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

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Abstract
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 (S 1, S r1, S 2). 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 (S 1) and 117 by (S 2). Protein sequences deduced from cDNA clones encoding S1 and Sr1 proteins show 95% homology. 15 of the 25 residues that differ between these S 1and 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 S 2allele, 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 S 1, S r1and S 3, the second class S 2and an allele of the cultivar Roxy, and the third class contains at present only S 4. It is proposed that these classes reflect the origin of the S alleles from a few ancestral S sequence types.