COMPARATIVE CYTOGENOMICS OF A 431-KB BAC CONTIG BETWEEN Arabidopsis thaliana AND Brassica rapa.

¹ Dep't. of Agronomy and Plant Genetics Univ. of Minnesota 411 Bourlag Hall 1991 Upper Buford Circle St. Paul, MN 55108

² Dep't of Horticulture Univ. of Wisconsin-Madison 1575 Linden Drive Madison, WI 53706

³ Dep't of Genetics Harvard Univ. Massachusetts General Hospital Boston, MA 02114

⁴ Cereon Genomics LLC. 45 Sidney St Cambridge, MA 02139

A 431-kb sequenced Arabidopsis bacterial artificial chromosome (BAC) contig was used to compare this gene-rich region (~1 gene/ 3.5 kb) in the closely related Brassica rapa. This contig is from the 79 cM region of chromosome II of A. thaliana and consists of six BAC clones. The BACs were physically mapped, using fluorescence in situ hybridization (FISH) on chromosomes and extended DNA fibers from B. rapa and compared to the sequencing data and fiber-FISH data obtained from Arabidopsis. Three adjacent BACs were simultaneously hybridized with alternating fluorophores to determine the size of the FISH signals derived from the middle BAC in B. rapa. The fiber-FISH data suggests that in this genomic region there has been little or no expansion of intergenic sequences, and the chromosome data indicates duplication of most or all of this contig within the B. rapa genome. The FISH signals were also similar in appearance to the data obtained when these BACs were hybridized to Arabiopdsis suggesting similar gene spacing. Thus, the comparative physical mapping data using this contig suggest that the increase in genome size in B. rapa may be primarily due to duplication and not an increase in intergenic spacer sequences.