Centromeric localization of an S-RNase gene in Petunia hybrida Vilm.

S-RNase has been identified to be an S-allele-specific stylar determinant c ontributing to the self-incompatibility response in Solanaceae. In order to examine the physical location of the S-RNase gene, multi-color fluorescenc e in situ hybridization (FISH) using the S-B1-RNase cDNA probe and ribosoma l RNA gene (rDNA) probe was performed on an (SSB2)-S-B1 heterozygote of Pet unia hybrids. The S-B1-RNase gene was detected as a doublet signal close to the centromere of chromosome III. Next, we performed FISH using a large ge nome probe prepared from a lambda SB1-311 clone (20 kb) which contains the SB1-RNase gene and its 3' flanking region. This probe hybridized to the cen tromeric regions of all P. hybrida chromosomes. Sequence analysis of the la mbda SB1-311 clone revealed the presence of a repetitive sequence consistin g of a novel 666 bp unit sequence. A subclone (pBS-SB1B5) containing this u nit sequence also hybridized to all of the centromeric regions, confirming that this unit is the centromeric specific repetitive sequence. These data suggested that the S-B1-RNase gene is located very close to (within a dista nce of 12 kb from) the centromeric-specific repetitive sequence. Likewise, the pBS-SB1B5 probe hybridized to the centromeric regions of all chromosome s in P littoralis, another Petunia species. However, the probe did not hybr idize to the centromere of the chromosomes from other species in Solanaceae . These results suggested that this centromeric repetitive sequence might b e a genus-specific one.