Pollen Compatibility Research Articles

Island plants with newly discovered reproductive traits have higher capacity for uniparental reproduction, supporting Baker’s law

Uniparental reproduction is advantageous when lack of mates limits outcrossing opportunities in plants. Baker’s law predicts an enrichment of uniparental reproduction in habitats colonized via long-distance dispersal, such as volcanic islands. To test it, we analyzed reproductive traits at multiple hierarchical levels and compared seed-set after selfing and crossing experiments in both island and mainland populations of Limonium lobatum, a widespread species that Baker assumed to be self-incompatible because it had been described as pollen-stigma dimorphic, i.e., characterized by floral morphs differing in pollen-surface morphology and stigma-papillae shape that are typically self-incompatible. We discovered new types and combinations of pollen and stigma traits hitherto unknown in the literature on pollen-stigma dimorphism and a lack of correspondence between such combinations and pollen compatibility. Contrary to previous reports, we conclude that Limonium lobatum comprises both self-compatible and self-incompatible plants characterized by both known and previously undescribed combinations of reproductive traits. Most importantly, plants with novel combinations are overrepresented on islands, selfed seed-set is higher in islands than the mainland, and insular plants with novel pollen-stigma trait-combinations disproportionally contribute to uniparental reproduction on islands. Our results thus support Baker’s law, connecting research on reproductive and island biology.

The influence of the pollination compatibility type on the pistil S-RNase expression in European pear (Pyrus communis).

The S-RNase gene plays an essential role in the gametophytic self-incompatibility (GSI) system of Pyrus. It codes for the stylar-expressed S-RNase protein which inhibits the growth of incompatible pollen tubes through cytotoxicity and the induction of programmed cell death in the pollen tube. While research on the Pyrus GSI system has primarily focused on the S-RNase gene, there is still a lack of insight into its spatiotemporal expression profile and the factors that regulate it. Previous studies have suggested that S-RNase expression in the style is influenced by pollination and is dependent on the compatibility type. We here continue on this basic hypothesis by analyzing the spatiotemporal expression of the S-RNase alleles in Pyrus communis "Conference" styles in response to different types of pollination; namely, upon full- and semi-compatible pollination and upon incompatible selfing. The results revealed that temporal dynamics of S-RNase expression are influenced by the pollen's compatibility type, indicating the presence of a signaling mechanism between pollen and style to control S-RNase production during pollen tube growth. In our experiment, S-RNase expression continuously decreased after cross-pollination and in the unpollinated control. However, after a fully incompatible pollination, S-RNase expression remained constant. Finally, semi-compatible pollination showed a initially constant S-RNase expression for both alleles followed by a strong decrease in expression. Based on these results and previous findings, we propose a regulatory mechanism to explain the effect of pollination and the associated compatibility type on S-RNase expression in the style. This proposed mechanism could be used as a starting point for future research.

Three types of genes underlying the Gametophyte factor1 locus cause unilateral cross incompatibility in maize

Unilateral cross incompatibility (UCI) occurs between popcorn and dent corn, and represents a critical step towards speciation. It has been reported that ZmGa1P, encoding a pectin methylesterase (PME), is a male determinant of the Ga1 locus. However, the female determinant and the genetic relationship between male and female determinants at this locus are unclear. Here, we report three different types, a total of seven linked genes underlying the Ga1 locus, which control UCI phenotype by independently affecting pollen tube growth in both antagonistic and synergistic manners. These include five pollen-expressed PME genes (ZmGa1Ps-m), a silk-expressed PME gene (ZmPME3), and another silk-expressed gene (ZmPRP3), encoding a pathogenesis-related (PR) proteins. ZmGa1Ps-m confer pollen compatibility. Presence of ZmPME3 causes silk to reject incompatible pollen. ZmPRP3 promotes incompatibility pollen tube growth and thereby breaks the blocking effect of ZmPME3. In addition, evolutionary genomics analyses suggest that the divergence of the Ga1 locus existed before maize domestication and continued during breeding improvement. The knowledge gained here deepen our understanding of the complex regulation of cross incompatibility.

Pollen viability and incompatibility in indigenous rice bean (Vigna umbellata (Thunb.) Ohwi & Ohashi)

Pollen viability, germination and compatibility are essential in determining the success of pollination and seed setting of high-valued crops. Rice bean (Vigna umbellata (Thunb.) Ohwi & Ohashi) is an underutilized and unexplored indigenous legume with high potential for commercial production. In this study, pollen quality, viability, germination rate and incompatibility among selected six rice bean (V. umbellata) accessions from Barili, Cebu, Philippines were evaluated to determine the barriers and effective pollination habit for increased productivity while retaining the important traits, including high tolerance in poor soils, superior climatic resilience and resistance to pest and diseases. Results of acetocarmine calorimetric assay showed that rice beans’ (V. umbellata) pollens are highly viable, with accessions VU 004 (56.33 ± 4.91%) and VU 007 (54.34 ± 4.53%) having the optimum viability rate. Brewbaker and Kwack medium treated with 0.2 g.l-1 and 0.3 g.l-1 boric acid (H3BO4) enhanced the germination rate in vitro (11.56 ± 5.53% and 9.47 ± 6.50% respectively). Bud (14.96 ± 1.53%) and post-anthesis pollens (10.28 ± 0.94%) have optimum germination rate in 0.2 g.l-1 boric acid media, while anthesis pollens are suitable in media supplemented with sucrose and boric acid alone (12.20 ± 1.50%) and with 0.1 g.l-1 myo-inositol supplementation (8.49 ± 1.86%). Pollination test revealed that rice bean accessions have high self-compatibility (50.76 + 3.45%) and low cross-compatibility (26.57 + 2.49%). The findings provide an important background in understanding the pollen quality and intraspecific interaction among indigenous rice bean (V. umbellata) accessions in Barili, Cebu to improve production and hybridization.

‘Chemlali Mhassen’: New olive cultivar derived from crossbreeding program in Tunisia with high oil quality and productivity

The new olive cultivar ‘Chemlali Mhassen’ was derived from the autopollination of the Tunisian oil cultivar ‘Chemlali Sfax’. The main morphological differences between the two cultivars were observed on the endocarp (symmetry, position of maximum diameter, apex, base and surface). On the agronomic plan, this cultivar is distinguishable from the original cultivar due to its medium earliness of bearing (4 years), medium alternate bearing (0.44), early ripening, moderate sensitivity to verticillium and its high olive production per tree (7.7 kg). Concerning oil quality, ‘Chemlali Mhassen’ had higher performances than the original cultivar for oleic acid content (70 to 77 %) and lower contents for palmitic acid (9.2 to 11.5 %) and linoleic acid (9.3 to 14.7 %). Similar performances were recorded between the new and the original cultivars for rhizogenesis behavior and pollen compatibility.

Pollination Compatibility and Xenia in Camellia oleifera

Camellia oleifera, a major woody oil plant, has a low oil yield because of self-incompatibility. For commercial oil production, compatible pollen and optimal cross-pollination combinations are required. To evaluate the effects of pollination compatibility and pollen source on oil yield and quality, four C. oleifera cultivars—Huashuo (HS), Huajin (HJ), Huaxin (HX), and Xianglin XLC15 (XL)—were subjected to self-, cross-, and natural pollination. Pollen compatibility, oil yield, and quality indices were analyzed. There were no significant differences in pollen germination and tube growth between self- and cross-pollination. Following self-pollination, fertilization was unsuccessful, resulting in severe ovule dysplasia; cross-pollination decreased the ovule abortion rate. Pollen source significantly affected the fruit set, fruit traits, seed traits, and fatty acid content, implying xenia in C. oleifera. In cross-pollinated plants, HX pollen produced more seeds, and HJ pollen increased linoleic acid content relative to naturally pollinated plants. For the XL and HS combinations, linolenic acid contents were significantly higher than other pollination combinations. However, oleic acid content was not significantly affected by pollen source, in any of the cultivars. Cultivar HX was, therefore, the most effective pollen donor, and HS × HX was the optimal cross-pollination combination for improving oil yield and sustainability.

Pattern of inheritance of a self‐fertility gene in an autotetraploid perennial ryegrass (Lolium perenne) population

AbstractA mutation causing self‐fertility (SF) in perennial ryegrass was studied at the tetraploid level. The aim of this work was to determine a) whether SF remains functional in a tetraploid population and b) whether the SF mutation expresses dominance in heterozygous pollen grains. A tetraploidized plant carrying SF alleles was self‐pollinated to create a segregating F2 population. In the F2 individuals, pollen compatibility ranged between 38% and 84% showing that SF remained functional. The SF locus genotype was the main determinant of pollen compatibility explaining 78% of the variation. The observed segregation was significantly different from the expected under both SF being dominant or recessive models (P(χ2)≤0.001) and tended to be intermediate between them, indicating partial dominance or additive gene action. The frequency of the different genotypes suggested that pollen grains homozygous for the mutation have a competitive advantage over heterozygous pollen and that pollen compatibility is affected by the interaction with additional loci. The implications of our results for breeding polyploid grasses are discussed.

Unilateral incompatibility is linked to reduced pollen expression of a farnesyl pyrophosphate synthase.

Unilateral incompatibility is linked to reduced pollen expression of a farnesyl pyrophosphate synthase.

Fine mapping a self-fertility locus in perennial ryegrass.

A self-fertility locus was fine mapped to a 1.6cM region on linkage group 5 in a perennial ryegrass population. This locus was the main determinant of pollen self-compatibility. In grasses, self-incompatibility (SI) is characterized by a two-loci gametophytic (S and Z) mechanism acting together in the recognition and inhibition of self-pollen. Mutations affecting the expression of SI have been reported in a few grass species. In perennial ryegrass (Lolium perenne L.), a mutation independent from S and Z, and mapping on linkage group 5 (LG 5), was previously reported to produce self-fertile plants. Here, we describe fine mapping of the self-fertility (SF) gene in a perennial ryegrass population and determine whether there is any effect of other genomic regions on the pollen compatibility. The phenotypic segregation of SF showed a bimodal distribution with one mean at 49% pollen compatibility and the other at 91%. Marker-trait association analysis showed that only markers on LG 5 were significantly associated with the trait. A single gene model explained 82% of the observed variability and no effects of the other regions were detected. Using segregation and linkage analysis, the SF locus was located to a 1.6cM region on LG 5. The flanking marker sequences were aligned to rice and Brachypodium distachyon reference genomes to estimate the physical distance. We provide markers tightly linked to SF that can be used for introgression of this trait into advanced breeding germplasm. Moreover, our results represent a further step towards the identification of the SF gene in LG 5.

Simulation-based modeling of wild blueberry pollination

The high variability of wild (lowbush) blueberry plants in spatial and genetic structure, in combination with bee foraging behavior varying between species, and the complexity of these factors interacting over time and space, are major obstacles to understanding of pollination dynamics subject to environmental change. The bottom-up modeling paradigm provides an ideal approach to bridging the gap between known mechanisms of individual organisms and unknown spatial–temporal dynamics of pollination at the field scale. By linking empirical data to stochastically-based ecological process modeling, we present a spatially-explicit agent-based simulation model that enables exploration of how various factors, including plant spatial arrangements, outcrossing and self-pollination, bee species compositions and weather conditions, in isolation and combination, affect pollination efficiency throughout a blueberry bloom season. The firmly validated open-source model is a useful tool for hypothesis testing and theory development for wild blueberry pollination researches. Sensitivity analysis suggested that fruit set and resulting measures of productivity such as fruit mass and viable seeds per fruit were sensitive to parameterization of blueberry genotype or clone size and the amount of blueberry plant cover in a field. Fruit set due to pollen compatibility was sensitive to ovule number per flower and foraging bee density. Simulation experiments allowed us to compare bee pollination efficiencies at the bee taxon population level (honey bees, bumble bees, and native solitary bees), the effect of foraging distance from bee nest or colony site on fruit set, and test whether the mechanism of gametophytic self-incompatibility (pre- vs. post-zygotic decision making by the plant) in wild blueberry pollination at the field level matters in estimating yield.

Evolutionary history of two pollen self-incompatibility factors reveals alternate routes to self-compatibility within Solanum.

Self-incompatibility (SI) prevents self-fertilization and reduces inbreeding. While SI is common in plants, transitions to self-compatibility (SC) occur frequently. Little is known about the genetic changes and evolutionary steps underlying these shifts. In the Solanaceae, SI is gametophytic, with specificity determined by S-RNases in the pistil and S-locus F-box proteins (SLFs) in pollen. We examined the role of two pollen factors, Cullin1 (CUL1) and SLF-23, in SI → SC transitions in wild tomato species from the Arcanum species group (Solanum arcanum, S. neorickii, and S. chmielewskii). Pollen compatibility was assessed on tester lines that reject pollen lacking functional SLF-23 or CUL1. Complementation tests, gene sequencing, and phylogenetic analyses were used to characterize both functional and nonfunctional alleles. We found evidence for multiple independent SI → SC transitions. In S. arcanum and S. chmielewskii, SC is caused by loss of pistil S-RNase activity, while in S. neorickii SC is associated with expression of a functional SLF-23 that recognizes the S9 type S-RNase expressed in its pistils. Interestingly, we found identical deletion mutations in CUL1 exon 7 of S. chmielewskii as previously seen in S. habrochaites. Mating system transitions in the Arcanum group have occurred via both pistil loss-of-function and pollen gain-of-function SC mutations. Mutations common to S. chmielewskii and S. habrochaites must have arisen in a common ancestor, possibly to the entire tomato clade, then became fixed in different lineages after loss of pistil-side SI function.

Pentaploid Wheat Hybrids: Applications, Characterisation, and Challenges.

Interspecific hybridisation between hexaploid and tetraploid wheat species leads to the development of F1 pentaploid hybrids with unique chromosomal constitutions. Pentaploid hybrids derived from bread wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum spp. durum Desf.) crosses can improve the genetic background of either parent by transferring traits of interest. The genetic variability derived from bread and durum wheat and transferred into pentaploid hybrids has the potential to improve disease resistance, abiotic tolerance, and grain quality, and to enhance agronomic characters. Nonetheless, pentaploid wheat hybrids have not been fully exploited in breeding programs aimed at improving crops. There are several potential barriers for efficient pentaploid wheat production, such as low pollen compatibility, poor seed set, failed seedling establishment, and frequent sterility in F1 hybrids. However, most of the barriers can be overcome by careful selection of the parental genotypes and by employing the higher ploidy level genotype as the maternal parent. In this review, we summarize the current research on pentaploid wheat hybrids and analyze the advantages and pitfalls of current methods used to assess pentaploid-derived lines. Furthermore, we discuss current and potential applications in commercial breeding programs and future directions for research into pentaploid wheat.

Mutations in two pollen self-incompatibility factors in geographically marginal populations of Solanum habrochaites impact mating system transitions and reproductive isolation.

Self-incompatibility (SI) is a mechanism that prevents inbreeding in many plant species. The mutational breakdown of SI occurs frequently, yet relatively little is known about the evolutionary steps involved in the progressive loss of pistil and pollen SI function. In Solanaceae, SI is the S-RNase-based gametophytic type. We used SI and SC populations of the wild tomato species Solanum habrochaites to study natural variation for two pollen SI factors: a Cullin1 (CUL1) protein and an S-locus F-box protein (SLF-23). Pollen compatibility was assessed on an allotriploid tester line encoding an S-RNase recognized by SLF-23. Both pollen factors are required for compatibility on this tester line. Complementation tests and gene sequencing were used to identify mutations in CUL1 or SLF-23. We detected loss-of-function mutations in CUL1 and/or SLF-23 in SC populations collected near the northern and southern geographic margins of this taxon's natural range. Nonmarginal SC and all SI accessions expressed mostly functional alleles of these pollen factors. Comparison of the CUL1 sequences identified several shared deletion mutations present in both northern and southern margin SC accessions. Loss-of-function mutations in CUL1 and SLF-23 likely became fixed relatively late during SI to SC transitions, after loss of pistil SI function. Mutations in CUL1 establish unilateral incompatibility with SI populations and strengthen reproductive isolation. Point mutations common to northern and southern SC biotypes likely derive from shared ancestral variants found in more central SI populations.

HIBRIDAÇÕES CONTROLADAS INTER E INTRAESPECÍFICAS PARA O MELHORAMENTO GENÉTICO DE PORTA-ENXERTOS DE PEREIRA

RESUMO O objetivo da pesquisa foi a averiguação da compatibilidade de pólen em pereira com marmeleiro (hibridações interespecíficas)e com cultivares de pereira (hibridações intraespecíficas). Foram usadas: ‘Packham’s Triumph’ (genitorfeminino) e ‘William’s’ e ‘Clapp’s Favorite’, ‘Maçã’ e ‘Portugal’ (genitores masculinos). Foi determinada frutificação efetiva (%) no campo aos 40 dias após as hibridações e a germinação in vivopor meio deatribuição de notas, seguindo escala de posição do tubo polínico no ovário. Obteve-se maior frutificaçãoefetiva entre as pereiras ‘Packham’s Triumph’ x ‘Clapp’s Favorite’. Nos cruzamentos interespecíficos, a maiorfrutificação efetiva foi obtida entre ‘Packham’s Triumph’ x ‘Portugal’. Na germinação in vivo, o estádio de desenvolvimento do tubo polínico apresentou variações significativas entre as combinações, porém não se obteve penetração de tubo polínico no óvulo. Como se observou frutificação em todos os cruzamentos, existe possível compatibilidade entre os genótipos estudados. Os resultados indicam que as cultivares doadoras de pólen da espécie P. communis são compatíveis com ‘Packham’s Triumph’. Outros estudos devem ser realizados com pólen de marmeleiro a fim de confirmar essa compatibilidade.

A preliminary study on pollen compatibility of some hazelnut cultivars in Iran

Pollen incompatibility is a major problem among hazelnut cultivars and can result in considerable crop loss in hazelnut orchards. Thus, identifying the level of self-compatibility of a cultivar and compatibility between cultivars are crucial aspects to selecting the proper pollenizers. Controlled self- and cross-pollinations were carried out in three local and four imported hazelnut cultivars. Based on cluster set, partial self-compatibility was found in cultivars Pashmine and Shastak, and complete self-compatibility in ‘Tabestane’. The best pollenizers for the cultivars Pashmine, Tabestane, Shastak, Barcelona, Segorbe, Daviana and Merveille were Segorbe, Barcelona, Segorbe, Shastak, Shastak, Pashmine and, Pashmine, respectively.

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

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.

PollenCALC: Software for estimation of pollen compatibility of self-incompatible allo- and autotetraploid species

BackgroundSelf-incompatibility (SI) is a biological mechanism to avoid inbreeding in allogamous plants. In grasses, this mechanism is controlled by a two-locus system (S-Z). Calculation of male and female gamete frequencies is complex for tetraploid species. We are not aware of any software available for predicting pollen haplotype frequencies and pollen compatibility in tetraploid species.ResultsPollenCALC is a software tool written in C++ programming language that can predict pollen compatibility percentages for polyploid species with a two-locus (S, Z) self-incompatibility system. The program predicts pollen genotypes and frequencies based on defined meiotic parameters for allo- or autotetraploid species with a gametophytic S-Z SI system. These predictions can be used to obtain expected values for for diploid and for (allo- or autotetraploidy SI grasses.ConclusionThe information provided by this calculator can be used to predict compatibility of pair-crosses in plant breeding applications, to analyze segregation distortion for S and Z genes, as well as linked markers in mapping populations, hypothesis testing of the number of S and Z alleles in a pair cross, and the underlying genetic model.

The relationship between reproductive growth and blank fruit formation in Corylus heterophylla Fisch

The reason leading to the high blank fruit ratio of Corylus heterophylla Fisch was elucidated by investigating pollen compatibility, ovule and embryo development. It was showed that the female flowers bloomed in the middle of April and the ovary did not develop until 1 month later. In late May, two ovules were found in an ovary. On 22 June, the ovule wall differentiated into integument, and nucellus endosperm were clearly observed in the ovules, suggesting the starting of fertilization event. Globular, heart, torpedo and cotyledon embryo developed step by step from 28 June to 12 July. The ovule grew rapidly in filled fruit since 12 July. Ovule in blank fruit ceased growth from 12 July although the full embryo with cotyledon could be observed. The blank fruit could be distinguished from the filled one for its undeveloped ovule and large amount of parenchyma. There was no significant difference in ovary size between the filled and empty nuts, but the weight of blank nut was only about one half of the filled one. It is concluded that formation of blank fruit of C. heterophylla Fisch is closely related to embryo abortion, but not incompatibility between the pollen and stigma.

Compatibility and incompatibility in S-RNase-based systems.

S-RNase-based self-incompatibility (SI) occurs in the Solanaceae, Rosaceae and Plantaginaceae. In all three families, compatibility is controlled by a polymorphic S-locus encoding at least two genes. S-RNases determine the specificity of pollen rejection in the pistil, and S-locus F-box proteins fulfill this function in pollen. S-RNases are thought to function as S-specific cytotoxins as well as recognition proteins. Thus, incompatibility results from the cytotoxic activity of S-RNase, while compatible pollen tubes evade S-RNase cytotoxicity. The S-specificity determinants are known, but many questions remain. In this review, the genetics of SI are introduced and the characteristics of S-RNases and pollen F-box proteins are briefly described. A variety of modifier genes also required for SI are also reviewed. Mutations affecting compatibility in pollen are especially important for defining models of compatibility and incompatibility. In Solanaceae, pollen-side mutations causing breakdown in SI have been attributed to the heteroallelic pollen effect, but a mutation in Solanum chacoense may be an exception. This has been interpreted to mean that pollen incompatibility is the default condition unless the S-locus F-box protein confers resistance to S-RNase. In Prunus, however, S-locus F-box protein gene mutations clearly cause compatibility. Two alternative mechanisms have been proposed to explain compatibility and incompatibility: compatibility is explained either as a result of either degradation of non-self S-RNase or by its compartmentalization so that it does not have access to the pollen tube cytoplasm. These models are not necessarily mutually exclusive, but each makes different predictions about whether pollen compatibility or incompatibility is the default. As more factors required for SI are identified and characterized, it will be possible to determine the role each process plays in S-RNase-based SI.

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.