Detection and estimation of linkage for a co-dominant structural gene locus linked to a gametophytic self-incompatibility locus

Summary The operation of gametophytic self-incompat ibility systems may lead to disturbed segregation ratios for genes at loci linked to the self-incompatibility loci. An exhaustive consideration of the types of crosses, methods of linkage estimation, progeny sizes and controls needed for accurate analysis of disturbed segregation ratios is presented. Examples of the application of these methods are included.     

Quantitatively determined self-incompatibility

Summary It has been claimed that Borage (Borago officInalis L.) has a multifactorial self-incompatibility system. Such systems may have a high level of ineffective pollination, and we show that this is the case in borage. The ranking of seed set from highest to lowest is as follows: bee-pollination; natural pollination in the absence of bees; artificial cross-pollination between unrelated plants; artificial cross-pollination between related plants; artificial self-pollination. In diallel crosses, significant parental effects were detected but no consistent patterns of seed set, which suggest a simple self-incompatibility system, were detected. The level of outcrossing with natural pollination was very variable but greater than 50%. Thus, there appears to be no straightforward self-incompatibility system in borage.     

Functional fragments of a relictual gametophytic self-incompatibility system are associated with the loci determining flower type of the heterostylous outcrosser Fagopyrum esculentum Moench. and the homostylous selfer F. homotropicum ohnishi

Functional fragments of presumably a relictual gametophytic self-incompatibility system (GSI) linked with the loci determining flower type were discovered by genetic analysis of an unilateral pre-zygotic barrier between the short-styled (thrum) morph of a heterostylous cross-pollinated species, Fagopyrum esculentum Moench., and a self-pollinator with homostylous flowers, F. homotropicum Ohnishi (asseccion C9139). The relic genes of GSI were revealed only in interspecific crosses. However, this is a direct experimental confirmation of a hypothesis proposed by Lewis (1954) which combined the heterostyly supergene components (G, P and A) with "pistil" and "pollen" parts of the S-locus of homomorphic self-incompatibility systems (I1 and I2). Also, this result provides strong evidence for the evolution of heterostyly upon the ruins of a gametophytic self-incompatibility system.     

Segregation distortion of T-DNA markers linked to the self-incompatibility (S) locus in Petunia hybrida

In plants with a gametophytic self-incompatibility system the specificity of the pollen is determined by the haploid genotype at the self-incompatibility (S) locus. In certain crosses this can lead to the exclusion of half the gametes from the male parent carrying a particular S-allele. This leads to pronounced segregation distortion for any genetic markers that are linked to the S-locus. We have used this approach to identify T-DNA insertions carrying a maize transposable element that are linked to the S-locus of Petunia hybrida. A total of 83 T-DNA insertions were tested for segregation distortion of the selectable marker used during transformation with Agrobacterium. Segregation distortion was observed for 12 T-DNA insertions and at least 8 of these were shown to be in the same linkage group by intercrossing. This indicates that differential transmission of a single locus (S) is probably responsible for all of these examples of T-DNA segregation distortion. The identification of selectable markers in coupling with a functional S-allele will allow the preselection of recombination events around the S-locus in petunia. Our approach provides a general method for identifying transgenes that are linked to gametophytic self-incompatibility loci and provides an opportunity for transposon tagging of the petunia S-locus.     

The inheritance of modifiers conferring self-fertility in the partially self-incompatible perennial, Campanula rapunculoides L. (Campanulaceae)

The role of partial self-incompatibility in plant breeding system evolution has received little attention. Here, we examine the genetic basis of modifiers conferring self-fertility in the creeping bellflower, Campanula rapunculoides L. (Campanulaceae), a partially self-incompatible herb. A survey of 35 individuals from two natural populations indicates that 45% of them are strongly self-incompatible, 40% intermediately self-incompatible, and 15% weakly self-incompatible and that some plants show a strong breakdown in self-incompatibility over floral age. We generated 101 F1 families by random crossing among 31 parental plants and estimated the heritability of self-fertility in day 1 and day 4 female-phase flowers, the genetic correlation between day 1 and day 4 self-fertility, and the coefficient of additive genetic variance of self-fertility. We use linear regression and data from additional crosses to examine whether there are significant maternal effects in the expression of self-fertility. We use Fain's test to determine if a major gene influences self-fertility and, finding no evidence, use data from additional crosses on an F2 generation to estimate the mean number and dominance of genes conferring self-fertility. These analyses indicate that the heritability (h2) of self-fertility is 0.24 in day 1 female-phase flowers and 0.44 in day 4 flowers, self-fertility is primarily additive but shows some recessive effects, and self-fertility is estimated to be controlled by four genetic factors. In addition, we have evidence that there may be maternal effects for self-fertility, especially for weakly self-incompatible plants. The significance of these results in the context of mating system evolution is discussed.     

On the Evolution of Genetic Incompatibility Systems. VI. a Three-Locus Modifier Model for the Origin of Gametophytic Self-Incompatibility

Recent genetic analyses have demonstrated that self-incompatibility in flowering plants derives from the coordinated expression of a system of loci. To address the selective mechanisms through which a genetic system of this kind evolves, I present a three-locus model for the origin of gametophytic self-incompatibility. Conventional models assume that a single locus encodes all physiological effects associated with self-incompatibility and that the viability of offspring depends only on whether they were derived by selfing or outcrossing. My model explicitly represents the genetic determination of offspring viability by a locus subject to symmetrically overdominant selection. Initially, the level of expression of the proto-S locus is insufficient to induce self-incompatibility. Weak gametophytic self-incompatibility arises upon the introduction of a rare allele at an unlinked modifier locus which enhances the expression of the proto-S locus. While conventional models predict that the origin of self-incompatibility requires at least two- to threefold levels of inbreeding depression, I find that the comparatively low levels of inbreeding depression generated by a single overdominant locus can ensure the invasion of an enhancer of self-incompatibility under sufficiently high rates of receipt of self-pollen. Associations among components of the incompatibility system promote the origin of self-incompatibility. Enhancement of heterozygosity at the initially neutral proto-S locus improves offspring viability through associative overdominance. Further, the modifier that enhances the expression of self-incompatibility develops a direct association with heterozygosity at the overdominant viability locus. These results suggest that the evolutionary processes by which incompatibility systems originate may differ significantly from those associated with their breakdown. The genetic mechanism explored here may apply to the evolution of other systems that restrict reproduction, including maternal-fetal incompatibility in mammals.     

The effect of interplant distance on mating success in federally threatened, self-incompatible Hymenoxys herbacea = H. acaulis var. glabra (Asteraceae)

We investigated large- and fine-scale effects of interplant distance on compatibility, seed set, and seed germination in a rare, self-incompatible perennial, Lakeside daisy (Hymenoxys herbacea = H. acaulis var. glabra). Plants were collected at the Marblehead Peninsula. Ohio, and transplanted to a greenhouse where they were hand-pollinated. For the large-scale analysis, 110 crosses were classified in three categories: Near crosses (0.75-6.70 m), Far crosses (17-72 m), and Very Far crosses (>900 ml. There was no significant effect of interplant distance on compatibility, seed set, or seed germination in these crosses. For the fine-scale analysis, we made 44 crosses with interplant distances ranging from 0.75 to 10 m. At this scale, interplant distance explained 10.9% of the variance of the seed/floret ratio, suggesting that local genetic structure may result in a modest amount of biparental inbreeding. We found no fine-scale effects of interplant distance on compatibility or percentage of seed germination, but it is possible that biparental inbreeding could affect later stages of the life cycle not included in this study. For all distance classes, >80% of the crosses were compatible, indicating that lack of compatibility between mates is not likely to limit seed production. Apparently, presumed population bottlenecks have not been severe enough for genetic drift to eliminate substantial numbers of self-incompatibility alleles.     

Signaling in pollen-pistil interactions.

A complex set of cell interactions is required to achieve fertilization. The pollen grain must be recognized by the pistil, take up water, and grow a pollen tube directionally through the style in order to deliver the sperm to the ovule. In many families of flowering plants, self-fertilization can be prevented by recognition mechanisms that allow self-pollen rejection by the pistil. The self-incompatibility response is under the genetic control of a single multi-allelic locus, the (Self-incompatibility) locus. There are two major classes of self-incompatibility systems. Gametophytic self-incompatibility has been well characterized in the Solanaceae and in the Papaveraceae, while sporophytic self-incompatibility has been well characterized in the Brassicaceae. In this review article, we present recent advances in understanding the signals mediating pollen recognition and pollen tube growth, in both compatible and incompatible interactions.     

Do homomorphic and heteromorphic self-incompatibility systems have the same sporophytic mechanism?

The view put forward by some authors that flowering plant self-incompatibility mechanisms of the homomorphic sporophytic and heteromorphic sporophytic types have a close evolutionary relationship, with one form being evolved from the other, or both forms directly evolved from ancestors with homomorphic gametophytic incompatibility, is challenged. A review is provided of the various facets of each of the three main self-incompatibility systems, including a detailed summary of our current knowledge of the rejection mechanism, to demonstrate that the implicit assumption that these systems have a common S locus, and also evolutionary theories linking the systems, need to be treated with considerable caution.     

Unilateral incompatibility within the brassicaceae: further evidence for the involvement of the self-incompatibility (S)-locus

Unilateral pollen-pistil incompatibility within the Brassicaceae has been re-examined in a series of interspecific and intergeneric crosses using 13 self-compatible (SC, Sc) species and 12 self-incompatible (SI) species from ten tribes. SC x SC crosses were usually compatible, SI x SC crosses showed unilateral incompatibility, while SI x SI crosses were often incompatible or unilaterally incompatible. Unilateral incompatibility (UI) is shown to be overcome by bud pollination or treating stigmas with cycloheximide — features in common with self-incompatibility. Treating stigmas with pronase prevents pollen tubes from penetrating the stigma in normally compatible intra-and interspecific pollinations. The results presented show that the presence of an incompatibility system is important in predicting the outcome of interspecific and intergeneric crosses and, combined with the physiological similarities between UI and SI, would suggest an involvement of the S-locus in UI.     

Functional fragments of a relictual gametophytic self-incompatibility system are associated with the loci determining flower type of the heterostylous outcrosser Fagopyrum esculentum Moench. and the homostylous selfer F. homotropicum Ohnishi

Functional fragments of presumably a relictual gametophytic self-incompatibility system (GSI) linked with the loci determining flower type were discovered by genetic analysis of an unilateral pre-zygotic barrier between the short-styled (thrum) morph of a heterostylous cross-pollinated species, Fagopyrum esculentum Moench., and a self-pollinator with homostylous flowers, F. homotropicum Ohnishi (asseccion C9139). The relic genes of GSI were revealed only in interspecific crosses. However, this is a direct experimental confirmation of a hypothesis proposed by Lewis (1954) which combined the heterostyly supergene components (G, P and A) with “pistil” and “pollen” parts of the S-locus of homomorphic self-incompatibility systems (I ₁ and I ₂). Also, this result provides strong evidence for the evolution of heterostyly upon the ruins of a gametophytic self-incompatibility system.     

The molecular biology of self-incompatibility systems in flowering plants

Self-incompatibility is a common mechanism by which flowering plants can exert some control over the process of fertilization. Typically, the self-incompatibility response involves the recognition and rejection of self-incompatible pollen which leads to a block in self-fertilization and, as a consequence, promotes outcrossing. In recent years, considerable progress has been made in the molecular understanding of several self-incompatibility systems. Interestingly, a common mechanism for self-incompatibility is not employed by all flowering plants, but in fact quite diverse mechanisms have been recruited for the rejection of self-incompatible pollen. In this review, the recent advances in the self-incompatibility systems of the Solanaceae, Papaveraceae, and Brassicaceae will be described as well as some of the molecular work that is emerging for the Poaceae and the heteromorphic self-incompatibility systems.     

The ancestral graph and gene genealogy under frequency-dependent selection.

Minority-advantage frequency-dependent selection has been proposed as the cause for the high level of observed polymorphism in some self/nonself-recognition systems. We present a mathematically rigorous derivation of the ancestral graph for a sample of genes that evolved according to a haploid infinite-alleles model of minority-advantage frequency-dependent selection. In the case of sufficiently weak selection, the gene genealogy can be extracted from the ancestral graph. We demonstrate that the gene genealogy under this model is identical to that obtained for a diploid model with heterozygote advantage. The case of strong selection is exemplified by a one-locus haploid self-incompatibility system; in this context, we investigate the number of alleles that can be maintained in a spatial versus a non-spatial habitat. Finally, we compare gametophytic self-incompatibility to the haploid self-incompatibility model.     

INCOMPATIBILITY IN AMSINCKIA GRANDIFLORA (BORAGINACEAE): DISTRIBUTION OF CALLOSE PLUGS AND POLLEN TUBES FOLLOWING INTER- AND INTRAMORPH CROSSES

The distribution of callose plugs and pollen tubes was investigated following inter- and intramorph crosses of Amsinckia grandiflora (Boraginaceae), a distylous species possessing cryptic self-incompatibility. Callose plug distribution provided a good indication of the distribution of pollen tubes. Compared to intramorph crosses, many more callose plugs and pollen tubes were found in basal stylar regions following intermorph crosses, indicating that differential pollen tube growth is a likely cause of cryptic self-incompatibility. The incompatibility response differed for the floral morphs: in the pin (long-styled) morph pollen tubes were most likely to cease growth in the midstylar region, while inhibition was more likely to occur in the upper stylar region of the thrum (short-styled) morph. There was no evidence of stigmatic inhibition of pollen tubes for either morph, although the incompatibility response in the Boraginaceae is normally located in the stigmatic region.     

Self-compatibility in aLycopersicon peruvianum variant (LA2157) is associated with a lack of style S-RNase activity

A series of crosses between a naturally-occurring self-compatible accession ofLycopersicon peruvianum and a closely-related self-incompatible accession were used to demonstrate that the mutation to self-compatibility is located at the S-locus. Progeny of the crosses contain abundant style proteins of about 30 kDa that segregate with the S₆and S₇-alleles from the SI parent and the S_c-allele from the SC parent. The S₆and S₇-associated proteins have ribonuclease activity whereas the S_c-associated protein is not an active ribonuclease. This finding indicates that S-RNases are determinants of self-incompatibility in the style and that the ribonuclease activity is essential for their function.     

Is Eucalyptus Cryptically Self-incompatible?

BACKGROUND AND AIMS: The probability that seeds will be fertilized from self- versus cross-pollen depends strongly on whether plants have self-incompatibility systems, and how these systems influence the fate of pollen tubes. METHODS: In this study of breeding systems in Eucalyptus urophylla and Eucalyptus grandis, epifluorescence microscopy was used to study pollen tube growth in styles following self- and cross-pollinations. KEY RESULTS: Pollen tubes from self-pollen took significantly longer than those from cross-pollen to grow to the base of the style in both E. urophylla (120 h vs. 96 h) and E. grandis (96 h vs. 72 h). In addition, both species exhibited reduced seed yields following self-pollination compared with cross-pollination. CONCLUSIONS: The present observations suggest that, in addition to a late-acting self-incompatibility barrier, cryptic self-incompatibility could be a mechanism responsible for the preferential out-crossing system in these two eucalypt species.     

Transmission ratio distortion in Arabidopsis lyrata: effects of population divergence and the S-locus

We investigated transmission ratio distortion within an Icelandic population of Arabidopsis lyrata using 16 molecular markers unlinked to the S-locus. Transmission ratio distortion was found more often than expected by chance at the gametic level, but not at the genotypic or zygotic level. The gametic effect may be due to meiotic drive or selection acting postmeiotically. At the gametic level, 10.9% of the tests were significant, which is substantially lower than earlier observed in an interpopulation cross (allowing for differences in power)-suggesting that the high level of transmission ratio distortion in the interpopulation cross is due to population divergence. It is also substantially lower than previously observed in intrapopulation crosses at the self-incompatibility locus, suggesting inherent fitness differences of the self-incompatibility alleles. We discuss the possible role of deleterious alleles accumulating at loci under balancing selection. Zygotic effects play a larger role in the interpopulation cross than in the intrapopulation crosses suggesting that Dobzhansky-Muller incompatibilities may be accumulating between the widely diverged populations.     

Inheritance and genetic mapping of self-incompatibility in Coffea canephora Pierre

Cross-compatibility behaviour of doubled haploid (DH) and hybrid genotypes of Coffea camphora was established using both phenotypic bioassay and in situ seed-set examination. The availability of DHs provided the opportunity of working with genetically homogenous pollen and female parents. The aniline blue fluorescence (ABF) method was applied to detect callose accumulation in pollen and pistil. Clear cross-compatibility/incompatibility situations were observed and confirmed by in situ seed-set analysis. Cross-compatibility analysis of hybrid combinations involving different DHs corroborated the crossing behaviour observed at the DH level. Expression of the self-incompatibility system did not appear to be affected by the low vigour of the DH. The crossing-behaviour distribution observed within DHs derived from clone IF200 confirmed that self-incompatibility in C. canephora is a gametophytic self-incompatibility system controlled by a single locus (S-locus). Reduced seed-set developments following incompatible crosses may indicate the occurrence of pseudo-incompatibility. Molecular marker linkage analysis showed that the S-locus is associated with an RFLP marker on linkage group 9. The availability of a linked DNA marker should facilitate the genetic analysis of self-incompatibility in relation to coffee breeding programmes.     

Transient SI and the dynamics of self-incompatibility alleles: a simulation model and empirical test

A stochastic computer simulation model was created to compare the combined effects of selection and genetic drift on the dynamics of S-alleles under full sporophytic self-incompatibility (SI) versus transient SI, a form of partial SI in which flowers become self-compatible as they age. S-alleles were lost more rapidly with transient than with full SI, as is expected with weakened frequency-dependent selection. Based on these results, equilibrium S-allele diversity is expected to be lower with partial SI for populations of comparable size and migration rates. Consistent with model results, a comparison of the proportion of incompatible crosses in full diallel experiments for a fully SI and a transiently SI species in the annual genus Leptosiphon suggests that S-allele diversity is lower in the partially SI species. Results of the simulation model indicate that the transmission advantage of self-fertilization can have complex effects on S-allele dynamics in partial SI systems.     

Single gene control of postzygotic self-incompatibility in poke milkweed, Asclepias exaltata L

Most individuals of Asclepias exaltata are self-sterile, but all plants lack prezygotic barriers to self-fertilization. To determine whether postzygotic rejection of self-fertilized ovules is due to late-acting self-incompatibility or to extreme, early acting inbreeding depression, we performed three diallel crosses among self-sterile plants related as full-sibs. The full-sibs segregated into four compatibility classes, suggesting that late acting self-incompatibility is controlled by a single gene (S-locus). Crosses between plants sharing one or both alleles at the S-locus are incompatible. An additional diallel cross was done among full-sib progeny from a cross of a self-sterile and a self-fertile plant. These progeny grouped into two compatibility classes, and plants within classes displayed varying levels of self-fertility. This suggests that the occasional self-fertility documented in natural pollinations is caused by pseudo-self-fertility alleles that alter the functioning of the S-locus.