Fine mapping of ui6.1, a gametophytic factor controlling pollen-side unilateral incompatibility in interspecific solanum hybrids.

Unilateral incompatibility (UI) is a prezygotic reproductive barrier in plants that prevents fertilization by foreign (interspecific) pollen through the inhibition of pollen tube growth. Incompatibility occurs in one direction only, most often when the female is a self-incompatible species and the male is self-compatible (the "SI x SC rule"). Pistils of the wild tomato relative Solanum lycopersicoides (SI) reject pollen of cultivated tomato (S. lycopersicum, SC), but accept pollen of S. pennellii (SC accession). Expression of pistil-side UI is weakened in S. lycopersicum x S. lycopersicoides hybrids, as pollen tube rejection occurs lower in the style. Two gametophytic factors are sufficient for pollen compatibility on allotriploid hybrids: ui1.1 on chromosome 1 (near the S locus), and ui6.1 on chromosome 6. We report herein a fine-scale map of the ui6.1 region. Recombination around ui6.1 was suppressed in lines containing a short S. pennellii introgression, but less so in lines containing a longer introgression. More recombinants were obtained from female than male meioses. A high-resolution genetic map of this region delineated the location of ui6.1 to approximately 0.128 MU, or 160 kb. Identification of the underlying gene should elucidate the mechanism of interspecific pollen rejection and its relationship to self-incompatibility.