Pollen tube growth in the self-compatible sweet cherry genotype, ‘Cristobalina’, is slowed down after self-pollination

Abstract

Sweet cherry is a self-incompatible fruit tree species in the Rosaceae. As other species in the family, sweet cherry exhibits S-RNase-based gametophytic self-incompatibility. This mechanism is genetically determined by the S-locus that encodes the pollen and pistil determinants, SFB and S-RNase, respectively. Several self-compatible sweet cherry genotypes have been described and most of them have mutations at the S-locus leading to self-compatibility. However, ‘Cristobalina’ sweet cherry is self-compatible due to a mutation in a pollen function modifier that is not linked to the S-locus. To investigate the physiology of self-compatibility in this cultivar, S-locus segregation in crosses involving ‘Cristobalina’ pollen, and pollen tube growth in self- and cross-pollinations, were studied. In the crosses with genotypes sharing only one S-haplotype, the non-self S-haplotype was inherited more frequently than the self S-haplotype. Pollen tube growth studies revealed that the time to travel the whole length of the style was longer for self-pollen tubes than for cross-pollen tubes. Together, these results suggest that ‘Cristobalina’ pollen tube growth is slower after self-pollination than after cross-pollination. This reproductive strategy would allow self-fertilisation in the absence of compatible pollen but would promote cross-fertilisation if cross-compatible pollen is available, a possible case of cryptic self-incompatibility. This bet-hedging strategy might be advantageous for an ecotype that is native to the mountains of the Spanish Mediterranean coast, in the geographical limits of the distribution of this species. ‘Cristobalina’ blooming takes place very early in the season when mating possibilities are scarce and, consequently, self-compatibility may be the only possibility for this genotype to produce offspring.