Self-incompatibility is a naturally occuring process–a plant will reject the pollen from a genetically related plant. It is controlled by a genetic system that allows flowering plants to recognize pollen that is too similar to its own, preventing self-fertilization. Because many commercial almond cultivars are related, it is important for growers to understand the underlying genetics that determine which cultivars can effectively cross-pollinate. Incompatible cultivars, when planted together, may not produce a marketable crop.

The almond cultivar 'Jeffries', a natural mutant of the cultivar 'Nonpariel', has an unusual characteristic: it will accept pollen from any cultivar except itself. Thus, it can serve as pollinator for many other almond cultivars. Research demonstrated that self-incompatibility is controlled by which gene variants occur at a specific location on the chromosome, the 'S-locus'. The S-locus genes are found to occur as different types, called haplotypes, which recognize different pollen characteristics.

Team members investigated the nature of the mutations that occurred at the 'Nonpariel' S-locus to produce 'Jeffries'. They determined the DNA sequence for a region on the S-locus of 'Jeffries'. This region of DNA codes for the S-RNase gene, which must be expressed for an almond flower to recognize and reject self-pollen. This research identified a likely candidate for another S-locus gene, the 'S pollen' gene, using PCR (polymerase chain reaction). PCR, a process in which a particular DNA sequence is rapidly copied, provides a large sample for genetic analysis.

Once the genetic system was understood, PCR was used to determine the DNA sequence at the S-locus for 44 popular California almond cultivars. Based on the sequences, the 44 cultivars were then assigned to cross-incompatibility groups (CIGs).

Table of Cross Incompatibility Groups of Almond Cultivar

Publications

Barckley, K.K., S.L. Uratsu, T.M. Gradziel and A.M. Dandekar. 2006. Multidimensional analysis of S-alleles from cross-incompatible groups of California almond cultivars. J. Am. Soc. Hort. Sci. 131: 632-636.

Kiochiro Ushijima, K., H. Sassa, A.M. Dandekar, T.M. Gradziel, R. Tao, and H. Hirano. 2003. Structural and transcriptional analysis of self-incompatibility (S) locus of almond (Prunus dulcis): identification of a pollen-expressed F-box gene with haplotype-specific polymorphism. Plant Cell 15(3): 771-781.

Ushijima, K., H. Sassa, M. Tamura, M. Kusaba, R. Tao, T.M. Gradziel, A.M. Dandekar and H. Hirano. 2001. Characterization of the S-locus region of almond (Prunus dulcis): analysis of a somaclonal mutant and a cosmid contig of an S haplotype. Genetics 158:179-186.