Sequence variability and gene structure at the self-incompatibility locus of Solanum tuberosum

Allelic complexity is a key feature of self-incompatibility (S) loci in gametophytic plants. We describe in this report the allelic diversity and gene structure of the S locus in Solanum tuberosum revealed by the isolation and characterization of genomic and cDNA clones encoding S-associated major pistil proteins from three alleles (S1, Sr1, S2). Genomic clones encoding the S1 and S2 proteins provide evidence for a simple gene structure: Two exons are separated by a small intron of 113 (S1) and 117 bp (S2). Protein sequences deduced from cDNA clones encoding S1 and Sr1 proteins show 95% homology, 15 of the 25 residues that differ between these S1 and Sr1 alleles are clustered in a short hypervariable protein segment (amino acid positions 44-68), which corresponds in the genomic clones to DNA sequences flanking the single intron. In contrast, these alleles are only 66% homologous to the S2 allele, with the residues that differ between the alleles being scattered throughout the sequence. DNA crosshybridization experiments identify a minimum of three classes of potato S alleles: one class contains the alleles S1, Sr1 and S3, the second class S2 and an allele of the cultivar Roxy, and the third class contains at present only S4. It is proposed that these classes reflect the origin of the S alleles from a few ancestral S sequence types.