Interspecific Pollen Rejection in Tomato and Relationship to Self-incompatibility

Cultivated tomato can by crossed with nearly all of its wild relatives, but many crosses succeed only if the cultivar is used as female.  In these cases, the reciprocal crosses are blocked by unilateral incompatibility (UI), a prezygotic reproductive barrier that prevents fertilization by foreign (interspecific) pollen through the inhibition of pollen tube growth.  UI occurs most often when the female is a self-incompatible species and the male is self-compatible (the ‘SI x SC rule’).  Pistils of Solanum lycopersicoides (SI) reject pollen of cultivated tomato (SC) in the upper style, whereas on pistils of S. lycopersicum x S. lycopersicoides hybrids, rejection occurs lower in the style, indicating a weakened UI. On allotriploid hybrids (i.e. two tomato and one S. lycopersicoides genomes), two pollen factors introgressed from S. pennellii are sufficient for full compatibility: ui1.1 on chromosome 1, and ui6.1 on chromosome 6. We identified the ui6.1 factor as a pollen-expressed Cullin1 (Cul1) gene with similarity to SI-related proteins reported in Petunia. Cultivated tomato and related red or orange-fruited species (all SC) exhibit the same loss of function mutation in this gene, whereas the green-fruited species (mostly SI) contain a functional allele. The ui1.1 factor has been mapped to a ~20 Mb region containing the S-locus. A large number of SLF-like genes were identified in this region, from which several are being evaluated as candidates for ui1.1.