Rejection of S-heteroallelic pollen by a dual-specific S-RNase in Solanum chacoense predicts a multimeric SI pollen component

S-heteroallelic pollen (HAP) grains are usually diploid and contain two dif ferent S-alleles. Curiously, HAP produced by tetraploids derived from self- incompatible diploids are typically self-compatible. The two different hypo theses previously advanced to explain the compatibility of HAP are the lack of pollen-S expression and the "competition effect" between two pollen-S g ene products expressed in a single pollen grain. To distinguish between the se two possibilities, we used a previously described dual-specific S-11/13- RNase, termed IIVapb-RNase, which can reject two phenotypically distinct po llen (P-11 and P-13). Since the HVapb-RNase does not distinguish between di e two pollen types (it recognizes both), P11P13 MP should be incompatible w ith the HVapb-RNase in spite of the competition effect. We show, here that P11P13 HAP is accepted by S11S13 styles, but is rejected by the S-11/13-RNa se, which demonstrates that the pollen-S genes must be expressed in HAP. A model involving tetrameric pollen-S is proposed to explain both the compati bility of P11P13 HAP on S11S13-containing styles and the incompatibility of P11P13 HAP on styles containing the HVapb-RNase.