identified, none of these showed similarity to known disease resistance genes. Of these, 12 ORFs showed homology to genes involved in biotic stress resistance reactions or to transcription factors. This paper demonstrates how genome specific probes can be employed for cloning an alien gene introgression into a cultivated species.