SELF-INCOMPATIBILITY IN THE GRASSES - EVOLUTIONARY RELATIONSHIP OF THE S-GENE FROM PHALARIS-COERULESCENS TO HOMOLOGOUS SEQUENCES IN OTHER GRASSES

Self-incompatibility is widespread in the grasses and it is proposed t hat the grasses share a common incompatibility mechanism that is disti nct from those operating in the dicotyledonous species studied in grea t detail. Where good genetic data are available, all grass species app ear to have an incompatibility mechanism controlled by two unlinked lo ci, S and Z. A putative S gene has been cloned from Phalaris coerulesc ens. This gene is characterized by two major domains: an allele specif icity domain and a thioredoxin catalytic domain. A family of sequences with varying degrees of homology to this gene has been identified amo ng 15 grass species covering all subfamilies of the Poaceae. These S-r elated sequences appear to be present in the grass family regardless o f self-compatibility. Evidence is presented to show that at least one of the sequences is transcribed, suggesting a functional gene. In cont rast to the high expression of the S gene in Phalaris pollen, expressi on of the related gene in the pollen (or anthers) of the grass species examined was so low that RNA gel blot analysis failed to display a si gnificant signal. However, reverse transcription-based polymerase chai n reaction (RT-PCR) successfully amplified the region corresponding to the S thioredoxin domain from 10 of the grass species. With grasses o ther than Phalaris, RT-PCR showed limited success in amplifying the re gion corresponding to the S variable portion at the 5' end of the Phal aris S gene. Sequencing of the PCR-amplified S thioredoxin region from wheat, barley, rye and Dactylis revealed that this is a highly conser ved gene with 93-97% sequence similarity with the corresponding Phalar is S gene. The conservation of sequence and ubiquitous expression of t he gene across the grass family strongly suggest that the S-related ge ne is carrying out a significant biological function in the Poaceae. O n the basis of these findings, a model for the evolution of the S self -incompatibility gene in the grasses is proposed.