Palaeohexaploid Ancestry for Caryophyllales Inferred from Extensive Gene-Based Physical and Genetic Mapping of the Sugar Beet Genome (Beta vulgaris)

Sugar beet (Beta vulgaris) is an important crop plant accounting for 30% of the world's sugar production annually. The genus Beta is a distant relative of currently sequenced taxa within the core eudicotyledons, and genomic characterization of sugar beet is essential in order to make its genome accessible to molecular dissection. Here, we present comprehensive genomic information in genetic and physical maps covering all nine chromosomes. Based on this information we identified the proposed ancestral linkage groups of rosids and asterids within the sugar beet genome. We generated an extended genetic map comprising 1127 single nucleotide polymorphism markers prepared from expressed sequence tags and bacterial artificial chromosome (BAC) end sequences. To construct a genome-wide physical map we hybridized gene-derived oligomer probes against two BAC libraries with 9.5 fold cumulative coverage of the 758 Mbp genome. More than 2500 probes and clones were integrated both in genetic maps and the physical data. The final physical map encompasses 535 chromosomally anchored contigs containing 8361 probes and 22,815 BAC clones. By using the gene order established with the physical map we detected regions of synteny between sugar beet (order Caryophyllales) and rosid species involving 1400-2700 genes in the sequenced genomes of Arabidopsis, poplar, grapevine, and cacao. The data suggest that Caryophyllales share the palaeohexaploid ancestor proposed for rosids and asterids. Taken together, we here provide extensive molecular resources for sugar beet enabling future high-resolution trait mapping, gene identification, and cross-referencing to regions sequenced in other plant species. (c) 2011 The Authors. The Plant Journal(c) 2011 Blackwell Publishing Ltd