TESTING THE SI x SC RULE: POLLEN-PISTIL INTERACTIONS IN INTERSPECIFIC CROSSES BETWEEN MEMBERS OF THE TOMATO CLADE (SOLANUM SECTION LYCOPERSICON, SOLANACEAE)

Premise of the study: Interspecific reproductive barriers (IRBs) act to ensure species integrity by preventing hybridization. Previous studies on interspecific crosses in the tomato clade have focused on the success of fruit and seed set. The SI x SC rule (SI species x SC species crosses are incompatible, but the reciprocal crosses are compatible) often applies to interspecific crosses. Because SI systems in the Solanaceae affect pollen tube growth, we focused on this process in a comprehensive study of interspecific crosses in the tomato clade to test whether the SI x SC rule was always followed. Methods: Pollen tube growth was assessed in reciprocal crosses between all 13 species of the tomato clade using fluorescence microscopy. Key results: In crosses between SC and SI species, pollen tube growth follows the SI x SC rule: interspecific pollen tube rejection occurs when SI species are pollinated by SC species, but in the reciprocal crosses (SC x SI), pollen tubes reach ovaries. However, pollen tube rejection occurred in some crosses between pairs of SC species, demonstrating that a fully functional SI system is not necessary for pollen tube rejection in interspecific crosses. Further, gradations in the strength of both pistil and pollen IRBs were revealed in interspecific crosses using SC populations of generally SI species. Conclusion: The SI x SC rule explains many of the compatibility relations in the tomato clade, but exceptions occur with more recently evolved SC species and accessions, revealing differences in strength of both pistil and pollen IRBs.