Rejection of S-Heteroallelic Pollen by a Dual-Specific S-RNase in Solanum chacoense Predicts a Multimeric SI Pollen Component

Corresponding author: Mario Cappadocia, Biology Department, University of Montreal, Montreal, Quebec H1X 2B2, Canada., mario.cappadocia{at}umontreal.ca (E-mail)

Communicating editor: D. CHARLESWORTH

S-heteroallelic pollen (HAP) grains are usually diploid and contain two different S-alleles. Curiously, HAP produced by tetraploids derived from self-incompatible diploids are typically self-compatible. The two different hypotheses previously advanced to explain the compatibility of HAP are the lack of pollen-S expression and the "competition effect" between two pollen-S gene products expressed in a single pollen grain. To distinguish between these two possibilities, we used a previously described dual-specific S_11/13-RNase, termed HVapb-RNase, which can reject two phenotypically distinct pollen (P₁₁ and P₁₃). Since the HVapb-RNase does not distinguish between the two pollen types (it recognizes both), P₁₁P₁₃ HAP should be incompatible with 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-RNase, which demonstrates that the pollen-S genes must be expressed in HAP. A model involving tetrameric pollen-S is proposed to explain both the compatibility of P₁₁P₁₃ HAP on S₁₁S₁₃-containing styles and the incompatibility of P₁₁P₁₃ HAP on styles containing the HVapb-RNase.

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