Approaching the self-incompatibility locus Z in rye (Secale cereale L.) via comparative genetics.

Using barley and wheat expressed sequence tags as well as rice genomic sequence and mapping information, we revisited the genomic region encompassing the self-incompatibility (SI) locus Z on rye chromosome 2RL applying a comparative approach. We were able to arrange 12 novel sequence-tagged site (STS) markers around Z, spanning a genetic distance of 32.3 cM, with the closest flanking markers mapping at a distance of 0.5 cM and 1.0 cM from Z, respectively, and one marker cosegregating with Z, in a testcross population of 204 progeny. Two overlapping rice bacterial artifical chromosomes (BACs), OSJNBa0070O11 and OSJNBa0010D21, were found to carry rice orthologs of the three rye STS markers from the 1.5-cM interval encompassing Z. The STS-marker orthologs on these rice BACs span less than 125,000 bp of the rice genome. The STS marker TC116908 cosegregated with Z in a mapping population and revealed a high degree of polymorphism among a random sample of rye plants of various origin. TC116908 was shown via Southern hybridization to correspond to gene no. 10 (OSJNBa0070O11.10) on rice BAC OSJNBa0070O11. Reverse transcription-PCR with a TC116908-specific primer pair resulted in the amplification of a fragment of the expected size from the rye pistil but not from leaf cDNA. OSJNBa0070O11.10 was found to show a highly significant sequence similarity to AtUBP22, a ubiquitin-specific protease (UBP). TC116908 likely represents a putative UBP gene that is specifically expressed in rye pistils and cosegregates with Z. Given that the ubiquitination of proteins is emerging as a general mechanism involved in different SI systems of plants, TC116908 appears to be a promising target for further investigation with respect to its relation to the SI system of the grasses.