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, S_r1, 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 by (S₂). Protein sequences deduced from cDNA clones encoding S₁ and S_r1 proteins show 95% homology. 15 of the 25 residues that differ between these S₁and S_r1alleles 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 S₂allele, 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, S_r1and S₃, the second class S₂and 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.