The self-incompatibility (S) locus in Petunia hybrida is located on chromosome III in a region, syntenic for the Solanaceae

Seven independent transformants were previously shown to carry T-DNA insert s that are genetically linked to the self-incompatibility (S)-locus in Petu nia hybrida. These T-DNAs provided reliable targets for fluorescent in situ hybridisation (FISH) localisation. The T-DNA loci were found to be distrib uted over the entire long arm of chromosome ill. The most tightly linked T- DNA loci were located in a subcentromeric position. Independent confirmatio n of this assignment of the S-locus to chromosome III was obtained by RFLP analysis of a cDNA marker linked to the S-locus of potato (CP100). Tight li nkage was found between CP100 and a peroxidase isozyme locus previously map ped to chromosome ill. The co-localisation of CP100, peroxidase and the S-l ocus in petunia reveals synteny around the S-locus between four members of the Solanaceae family. The implications of a subcentromeric localisation of the S-locus are discussed in relation to chromosome fragments associated w ith self-compatible mutants.