Unilateral Interspecific Incompatibility Research Articles

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.

A farnesyl pyrophosphate synthase gene expressed in pollen functions in S-RNase-independent unilateral incompatibility.

Multiple independent and overlapping pollen rejection pathways contribute to unilateral interspecific incompatibility (UI). In crosses between tomato species, pollen rejection usually occurs when the female parent is self-incompatible (SI) and the male parent self-compatible (SC) (the 'SI×SC rule'). Additional, as yet unknown, UI mechanisms are independent of self-incompatibility and contribute to UI between SC species or populations. We identified a major quantitative trait locus on chromosome 10 (ui10.1) which affects pollen-side UI responses in crosses between cultivated tomato, Solanum lycopersicum, and Solanum pennelliiLA0716, both of which are SC and lack S-RNase, the pistil determinant of S-specificity in Solanaceae. Here we show that ui10.1 is a farnesyl pyrophosphate synthase gene (FPS2) expressed in pollen. Expression is about 18-fold higher in pollen of S. pennellii than in S. lycopersicum. Pollen with the hypomorphic S. lycopersicum allele is selectively eliminated on pistils of the F1 hybrid, leading to transmission ratio distortion in the F2 progeny. CRISPR/Cas9-generated knockout mutants (fps2) in S.pennelliiLA0716 are self-sterile due to pollen rejection, but mutant pollen is fully functional on pistils of S. lycopersicum. F2 progeny of S. lycopersicum×S. pennellii (fps2) show reversed transmission ratio distortion due to selective elimination of pollen bearing the knockout allele. Overexpression of FPS2 in S.lycopersicum pollen rescues the pollen elimination phenotype. FPS2-based pollen selectivity does not involve S-RNase and has not been previously linked to UI. Our results point to an entirely new mechanism of interspecific pollen rejection in plants.

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.

Interspecific hybridization between Brassica napus and Brassica rapa ssp. chinensis genotypes through embryo rescue and their evaluation for crossability

The genus Brassica contains several important crop species that are used for a variety of purposes. Brassica napus and Brassica rapa are the most important ones for their use as oilseed crops. In Brassica , interspecific hybridization is a potential and useful method for transferring valuable traits between species of commercial interest. For breeding Brassicas resistant to some diseases, i.e. clubroot, attempts of transferring resistant genes (CR) through interspecific hybridization have also been reported. The main step in the introgression process is the production of interspecific hybrids between the two species. However, the cross-incompatibility occurring in wide hybridization might hamper the possibility of obtaining hybrid progenies. For that reason, the crossability study is essential and may give an insight into the cross-compatibility relationship among the species, the direction of success of crossing, and the crossability barriers of some combinations, if any. In the present study, interspecific reciprocal crosses between B. napus and B. rapa ssp. chinensis genotypes were carried out in order to determine their crossability and to produce their F1 hybrids. Crossability was analyzed based on the pollen germination index (PGI) and the development of hybrid embryos in in vitro cultures. It was observed that the unilateral interspecific incompatibility occurred in crosses between B. napus × B. rapa ssp. chinensis. When B. napus cultivars were used as a maternal parent, pollen grains of B. rapa germinated well on the stigmas, while only a small number of pollen tubes could elongate near the ovules, resulting in a low development of hybrid embryos. Using the B. rapa ssp. chinensis as the pistillate parent in the crosses, it was found that the crossability was relatively higher than that in the case of the reciprocal cross. This resulted in a greater number of embryos obtained. Generally, it was concluded that the selection of parental components for hybridization is an important step for obtaining breeding success.

The Role of a Pollen-Expressed Cullin1 Protein in Gametophytic Self-Incompatibility in Solanum

We previously isolated a pollen factor, ui6.1, which encodes a Cullin1 protein (CUL1) that functions in unilateral interspecific incompatibility (UI) in Solanum. Here we show that CUL1 is also required for pollen function in self-incompatibility (SI). We used RNA interference (RNAi) to reduce CUL1 expression in pollen of Solanum arcanum, a wild SI tomato relative. Hemizygous T0 plants showed little or no transmission of the transfer DNA (T-DNA) through pollen when crossed onto nontransgenic SI plants, indicating that CUL1-deficient pollen are selectively eliminated. When crossed onto a related self-compatible (SC) accession lacking active S-RNase, pollen transmission of the T-DNA followed Mendelian ratios. These results provide further evidence for functional overlap between SI and UI on the pollen side and suggest that CUL1 mutations will reinforce SI-to-SC transitions in natural populations only if preceded by loss of pistil S-RNase expression.

Genetic evidence that two independent S-loci control RNase-based self-incompatibility in diploid strawberry

The self-incompatibility mechanism that reduces inbreeding in many plants of the Rosaceae is attributed to a multi-allelic S locus which, in the Prunoideae and Maloideae subfamilies, comprises two complementary genes, a stylar-expressed S-RNase and a pollen-expressed SFB. To elucidate incompatibility in the subfamily Rosoideae, stylar-specific RNases and self-(in)compatibility status were analysed in various diploid strawberries, especially Fragaria nubicola and F. viridis, both self-incompatible, and F. vesca, self-compatible, and in various progenies derived from them. Unexpectedly, two unlinked RNase loci, S and T, were found, encoding peptides distinct from Prunoideae and Maloideae S-RNases; the presence of a single active allele at either is sufficient to confer self-incompatibility. By contrast, in diploid Maloideae and Prunoideae a single locus encodes S-RNases that share several conserved regions and two active alleles are required for self-incompatibility. Our evidence implicates the S locus in unilateral inter-specific incompatibility and shows that S and T RNases can, remarkably, confer not only allele-specific rejection of cognate pollen but also unspecific rejection of Sn Tn pollen, where n indicates a null allele, consistent with the the presence of the pollen component, SFB, activating the cognitive function of these RNases. Comparison of relevant linkage groups between Fragaria and Prunus suggests that Prunus S-RNases, unique in having two introns, may have resulted from gene conversion in an ancestor of Prunus. In addition, it is shown that there is a non-S locus that is essential for self-incompatibility in diploid Fragaria.

Stylar glycoproteins bind to S‐RNase in vitro

S-RNases determine the specificity of S-specific pollen rejection in self-incompatible plants of the Solanaceae, Rosaceae, and Scrophulariaceae. They are also implicated in at least two distinct types of unilateral interspecific incompatibility in Nicotiana. However, S-RNase itself is not sufficient for most types of pollen rejection, and evidence for its direct interaction with pollen tubes is limited. Thus, non-S-RNase factors also are required for pollen rejection. As one approach to identifying such factors, we tested whether SC10-RNase from Nicotiana alata would bind to other stylar proteins in vitro. SC10-RNase was immobilized on Affi-gel, and binding proteins were analyzed by SDS-PAGE and immunoblotting. In addition to SC10-RNase and a small protein similar to lily chemocyanin, the most prominent binding proteins include NaTTS, 120K, and NaPELPIII, these latter three being arabinogalactan proteins previously shown to interact directly with pollen tubes. We also show that SC10-RNase and these glycoproteins migrate as a complex in a native PAGE system. Our hypothesis is that S-RNase forms a complex with these glycoproteins in the stylar ECM, that the glycoproteins interact directly with the pollen tubes and thus that the initial interaction between the pollen tube and S-RNase is indirect.

The S-locus and unilateral incompatibility.

Plants have many ways to regulate the type of pollen that arrives on the stigma surface. Once there, further control mechanisms regulate compatibility. The latter controls are largely based on biochemical interactions that support compatible pollination and prevent incompatible matings. S-RNase-based self-incompatibility (SI) systems are the most phylogenetically widespread mechanisms for controlling pollination. Studies of Nicotiana establish a firm link between SI and unilateral interspecific incompatibility. Although implicated in both inter- and intraspecific compatibility, S-RNase operates through at least three distinct genetic mechanisms that differ in their dependence on non-S-RNase factors. Identification and characterization of these non-S-RNase factors is currently an area of active research. Searching for genetic and biochemical interactions with S-RNase can identify candidate non-S-RNase factors. HT-protein is one factor that is required for S-allele-specific pollen rejection in the Solanaceae. Major style arabinogalactan proteins such as TTS interact biochemically with S-RNase. These glycoproteins are known to interact with compatible pollen tubes and have long been suggested as possible recognition molecules. Their binding to S-RNase implies a link between stylar systems for compatibility and incompatibility. Thus, genetic and biochemical studies suggest a highly networked picture of pollen-pistil interactions.

アルストロメリア野生種１５種の交雑親和性

Diallel cross and ovule culture were carried out among 15 Alstroemeria species. Ovules harvested 14 days after pollination were cultured on 1/2MS medium containing 3% sucrose at 20°C under a16-hr photoperiod. Embryo germination occurred in 196 out of 210 combinations of the diallel. A. angustifolia, A. ligtu, A. magenta and A. pelegrina showed wide cross-compatibility with not only Chilean but Brazilian species as well. When A. aurea, A. diluta, A. garaventae and A. zoellneri each served as a female parent, the percentage embryo germination was low in some crosses, but some germination took place in all combinations. A. inodora, A. maginfica, A. psittacina and A. pulchra exhibited unilateral interspecific incompatibility in many combinations.

Cross-Compatibility of Petunia exserta with Other Petunia taxa.

Cross-compatibility of Petunia exserta, a natural species having red flowers, with 46 taxa of Petunia sensu Jussieu (20 taxa of Petunia sensu Wijsman plus 27 taxa of Calibrachoa) was examined to obtain basic information for the breeding of garden petunias. Petunia exserta completely failed to set capsules when crossed with any species of Calibrachoa. Petunia exserta exhibited unilateral interspecific incompatibility with 15 taxa of Petunia sensu Wijsman : As a seed parent, it could be crossed with all taxa of Petunia except P. occidentalis, but the reciprocal crossings were successful with only five taxa. In the F2 generation of P. exserta hybrid crossed with P. axillaris subsp. axillaris having white flower, individuals having red flowers segregated. F2 progenies contained delphinidin glycosides as the major floral anthocyanin similar to P. exserta. The use of P. exserta as an additional gene resource for improving red flowered garden petunias is discussed.

Stylar water potential and unilateral interspecific incompatibility inSolanaceae

Water and osmotic potentials were measured in young and mature styles of selfcompatible (SC) and selfincompatible (SI) species ofSolanaceae. For mature flowers, the stylar water and osmotic potentials were considerably lower in SI speciesNicotiana alata andPetunia hybrida than in SC speciesN. sylvestris andN. tabacum. Stylar water content (in % fresh mass) was also significantly lower in SI species (N. alata, P. hybrida, diploid clones of potato) than in SC species (N. acuminata, N. glauca, N. paniculata, N. sylvestris, N. tabacum). For flower buds younger than 2 d before anthesis, no differences in stylar water potential betweenN. alata (SI) andN. tabacum (SC) were observed. During further flower bud development the stylar water potential inN. alata decreased more expressively than inN. tabacum. Pollen culture in media with various concentrations of sucrose or polyethylene glycol revealed that optimal water potential for pollen tube growth was lower inN. alata than inN. tabacum. The differences were similar as for differences in stylar water potential. The results are considered in relation to possible involvement of stylar water potential in unilateral interspecific incompatibility inSolanaceae.

BRIDGE SPECIES FOR INTROGRESSION OF WILD GERMPLASM IN SWEET POTATO BREEDING.

The species of section Batatas represent the primary potential source of wild germplasm for sweet potato breeding. Their wide distribution and adaptation make their use desirable, but their direct use is constrained by their ploidy level (2x, and 4x, in contrast with cultivated I. batatas which is 6x) and lack of storage root production.We investigated 4x clones of interspecific origin (6x I. batatas 2x I. trifida) as bridges to introgress genes from several wild species of the section Batatas to I. batatas. Six 2x wild species from different origins were reciprocally crossed to the 4x clones. The crosses were successful only when the diploid wild species were used as females (2x × 4x) indicating unilateral interspecific incompatibility. The unsuccessful types of crosses (4x × 2x) were repeated using wild hybrid males containing I. trifida in their parentage. This time only those crosses involving a hybrid having I. trifida as its female parent produced seeds. Progenies from the successful 2x × 4x crosses were diploid instead of the expected triploid. Triploid progenies were obtained only in the 4x × (2x hybrid) crosses. Some triploid progenies produced 2n pollen in varying frequencies. These results show different levels of crossing barriers among Ipomoea species and give us alternative pathways for overcoming them. I. trifida was identified as a possible bridge species.

Unilateral incompatibility and the effects of interspecific pollination for Erythronium americanum and Erythronium albidum (Liliaceae)

To determine whether interspecific pollen transfer could reduce seed production by two sympatric lilies, Erythronium albidum and Erythronium americanum, we hand-pollinated flowers with mixtures of conspecific and heterospecific pollen. These species exhibited typical unilateral interspecific incompatibility, i.e., pollen tubes from the self-infertile species (E. americanum) grew apparently unimpeded in styles of the self-fertile species (E. albidum), whereas the reverse cross resulted in an incompatibility reaction. Because of this asymmetrical relation and faster growth by heterospecific pollen tubes in E. albidum stigmas than by conspecific tubes, pollination with abundant heterospecific pollen reduced fruit and seed production by E. albidum, but not by E. americanum, as long as the stigma also received some conspecific pollen. Unilateral incompatibility could benefit self-infertile species in reproductive interactions with closely related self-fertile species; however, this benefit remains to be demonstrated for naturally pollinated plants. Key words: Erythronium albidum, Erythronium americanum, interspecific pollen transfer, pollination, unilateral incompatibility.

EVOLUTION OF UNILATERAL INCOMPATIBILITY IN FLOWERING PLANTS: FURTHER EVIDENCE IN FAVOUR OF TWIN SPECIFICITIES CONTROLLING INTRA‐AND INTERSPECIFIC INCOMPATIBILITY

SummaryThe breeding system in Nicotiana glauca is unique in that the species comprises mainly self‐compatible (S.C.) green‐flowered plants but also includes rare self‐incompatible (S.I.) red‐flowered races. The S.C. plants of Nicotiana glauca show unilateral interspecific incompatibility with other S.I. species of Nicotiana, a widely occurring characteristic normally associated with S.C. species with relatively long‐established self‐compatibility.The present study of S.I. and S.C. strains of Nicotiana glauca showed that the ♀ S.I. × S.C. ♂ type of unilateral incompatibility (U.I.) occurs within this species, and the U.I. is a function of the S alleles. Of the nine SI alleles studied seven, when present in the homozygous state in the style, made that style reject Sc (Nicotiana glauca) pollen whereas two alleles allowed styles to accept this pollen. Similar polymorphism at the Nicotiana glauca S locus was also found in relation to Sf pollen from the S.C. species Nicotiana langsdorffii. Sl alleles in Nicotiana glauca were classified differently for the polymorphisms in relation to Sc and Sf pollen.The results show that the Sc allele is sporophytically determined causing allelic interaction in the pollen and style. Dominance, co‐dominance and competitive interaction were observed. In contrast the St allele of Nicotiana langsdorffii is gametophytically determined, stylar alleles always showing independent action.Within the same species Nicotiana glauca, there is both gametophytic and sporophytic determination of 5 alleles. Whereas in species with sporophytic self‐incompatibility allelic interaction is predetermined in the pollen and style, in this case allelic interaction is not determined until after pollen/stigma contact and is complementary.The occurrence within the same species of (1) the unilateral S.I. × S.C. type of incompatibility, (2) both gametophytic and sporophytic determination, (3) late determination of allelic interaction after pollen/stigma contact, and (4) competitive interaction of 5 alleles, is all explained on the basis of the author's theory of ‘twin specificities’ controlling intra‐and interspecific incompatibility in flowering plants.Wider implications of the theory are discussed. Early evolution of specificity determination in prokaryotes provided a basis for cell to cell interaction that eventually led, on the one hand, to evolution of sex and genetic recombination systems and, on the other, to mechanisms for infection/disease protection, and graft‐incompatibility control. These phenomena are evident in both plant and animal kingdoms.

The effect of prepollination injection with stigmatic exudate on interspecific pollen tube growth in Lilium longiflorum thunb. styles

Prepollination injection of Lilium longiflorum styles with exudate from the stigmas of L. longiflorum significantly increased the length of pollen tubes normally inhibited by short-growth unilateral interspecific incompatibility (UI). However, stigmatic exudate injection had no effect on nor significantly retarded pollen tube growth of lily species expressing reciprocal interspecific incompatibility with L. longiflorum. These results suggest that the inhibition of short-growth pollen tubes in unilateral incompatible combinations is achieved through a different process than the inhibition-limiting pollen tube growth in reciprocal interspecific incompatibility.

Incompatibility in forest trees

Despite the great importance for forest tree breeding, very limited knowledge is yet available about the breeding systems of forest trees. Where incompatibility has been studied in the hardwoods; patterns have been observed which confirm the general rules detected for other angiosperms. Self- and interspecific incompatibility at the level of pollen tube growth has been reported for example in Betula and Alnus . In Alnus one case of unilateral interspecific incompatibility has been found. Self-incompatibility has, so far, not been reported from the conifers. Inter-specific incompatibility in the form of the arrestment of the pollen tube growth in the nucellus tissue has been observed in Picea and is particularly clear in Pinus crosses between the subgenera and Haploxylon and Diploxylon , but also within the Diploxylon -group. The nature of the incompatibility mechanism is still unknown, but serological differences related to the behaviour in the crosses has been detected in birch and pine pollen. It is suggested that the complex polysaccharidic composition of the cell walls and membranes might form a specific stereochemical basis for the incompatibility reaction. The presence of a combination of self-pollination, polyembryony and genetic load is discussed as an alternative mechanism favouring outbreeding in the Gymnosperms.

POLLEN TUBE GROWTH AND INCOMPATIBILITY FOLLOWING INTRA‐ AND INTER‐SPECIFIC POLLINATIONS IN LILIUM LONGIFLORUM

The growth of pollen tubes of a sampling of Lilium species in styles of L. longiflorum incubated at 24 C for 48 hr after pollination indicated two types of interspecific incompatibility. Pollen tubes of two self‐compatible species of section Leucolirion, the section including L. longiflorum, stopped growth abruptly upon reaching the stylar canal, were of abnormal morphology, and were incapable of continued growth with longer incubation. Pollen tubes of self‐compatible or self‐incompatible species of sections Sinomartagon, Pseudolirium, Liriotypus, and Daurolirion approached but did not exceed the length of intraspecific incompatible pollen tubes in styles of L. longiflorum. Pollen tube morphology was normal and tubes were capable of continued growth with additional incubation. Unilateral interspecific incompatibility occurred in reciprocal crosses between self‐incompatible L. longiflorum and self‐compatible L. regale and L. formosanum, but exceptions occurred in Aurelian hybrids. Incubation of interspecifically pollinated L. longiflorum styles at 39 C, which removes the self‐incompatibility reaction, had no effect on interspecific incompatibility.

INTERSPECIFIC INCOMPATIBILITY IN SOLANUM SPECIES

Pandey, K.K. (Crop Res. Div., D.S. & I.R., Lincoln, Christchurch, New Zealand.) Interspecific incompatibility in Solanum species. Amer. Jour. Bot. 49(8): 874–882. Illus. 1962.—A diallel cross involving 11 self‐incompatible and 3 self‐compatible species of Solanum was made to study the genetic basis of interspecific incompatibility. Interspecific incompatibility was not limited to crosses in which a self‐compatible species was used as the male parent onto a self‐incompatible species (unilateral incompatibility). A number of crosses between self‐incompatible species were incompatible. In one cross, Q vernei X verrucosum, a self‐compatible species was successful as a pollen parent with a self‐incompatible species. Unlike other hybrids between self‐compatible and self‐incompatible species, which are self‐incompatible, these F1 hybrids were self‐fertile, and cross‐fertile among themselves and with both parents. The self‐fertile S. polyadenium was cross‐incompatible as a female as well as a male parent with all other species. It is suggested that the unilateral incompatibility is a property of the allele SC which originated as a consequence of one kind of breakdown of the SI gene; the SC allele produces “bare” pollen growth substances which are inactivated in an incompatible style. It is proposed that the failure of the principle of unilateral interspecific incompatibility in solanaceous species may be due to the action of alleles at the second incompatibility locus revealed in certain Mexican species. It is assumed that the South American species are selected intraspecifically only for the action of S alleles but that in certain interspecific crosses and rarely in intraspecific crosses the alleles at the second locus may be expressed, thus interfering with the usual action of S alleles. The F1 hybrids Q verrucosum (self‐fertile) X simplicifolium (self‐sterile) were self‐incompatible at the tetraploid as well as the diploid level, and their cross‐compatibility behavior was consistent with the expected activity of the SC and SI alleles of the 2 parents respectively.

Interspecific Incompatibility in Solanum Species

Pandey, K.K. (Crop Res. Div., D.S. & I.R., Lincoln, Christchurch, New Zealand.) Interspecific incompatibility in Solanum species. Amer. Jour. Bot. 49(8): 874–882. Illus. 1962.—A diallel cross involving 11 self-incompatible and 3 self-compatible species of Solanum was made to study the genetic basis of interspecific incompatibility. Interspecific incompatibility was not limited to crosses in which a self-compatible species was used as the male parent onto a self-incompatible species (unilateral incompatibility). A number of crosses between self-incompatible species were incompatible. In one cross, Q vernei X verrucosum, a self-compatible species was successful as a pollen parent with a self-incompatible species. Unlike other hybrids between self-compatible and self-incompatible species, which are self-incompatible, these F1 hybrids were self-fertile, and cross-fertile among themselves and with both parents. The self-fertile S. polyadenium was cross-incompatible as a female as well as a male parent with all other species. It is suggested that the unilateral incompatibility is a property of the allele SC which originated as a consequence of one kind of breakdown of the SI gene; the SC allele produces “bare” pollen growth substances which are inactivated in an incompatible style. It is proposed that the failure of the principle of unilateral interspecific incompatibility in solanaceous species may be due to the action of alleles at the second incompatibility locus revealed in certain Mexican species. It is assumed that the South American species are selected intraspecifically only for the action of S alleles but that in certain interspecific crosses and rarely in intraspecific crosses the alleles at the second locus may be expressed, thus interfering with the usual action of S alleles. The F1 hybrids Q verrucosum (self-fertile) X simplicifolium (self-sterile) were self-incompatible at the tetraploid as well as the diploid level, and their cross-compatibility behavior was consistent with the expected activity of the SC and SI alleles of the 2 parents respectively.

Unilateral interspecific incompatibility in flowering plants

Unilateral interspecific incompatibility in flowering plants