| Gametophytic self-incompatibility (GSI) is a common mechanism for preventing inbreeding in flowering plants. Typically, GSI in diploid species breaks down due to polyploidy resulting in self-compatible (SC) tetraploid species. The diploid sweet cherry and the tetraploid sour cherry represent an exception, as sour cherry individuals can be either self-incompatible (SI) or SC. SI is undesirable for cultivation due to the inefficiencies of growing pollinator varieties and the reliance on bees to ensure adequate cross-pollination. Therefore, sour cherry breeders should develop SC selections. Without an understanding of the genetic control of SI and SC in sour cherry, breeders are not able to predict the SI or SC phenotype of seedlings prior to the production of flowers, which typically occurs 3--5 years after planting. The availability of molecular markers to predict the SI or SC phenotype of a seedling could save valuable field space and evaluation time. The goal of this dissertation was to determine the genetic control of SI and SC in sour cherry. To do this, it was first necessary to determine which S-haplotypes exist in sweet cherry, one of the progenitors of sour cherry. RFLP analyses were used to determine the banding profiles for 14 sweet cherry S-haplotypes. Sour cherry was then found to contain six of these sweet cherry S-haplotypes ( S1, S4, S6, S9, S12 and S13) in addition to six unique S-haplotypes (S6c, S6m, Sa, Sh, S d and Se). Using inter-specific crosses between sweet and sour cherry and self-pollinations of sour cherry, four of the six shared S-haplotypes (S1, S4, S6, and S9) and one of the unique S-haplotypes (Sb) were shown to be functional and capable of accomplishing S-haplotype-specific rejection of pollen. The other S-haplotypes in sour cherry (S13, S6c, S6m, Sa, S d, and Se) were shown to be non-functional and incapable of initiating S-haplotype-specific rejection of pollen. Finally, a hypothesis regarding the genetic control of SI and SC in sour cherry was developed through the analysis of S-haplotype segregation in 794 progeny from six sour cherry self-populations and 15 inter-specific crosses between sweet and sour cherry. SI and SC predictions were verified using additional self-pollination and crossing experiments. The data suggests that the partial breakdown of SI in sour cherry is due to the accumulation of non-functional S-haplotypes that are incapable of S-haplotype-specific rejection of pollen, rather than due to the competition between pollen- S products in heteroallelic pollen, which is commonly observed in the Solanaceae. The implications of these findings on sour cherry breeding and on our knowledge of GSI are discussed. |
