In sugar beet (Beta vulgaris L.), early bolting is caused by a single dominant gene, designated B. Twenty AFLP markers selected from a 7.8-cM segment of the B region on chromosome 2 were used to screen a YAC library, and a first-generation physical map including the B gene, made up of 11 YACs, was established. Because the genome coverage of the YAC library was low, a BAC library was constructed in the vector pBeloBAC11. This library consists of 57,600 clones with an average insert size of 116 kb, corresponding to 8.8 genome equivalents. Screening of the BAC library with chloroplast and mitochondrial DNA probes indicated that less than 0.1% of the clones contained organelle-derived DNA. To fill the gaps in the physical map around the B gene, the BAC library was screened with four AFLP markers and 10 YAC-derived probes. In total, 54 different BACs were identified. Overlaps between BACs were detected by using BAC termini amplified by PCR as probes, and by RFLP fingerprinting. In this way, a minimal tiling path of the central 4.6-cM region was constructed, which consists of 14 BACs. The B locus was localized to a 360-kb contig, a size which makes positional cloning of the gene feasible.

The Poales (includes the grasses) and Asparagales [includes onion (Allium cepa L.) and asparagus (Asparagus officinalis L.)] are the two most economically important monocot orders. The Poales are a member of the commelinoid monocots, a group of orders sister to the Asparagales. Comparative genomic analyses have revealed a high degree of synteny among the grasses; however, it is not known if this synteny extends to other major monocot groups such as the Asparagales. Although we previously reported no evidence for synteny at the recombinational level between onion and rice, microsynteny may exist across shorter genomic regions in the grasses and Asparagales. We sequenced nine asparagus BACs to reveal physically linked genic-like sequences and determined their most similar positions in the onion and rice genomes. Four of the asparagus BACs were selected using molecular markers tightly linked to the sex-determining M locus on chromosome 5 of asparagus. These BACs possessed only two putative coding regions and had long tracts of degenerated retroviral elements and transposons. Five asparagus BACs were selected after hybridization of three onion cDNAs that mapped to three different onion chromosomes. Genic-like sequences that were physically linked on the cDNA-selected BACs or genetically linked on the M-linked BACs showed significant similarities (e < -20) to expressed sequences on different rice chromosomes, revealing no evidence for microsynteny between asparagus and rice across these regions. Genic-like sequences that were linked in asparagus were used to identify highly similar (e < -20) expressed sequence tags (ESTs) of onion. These onion ESTs mapped to different onion chromosomes and no relationship was observed between physical or genetic linkages in asparagus and genetic linkages in onion. These results further indicate that synteny among grass genomes does not extend to a sister order in the monocots and that asparagus may not be an appropriate smaller genome model for plants in the Asparagales with enormous nuclear genomes.