Self‐incompatibility of <i>Prunus tenella</i> and evidence that reproductively isolated species of <i>Prunus</i> have different SFB alleles coupled with an identical <i>S</i> ‐RNase allele

<jats:title>Summary</jats:title> <jats:p> Many species of <jats:italic>Prunus</jats:italic> display an <jats:italic>S</jats:italic> ‐RNase‐based gametophytic self‐incompatibility (SI), controlled by a single highly polymorphic multigene complex termed the <jats:italic>S</jats:italic> ‐locus. This comprises tightly linked stylar‐ and pollen‐expressed genes that determine the specificity of the SI response. We investigated SI of <jats:italic>Prunus tenella</jats:italic> , a wild species found in small, isolated populations on the Balkan peninsula, initially by pollination experiments and identifying stylar‐expressed <jats:italic>RNase</jats:italic> alleles. Nine <jats:italic>P. tenella S</jats:italic> ‐ <jats:italic>RNase</jats:italic> alleles ( <jats:italic>S</jats:italic> <jats:sub> <jats:italic>1</jats:italic> </jats:sub> – <jats:italic>S</jats:italic> <jats:sub> <jats:italic>9</jats:italic> </jats:sub> ) were cloned; their sequence analysis showed a very high ratio of non‐synonymous to synonymous nucleotide substitutions ( <jats:italic>K</jats:italic> <jats:sub>a</jats:sub> / <jats:italic>K</jats:italic> <jats:sub>s</jats:sub> ) and revealed that <jats:italic>S</jats:italic> ‐ <jats:italic>RNase</jats:italic> alleles of <jats:italic>P. tenella</jats:italic> , unlike those of <jats:italic>Prunus dulcis</jats:italic> , show positive selection in all regions except the conserved regions and that between C2 and RHV. Remarkably, <jats:italic>S</jats:italic> <jats:sub> <jats:italic>8</jats:italic> </jats:sub> ‐RNase, was found to be identical to <jats:italic>S</jats:italic> <jats:sub> <jats:italic>1</jats:italic> </jats:sub> ‐RNase from <jats:italic>Prunus avium</jats:italic> , a species that does not interbreed with <jats:italic>P. tenella</jats:italic> and, except for just one amino acid, to <jats:italic>S</jats:italic> <jats:sub> <jats:italic>11</jats:italic> </jats:sub> of <jats:italic>P. dulcis.</jats:italic> However, the corresponding introns and <jats:italic>S‐RNase</jats:italic> – <jats:italic>SFB</jats:italic> intergenic regions showed considerable differences. Moreover, protein sequences of the pollen‐expressed SFB alleles were not identical, harbouring 12 amino‐acid replacements between those of <jats:italic>P. tenella</jats:italic> SFB <jats:sub>8</jats:sub> and <jats:italic>P. avium</jats:italic> SFB <jats:sub>1</jats:sub> . Implications of this finding for hypotheses about the evolution of new <jats:italic>S</jats:italic> ‐specificities are discussed. </jats:p>