Intramorph Incompatibility Research Articles

The intermorph breeding system is responsible for reproductive success in heterostylous Nyctanthes arbor‐tristis L.

AbstractHeterostyly is a specialized reproductive system associated with self‐ and intramorph incompatibility. It reinforces the intermorph breeding system in heterostylous plant species. However, distyly has not been comprehensively studied in Nyctanthes arbor‐tristis. The present study demonstrates the floral traits, breeding system, fruit and seed set, and morph ratio in distylous N. arbor‐tristis. The length of style and filament showed reciprocal herkogamy between long style (L‐morph) and short style (S‐morph). Flower size, corolla tube size, stamen and anther length, ovary length, and pollen diameter were larger in S‐morph. However, the style was longer in L‐morph. Our results demonstrated strongly incompatible self‐ and intramorph pollination combinations. The intermorph pollination was compatible, showing normal pollen tube growth throughout the style and ovary. The embryo formation was also higher in intermorph interaction. Moreover, the fruit setting was higher when the intermorph plants grew in the vicinity of natural conditions. Collectively, all data indicate that N. arbor‐tristis is a distylous species with a self‐incompatible breeding system, resulting in less seed formation in nature.

Skewed morph ratios lead to lower genetic diversity of the heterostylous Primula veris in fragmented grasslands.

Populations of heterostylous plant species are ideally composed of equal frequencies of two (distylous) or three (tristylous) morphologically different floral morphs. Intra-morph incompatibility helps to avoid inbreeding and to maintain genetic diversity, supporting plant fitness and long-term viability. Habitat fragmentation can lead to skewed morph ratios and thereby reduce the abundance of compatible mates. This, in turn, can result in a loss of genetic diversity. We tested whether the genetic diversity of heterostylous plants is affected by morph ratio bias using populations of the distylous grassland plant Primula veris in recently fragmented grasslands. We recorded morph frequencies and population sizes in 30 study populations of P. veris on two Estonian islands characterised by different degrees of habitat fragmentation. Examining variation of thousands of single nucleotide polymorphisms (SNPs) and heterostyly-specific genetic markers, we quantified overall and morph-specific genetic diversity and differentiation in these populations. Morph frequencies deviated more in smaller populations. Skewed morph ratios had a negative effect on the genetic diversity of P. veris in more fragmented grasslands. In the populations of better-connected grassland system, genetic differentiation among S-morphs was higher than among L-morphs. Our study shows that deviations from morph balance are stronger in small populations and have a negative impact on the genetic diversity of the distylous plant P. veris. Together with the direct negative effects of habitat loss and decreased population size on the genetic diversity of plants, morph ratio bias may intensify the process of genetic erosion, thus exacerbating the local extinction of heterostylous species.

Insights into distyly and seed morphology of the aquatic plant Nymphoides fallax Ornduff (Menyanthaceae)

Nymphoides fallax is a distylous tetraploid species, endemic to Mexican and Guatemalan highland wetlands. Two floral morphs have been previously reported for N. fallax, but no studies have quantified floral organ variations and seed viability. Here we describe the reproductive system and seed morphology, compare measurements of short-and long-styled flowers, and test seed viability. We studied flowers, fruits, and seeds using light and scanning electron microscopes. Morphs differ in sepal, petal, corolla tube, filament, anther, pollen, pistil, stigma, and stigmatic papillae size. Further differences include stigma and stigmatic papillae shape. Morphs have similar ovule and seed number, and the studied population is nearly isoplethic. Fruit and seed production was high, as was seed viability (86 %). We noted prominently projecting tubercles and caruncles not previously reported for N. fallax seeds. Seeds float on the water surface and seedlings are established in the field. Even when studies of other populations are needed, our findings suggest that sexual reproduction and intra-morph incompatibility may play a substantial role in this aquatic plant, although it is capable of strong vegetative reproduction. Understanding the role of distyly can contribute to elucidate N. fallax reproductive biology, and to establish conservation practices for the species and wetland ecosystems.

Identification of a gene encoding polygalacturonase expressed specifically in short styles in distylous common buckwheat (Fagopyrum esculentum)

Common buckwheat (Fagopyrum esculentum) is a heteromorphic self-incompatible (SI) species with two types of floral architecture: thrum (short style) and pin (long style). The floral morphology and intra-morph incompatibility are controlled by a single genetic locus, S. However, the molecular mechanisms underlying the heteromorphic self-incompatibility of common buckwheat remain unclear. To identify these mechanisms, we performed proteomic, quantitative reverse-transcription PCR, and linkage analyses. Comparison of protein profiles between the long and short styles revealed a protein unique to the short style. Amino-acid sequencing revealed that it was a truncated form of polygalacturonase (PG); we designated the gene encoding this protein FePG1. Phylogenetic analysis classified FePG1 into the same clade as PGs that function in pollen development and floral morphology. FePG1 expression was significantly higher in short styles than in long styles. It was expressed in flowers of a short-homostyle line but not in flowers of a long-homostyle line. Linkage analysis indicated that FePG1 was not linked to the S locus; it could be a factor downstream of this locus. Our finding of a gene putatively working under the regulation of the S locus provides useful information for elucidation of the mechanism of heteromorphic self-incompatibility.

Gene flow signature in the S-allele region of cultivated buckwheat

BackgroundBuckwheat (Fagopyrum esculentum Moench.) is an annual crop that originated in southern China. The nutritious seeds are used in cooking much like cereal grains. Buckwheat is an outcrossing species with heteromorphic self-incompatibility due to its dimorphic (i.e., short- and long-styled) flowers and intra-morph infertility. The floral morphology and intra-morph incompatibility are both determined by a single S locus. Plants with short-styled flowers are heterozygous (S/s) and plants with long-styled flowers are homozygous recessive (s/s) at this locus, and the S/S genotype is not found. Recently, we built a draft genome assembly of buckwheat and identified the 5.4-Mb-long S-allele region harbored by short-styled plants.In this study, the first report on the genome-wide diversity of buckwheat, we used a genotyping-by-sequencing (GBS) dataset to evaluate the genome-wide nucleotide diversity within cultivated buckwheat landraces worldwide. We also investigated the utility of the S-allele region for phylogenetic analysis of buckwheat.ResultsBuckwheat showed high nucleotide diversity (0.0065), comparable to that of other outcrossing plants, based on a genome-wide simple nucleotide polymorphism (SNP) analysis. Phylogenetic analyses based on genome-wide SNPs showed that cultivated buckwheat comprises two groups, Asian and European, and revealed lower nucleotide diversity in the European group (0.0055) and low differentiation between the Asian and European groups.The nucleotide diversity (0.0039) estimated from SNPs in the S-allele region is lower than that in genome-wide SNPs. Phylogenetic analysis based on this region detected three diverged groups, S-1, S-2, and S-3.ConclusionThe SNPs detected using the GBS dataset were effective for elucidating the evolutionary history of buckwheat, and led to the following conclusions: (1) the low nucleotide diversity of the entire genome in the European group and low differentiation between the Asian and European groups suggested genetic bottlenecks associated with dispersion from Asia to Europe, and/or recent intensified cultivation and selection in Europe; and (2) the high diversification in the S-allele region was indicative of gene flows from wild to cultivated buckwheat, suggesting that cultivated buckwheat may have multiple origins.

Late-acting self-incompatibility and a narrow floral tube as underlying stylar dimorphism in Narcissus (Amaryllidaceae)

Most heterostylous species show self- and intramorph incompatibility and models established for such taxa have traditionally been applied to the evolution of stylar dimorphism and heterostyly in Narcissus, a genus with late-acting self-incompatibility. The model of Lloyd and Webb (1992a,b) proposed that, in an approach-herkogamous ancestor, stylar dimorphism and heterostyly appeared consecutively as a result of two single mutations selected positively to enhance cross-pollination. Most polymorphic Narcissus are stylar dimorphic with two anther whorls, the lower positioned in the middle of a narrow floral tube, and style lengths that locate the stigmas above or below the low-level anthers. Here, I propose that in an ancestor with open-tubed flowers, late-acting self-incompatibility and variable style length, the narrowing of the floral tube increased self-pollination and ovule discounting in individuals with the stigma at the same height as the low-level anthers, imposing disruptive selection against this phenotype and causing the bimodal distribution of style lengths. This hypothesis stresses the need of avoiding self-interference for the selection of stylar dimorphism. It does not exclude the promotion of cross-pollination as a force for subsequent evolution of heterostyly in the genus nor the need of inter-morph pollination for the maintenance of polymorphism.

Both morph- and species-dependent asymmetries affect reproductive barriers between heterostylous species.

The interaction between floral traits and reproductive isolation is crucial to explaining the extraordinary diversity of angiosperms. Heterostyly, a complex floral polymorphism that optimizes outcrossing, evolved repeatedly and has been shown to accelerate diversification in primroses, yet its potential influence on isolating mechanisms remains unexplored. Furthermore, the relative contribution of pre‐ versus postmating barriers to reproductive isolation is still debated. No experimental study has yet evaluated the possible effects of heterostyly on pre‐ and postmating reproductive mechanisms. We quantify multiple reproductive barriers between the heterostylous Primula elatior (oxlip) and P. vulgaris (primrose), which readily hybridize when co‐occurring, and test whether traits of heterostyly contribute to reproductive barriers in unique ways. We find that premating isolation is key for both species, while postmating isolation is considerable only for P. vulgaris; ecogeographic isolation is crucial for both species, while phenological, seed developmental, and hybrid sterility barriers are also important in P. vulgaris, implicating sympatrically higher gene flow into P. elatior. We document for the first time that, in addition to the aforementioned species‐dependent asymmetries, morph‐dependent asymmetries affect reproductive barriers between heterostylous species. Indeed, the interspecific decrease of reciprocity between high sexual organs of complementary floral morphs limits interspecific pollen transfer from anthers of short‐styled flowers to stigmas of long‐styled flowers, while higher reciprocity between low sexual organs favors introgression over isolation from anthers of long‐styled flowers to stigmas of short‐styled flowers. Finally, intramorph incompatibility persists across species boundaries, but is weakened in long‐styled flowers of P. elatior, opening a possible backdoor to gene flow through intramorph pollen transfer between species. Therefore, patterns of gene flow across species boundaries are likely affected by floral morph composition of adjacent populations. To summarize, our study highlights the general importance of premating isolation and newly illustrates that both morph‐ and species‐dependent asymmetries shape boundaries between heterostylous species.

Disassortative mating prevails in style-dimorphic Narcissus papyraceus despite low reciprocity and compatibility of morphs.

Evolution to reduce inbreeding can favor disassortative (intermorph) over assortative (intramorph) mating in hermaphroditic sexually polymorphic plant species. Heterostyly enhances disassortative pollination through reciprocal placement of stigmas and anthers of morphs and appropriate pollinators. Stylar dimorphism in which there is not reciprocal anther placement may compromise disassortative mating, particularly when there is not intramorph incompatibility. Variable rates of disassortative mating along with differential female fecundity or siring success among floral morphs could lead to variation in morph ratio. We investigated mating patterns, female fecundity, and siring success of style-length morphs in Narcissus papyraceus, a self-incompatible but morph-compatible species with dimorphic (long- and short-styled) and monomorphic (long-styled) populations in central and north regions of its range, respectively. We established experimental populations in both regions and exposed them to ambient pollinators. Using paternity analysis, we found similar siring success of morphs and high disassortative mating in most populations. Female fecundity of morphs was similar in all populations. Although these results could not completely explain the loss of dimorphism in the species' northern range, they provided evidence for the evolutionary stability of stylar dimorphism in N. papyraceus in at least some populations. Our findings support the hypothesis that prevailing intermorph mating is key for the maintenance of stylar dimorphism.

Breeding systems of enantiostylous Cassiinae species (Fabaceae, Caesalpinioideae)

The floral morphology of enantiostylous species favours pollen flow between floral morphs because it relegates pollen deposition and capture to opposite sides of a pollinator's body. This functionality of floral morphology acts as a barrier to spontaneous self-pollination. Although numerous enantiostylous species exhibit this pattern, there are a number of variations, which might play important roles in the evolution of reproductive systems of atypical species. This study aimed to investigate the self-incompatibility and intramorph incompatibility mechanisms of monomorphic enantiostylous species, considering six species and one variety of Cassiinae. Data on floral morphology, pollinator behaviour, patterns of pollen deposition and capture, and breeding system were collected. All species exhibit heteranthery that results in distinct functions of stamens. Six bee species pollinated the flowers when grasping onto the anthers and generated vibratory movements that resulted in pollen release. Pollen deposition and capture occurred on opposite sides of a pollinator’s body for the majority of Cassiinae species. All of the evaluated species were self- and intramorph-compatible, except Senna martiana, that seemed to exhibit a case of late-acting self-incompatibility. The absence of physiological self-incompatibility mechanisms in most of the studied species reinforces the relevance of floral morphology and the interaction with pollinators for the reproductive success of monomorphic enantiostylous species.

Assessment of distyly syndrome in Primula palinuri Petagn. a rare species living on maritime vertical cliffs

The distinctive habitat of vertical cliffs provides refuges for many relict plant species. Primula palinuri Petagn. is the only maritime and Mediterranean species of the genus Primula. It only occurs on the cliffs of a narrow area of southern Italy and is classified as being at risk of extinction. Given that only fragmentary information on the biology and ecology of P. palinuri is available, we studied flower morphology, breeding system and other reproductive traits of this species to furnish data for the evaluation of its conservation status. We assessed that plants of P. palinuri develop either long-styled or short-styled flowers and both morphs are equally represented in the studied populations. Reciprocal position of anthers and stigmas is complemented by differences in pollen–ovule ratios, pollen size and exine ornamentation. In both morphs, about 90 % of open-pollinated flowers developed mature capsules and the average number of seeds per capsule was around sixty. Hand pollination experiments showed that reproductive success was high (92.86 and 100 % fruit set) following inter-morph crossings, but no fruit develops after same-morph pollination or selfing. We have ascertained that full distyly syndrome (distinct floral morphs, intra-morph incompatibility and morphs equally represented in the populations) occurs in P. palinuri. However, a further reduction of the severely fragmented range, any rapid change of the environmental conditions due to human impact on coastal areas or stochastic events of plant mortality may alter the proportion of the two morphs, affecting the fragile conservation status of the species.

Functional dioecy in Morinda parvifolia (Rubiaceae), a species with stigma-height dimorphism

The evolution of dioecy from heterostyly has been well documented, but detailed studies on this transitional process are rare. Here we report the occurrence of cryptic dioecy in a perennial liana species with stigma-height dimorphism, Morinda parvifolia Bartl. ex DC. (Rubiaceae). Floral morphology, ancillary characters and cross compatibility of long-styled (L-morph) and short-styled (S-morph) were examined. L-morph and S-morph display obvious pistil dimorphisms, with the stigma of S-morph lacking papillae cells. Both floral morphs show similar pollen morphology, although pollen viability is higher in S-morph than in L-morph. S-morph flowers produce viable pollen grains but much reduced stigma and set no fruits, functioning as males; L-morphs, although with viable pollen grains and receptive stigmas, exhibit strong self- and intramorph incompatibility, with self- and intramorph pollen tubes arrested in the stigma lobes and the upper part of style, respectively, resulting in L-morphs functioning only as females. The species thus has physiological androdioecy but functional dioecy. This might be the first case showing the possibility that androdioecy could be a mid-stage in the pathway of dioecy evolving from stigma-height dimorphism.

S-LOCUS EARLY FLOWERING 3 is exclusively present in the genomes of short-styled buckwheat plants that exhibit heteromorphic self-incompatibility.

The different forms of flowers in a species have attracted the attention of many evolutionary biologists, including Charles Darwin. In Fagopyrum esculentum (common buckwheat), the occurrence of dimorphic flowers, namely short-styled and long-styled flowers, is associated with a type of self-incompatibility (SI) called heteromorphic SI. The floral morphology and intra-morph incompatibility are both determined by a single genetic locus named the S-locus. Plants with short-styled flowers are heterozygous (S/s) and plants with long-styled flowers are homozygous recessive (s/s) at the S-locus. Despite recent progress in our understanding of the molecular basis of flower development and plant SI systems, the molecular mechanisms underlying heteromorphic SI remain unresolved. By examining differentially expressed genes from the styles of the two floral morphs, we identified a gene that is expressed only in short-styled plants. The novel gene identified was completely linked to the S-locus in a linkage analysis of 1,373 plants and had homology to EARLY FLOWERING 3. We named this gene S-LOCUS EARLY FLOWERING 3 (S-ELF3). In an ion-beam-induced mutant that harbored a deletion in the genomic region spanning S-ELF3, a phenotype shift from short-styled flowers to long-styled flowers was observed. Furthermore, S-ELF3 was present in the genome of short-styled plants and absent from that of long-styled plants both in world-wide landraces of buckwheat and in two distantly related Fagopyrum species that exhibit heteromorphic SI. Moreover, independent disruptions of S-ELF3 were detected in a recently emerged self-compatible Fagopyrum species and a self-compatible line of buckwheat. The nonessential role of S-ELF3 in the survival of individuals and the prolonged evolutionary presence only in the genomes of short-styled plants exhibiting heteromorphic SI suggests that S-ELF3 is a suitable candidate gene for the control of the short-styled phenotype of buckwheat plants.

Dimorphisms and self-incompatibility in the distylous species Palicourea demissa (Rubiaceae): possible implications for its reproductive output

Distyly has been interpreted as a mechanism that favors cross-fertilization. In this research we describe floral attributes and ancillary floral polymorphisms typically associated to heterostylous plants in Palicourea demissa (Rubiaceae), a distylous shrub of the Venezuelan Andes cloud forests. A hand-pollination experiment was done to evaluate self- and intramorph incompatibility and female reproductive output in both floral morphs. The studied population was morphologically distylous but morph differences in most ancillary floral polymorphisms and reciprocity of the sexual organ heights were found. The floral morphs were self-incompatible and did not differ in fruit set under controlled cross-pollination conditions, but at the population level they exhibited imperfect reciprocal herkogamy. Fruits and seeds of short-styled plants were larger than those of long-styled plants and fruit set was higher in short-styled plants under natural conditions, suggesting a higher reproductive potential among short-styled plants. Given the 1:1 morph ratio within the studied population, further evidence is needed to determine the influence of floral visitors and seed dispersers in the expression of heterostyly in P. demissa under natural conditions.

Cryptic self‐incompatibility and distyly in Hedyotis acutangula Champ. (Rubiaceae)

Distyly, floral polymorphism frequently associated with reciprocal herkogamy, self- and intramorph incompatibility and secondary dimorphism, constitutes an important sexual system in the Rubiaceae. Here we report an unusual kind of distyly associated with self- and/or intramorph compatibility in a perennial herb, Hedyotis acutangula. Floral morphology, ancillary dimorphisms and compatibility of the two morphs were studied. H. acutangula did not exhibit precise reciprocal herkogamy, but this did not affect the equality of floral morphs in the population, as usually found in distylous plants. Both pin and thrum pollen retained relatively high viability for 8 h. The pollen to ovule ratio was 72.5 in pin flowers and 54.4 in thrum flowers. Pistils of pin flowers remained receptive for longer than those of thrum flowers. No apparent difference in the germination rate of pin and thrum pollen grains was observed when cultured in vitro, although growth of thrum pollen tubes was much faster than that of pin pollen tubes. Artificial pollination revealed that pollen tube growth in legitimate intermorph crosses was faster than in either intramorph crosses or self-pollination, suggesting the occurrence of cryptic self-incompatibility in this species. Cryptic self-incompatibility functioned differently in the two morphs, with pollen tube growth rates after legitimate and illegitimate pollination much more highly differentiated in pin flowers than in thrum flowers. No fruit was produced in emasculated netted flowers, suggesting the absence of apomixis. Our results indicate that H. acutangula is distylous, with a cryptic self-incompatibility breeding system.

Distilia e homostilia em espécies de Palicourea Aubl. (Rubiaceae) do Cerrado do Brasil Central

Distyly is a type of reproductive system whose morphological and physiological features are used as a mechanism for increasing the xenogamy. Among the 25 Angiosperm families presenting this system, Rubiaceae is the richest. The objective of this study was to characterize the distylous system of four Palicourea species from the Cerrado of Central Brazil, Palicourea marcgravii A. St.-Hil., P. officinalis Mart., P. coriacea (Cham.) K. Schum. e P. macrobotrys (Ruiz & Pav.) Roem. & Schult. This study took place in the following areas of Distrito Federal: Parque Nacional de Brasilia, Estacao Ecologica de Aguas Emendadas e Reseva Ecologica do IBGE. The species Palicourea marcgravii and P. officinalis were typicall distylous due to the presence of two floral morphs, the in ratio, and also the self and intramorph incompatibility. However, P. coriacea and P. macrobotrys species were variants because homostylous individuals occur together with distylous morphs in first specie, and because the homostyle morph is dominant in the second. It is still unknown which evolutional drivers determine those differences. However, it is likely that intrinsic genetics characteristics of each taxon have a considerable influence, and also that species may respond differently to the same selective pressures, or they may not present any response.

Floral display size and spatial distribution of potential mates affect pollen deposition and female reproductive success in distylousPulmonaria officinalis(Boraginaceae)

In animal-pollinated plants, both the spatial distribution of flowering individuals and the number of flowers that an individual displays affect pollen deposition rates and female reproductive success. Heterostylous species are likely to be particularly sensitive to the contingencies of spatial distribution, as they are reproductively subdivided into distinct mating groups, which usually exhibit self- and intra-morph incompatibility and differ in floral morphology. In this paper, we explore the joint effects of both spatial distribution of potential mates and floral display size on morph-specific pollen deposition rates and seed set patterns in two natural populations of Pulmonaria officinalis, a distylous species with a weak self-incompatibility system. Both total stigmatic pollen load and the proportion of legitimate pollen decreased with increasing spatial isolation. Legitimate (intermorph) pollen transfer was, however, asymmetric and decreased more rapidly with decreasing proximity to a compatible legitimate mating partner in the S-morph than in the L-morph. Total stigmatic pollen loads per flower increased with increasing floral display size, indicating that large plants are disproportionately more visited than smaller individuals. However, because legitimate pollen deposition decreased with increasing floral display size, these results also suggest that larger numbers of flowers increase the degree of geitonogamous pollination. In both the L- and S-morph, seed set significantly decreased with increasing isolation from a legitimate mating partner, but in the L-morph seed set was less dependent on the spatial distribution of the S-morph. In addition, seed set significantly increased with floral display size in the L-morph, but not in the S-morph. These findings indicate that the spatial distribution of potential mates and variation in floral display size may cause morph-specific differences in pollen deposition rates and female reproductive success.

The Role of Reproductive Assurance in the Arctic: a Comparative Study of a Homostylous and Distylous Species Pair

ABSTRACTPollinator-dependent reproductive systems are predicted to be disadvantageous in the Arctic due to the unreliability of pollinator services. Observations in northwestern Alaska appear to support this prediction, where the common Primula eximia possesses reproductive assurance through homostyly and is outpacing the reproduction of its rare, distylous, and pollinator-dependent congener, P. tschuktschorum. We examined the reproductive ecology of these two taxa to determine if P. tschuktschorum is fully distylous with self and intramorph incompatibility, if P. eximia is homostylous with an advantage in fecundity over the rare species, and if there are reproductive interactions between the species. Our results indicate that while P. tschuktschorum is morphologically distylous, it displays only weak incompatibility reactions, but nonetheless has significant reproductive limitations relative to the fecundity of P. eximia, especially in the face of herbivory, which reduces the pool of compatible mates. Some hybridization is occurring between the species. These results support previous suggestions that reproductive assurance may be an important factor limiting the distribution of distyly where pollinator service is unreliable. Homostylous species are likely to be more successful and outcompete distylous progenitors, and selection pressures for reproductive assurance may lessen strong self and intramorph incompatibility reactions in distylous species.

Pollination Biology and the Impact of Floral Display, Pollen Donors, and Distyly on Seed Production in Arcytophyllum lavarum (Rubiaceae)

In animal-pollinated plants, two factors affecting pollen flow and seed production are changes in floral display and the availability of compatible mates. Changes in floral display may affect the number of pollinator visits and the availability of compatible mates will affect the probability of legitimate pollination and seed production. Distyly is a floral polymorphism where long-styled (pin) and short-styled (thrum) floral morphs occur among different individuals. Distylous plants frequently exhibit self and intra-morph incompatibility. Therefore changes in morph abundance directly affect the arrival of compatible pollen to the stigmas. Floral morph by itself may also affect female reproductive success because floral morphs may display differences in seed production. We explored the effects of floral display, availability of neighboring compatible mates, and floral morph on seed production in the distylous herb ARCYTOPHYLLUM LAVARUM. We found that floral display does not affect the mean number of seeds produced per flower. There is also no effect of the proportion of neighboring legitimate pollen donors on seed production in pin or thrum flowers. However, floral morphs differed in their female reproductive success and the thrum morph produced more seeds. Hand pollination experiments suggest that differences in seed production between morphs are the result of pollen limitation. Future research will elucidate if the higher seed production in thrum flowers is a consequence of higher availability of pollen donors in the population, or higher efficiency of the pin morph as pollen donor.

Factors affecting variation in seed production in the heterostylous herbPrimula modesta

AbstractSpatial and temporal variation in the seed production of a species and the various factors affecting seed production provide essential information for understanding biotic and abiotic controls on reproduction and for assessing population trends. We conducted a 4‐year study combining field observations and pollination experiments to evaluate the major factors causing variation in seed production ofPrimula modestaat two sites (fen and grassy sites) on Mt Asama in central Japan.Primula modestaswitched from a vegetative to reproductive life phase at a stage that depended on its accumulation of biomass, and the number of flowers produced by each plant was also highly correlated with plant size. Plants at the grassy site, where snow melts 2–3 weeks earlier than at the fen site, can accumulate more biomass and, thus, bear more flowers and ovules than those at the fen site. AlthoughP. modestais distylous with both self and intramorph incompatibility, pollen availability did not limit seed production at either site. Annual seed production was relatively constant at both sites over the four seasons except 2002, but it was influenced by many environmental factors, including stochastic events, and by antagonistic interactions with herbivores and pathogens. In 2002, many flowers failed to bloom at the fen site. This was likely caused by frost damage to flower buds because of an unusually early snowmelt in that year. Unusual climatic events may be a crucial factor causing spatial and temporal variation in seed production at our study sites. Longer‐term observations are needed to evaluate the effect of unusual climatic events on reproduction.

Breeding System and Floral Morphometry of Distylous Psychotria L. Species in the Atlantic Rain Forest, SE Brazil

General patterns of floral morphology and incompatibility mechanisms have been described for many distylous plants. The absence of these patterns in typically distylous groups, as observed especially in tropical environments, is interpreted as atypical distyly, or as a new reproductive strategy derived from it. Data are presented here on the morphological and compatibility relations between floral morphs of four Psychotria dimorphic species in the Atlantic rain forest in SE Brazil: Psychotria jasminoides, P. birotula, P. mapourioides, and P. pubigera. When significant differences were found, floral parts were larger in thrum flowers. Results of controlled crosses showed that most incompatible pollen tubes were arrested in the stigma, and only in a low proportion in the upper parts of the style. We conclude that, at the study site, the majority and most important morphological and mating features of typical distyly seem to be conserved in P. jasminoides and P. mapourioides, which presented reciprocal herkogamy, self and intramorph incompatibility, and a balanced morph ratio in the population. Typical distyly in P. birotula is supported by floral morphology, pollen tube data and morph ratio and, in P. pubigera, only by floral morphology and pollen tube data.