Unilateral incompatibility gene ui1.1 encodes an S-locus F-box protein expressed in pollen of Solanum species

Abstract

Unilateral interspecific incompatibility (UI) is a postpollination, prezygotic reproductive barrier that prevents hybridization between related species when the female parent is self-incompatible (SI) and the male parent is self-compatible (SC). In tomato and related Solanum species, two genes, ui1.1 and ui6.1, are required for pollen compatibility on pistils of SI species or hybrids. We previously showed that ui6.1 encodes a Cullin1 (CUL1) protein. Here we report that ui1.1 encodes an S-locus F-box (SLF) protein. The ui1.1 gene was mapped to a 0.43-cM, 43.2-Mbp interval at the S-locus on chromosome 1, but positional cloning was hampered by low recombination frequency. We hypothesized that ui1.1 encodes an SLF protein(s) that interacts with CUL1 and Skp1 proteins to form an SCF-type (Skp1, Cullin1, F-box) ubiquitin E3 ligase complex. We identified 23 SLF genes in the S. pennellii genome, of which 19 were also represented in cultivated tomato (S. lycopersicum). Data from recombination events, expression analysis, and sequence annotation highlighted 11 S. pennellii genes as candidates. Genetic transformations demonstrated that one of these, SpSLF-23, is sufficient for ui1.1 function. A survey of cultivated and wild tomato species identified SLF-23 orthologs in each of the SI species, but not in the SC species S. lycopersicum, S. cheesmaniae, and S. galapagense, pollen of which lacks ui1.1 function. These results demonstrate that pollen compatibility in UI is mediated by protein degradation through the ubiquitin-proteasome pathway, a mechanism related to that which controls pollen recognition in SI.

Keywords: S-locus F-box protein; Solanum lycopersicum; Solanum pennellii; interspecific incompatibility; self-incompatibility.

Fig. 1.

Genetic and physical maps of the ui1.1 region, showing positions of SLF genes in S. lycopersicum and S. pennellii. (A) Genetic map of the S-locus region on chromosome 1, from the EXPEN 2000 interspecific mapping population. (B) Genetic map of the ui1.1 region from a BC mapping population of 1,632 individuals. Recombination suppression reduced the genetic distance of this region to less than 25% of the reference map. (C) Physical map of markers based on the Tomato WGS Chromosomes (ver. SL2.31). (D) Physical map of SLF genes identified in the Tomato WGS Chromosomes. Note: 19 SLF genes were found this region, of which 4 were ruled out as ui1.1 candidates based on the positions of flanking markers. Open ovals, genes with likely loss-of-function mutations; shaded ovals, putative functional genes. (E) SLF genes identified in the S. pennellii genome. Hatched ovals, four S. pennellii SLF genes not present in S. lycopersicum.

Fig. 2.

Analysis of SLF mRNA levels by RT-PCR. Leaves and pollen of S. lycopersicum cv. VF36 and S. pennellii LA0716 were compared for expression levels of SLF genes. Pollen-specific expression was observed for all genes except four located outside the ui1.1 region. The constitutively expressed Actin gene is included as a control.

Fig. 3.

Compatibility phenotypes of SpSLF-23 transgenic plants on pistils of an allotriploid tester line. T₀ transgenic plants showed a compatible phenotype on pistils of allotriploid tester lines, demonstrating that SpSLF-23 is sufficient to confer ui1.1 function in pollen. The compatible phenotype cosegregated with the transgene in the T₁ generation. T₀, a representative T₀ transgenic plant; T₁(−), a T₁ plant lacking the transgene; T₁(+), a T₁ plant with the transgene. (Scale bar: 1 mm.) (Full data are in SI Appendix, Table S8.)

Fig. 4.

Survey of cultivated and wild tomato species for the presence or absence of SLF-23 orthologs. Five accessions from each species were tested for the presence of SLF-23 using gene-specific primers (SI Appendix, Table S2). The SC red- or orange-fruited species S. lycopersicum, S. galapagense, and S. cheesmaniae lack SLF-23 orthologs.