Stylar Dimorphism Research Articles

Comprehensive transcriptomic profiling reveals complex molecular mechanisms in the regulation of style-length dimorphism in Guettarda speciosa (Rubiaceae), a species with "anomalous" distyly.

The evolution of heterostyly, a genetically controlled floral polymorphism, has been a hotspot of research since the 19th century. In recent years, studies on the molecular mechanism of distyly (the most common form of heterostyly) revealed an evolutionary convergence in genes for brassinosteroids (BR) degradation in different angiosperm groups. This floral polymorphism often exhibits considerable variability that some taxa have significant stylar dimorphism, but anther height differs less. This phenomenon has been termed "anomalous" distyly, which is usually regarded as a transitional stage in evolution. Compared to "typical" distyly, the genetic regulation of "anomalous" distyly is almost unknown, leaving a big gap in our understanding of this special floral adaptation strategy. Here we performed the first molecular-level study focusing on this floral polymorphism in Guettarda speciosa (Rubiaceae), a tropical tree with "anomalous" distyly. Comprehensive transcriptomic profiling was conducted to examine which genes and metabolic pathways were involved in the genetic control of style dimorphism and if they exhibit similar convergence with "typical" distylous species. "Brassinosteroid homeostasis" and "plant hormone signal transduction" was the most significantly enriched GO term and KEGG pathway in the comparisons between L- and S-morph styles, respectively. Interestingly, homologs of all the reported S-locus genes either showed very similar expressions between L- and S-morph styles or no hits were found in G. speciosa. BKI1, a negative regulator of brassinosteroid signaling directly repressing BRI1 signal transduction, was identified as a potential gene regulating style length, which significantly up-regulated in the styles of S-morph. These findings supported the hypothesis that style length in G. speciosa was regulated through a BR-related signaling network in which BKI1 may be one key gene. Our data suggested, in species with "anomalous" distyly, style length was regulated by gene differential expressions, instead of the "hemizygous" S-locus genes in "typical" distylous flowers such as Primula and Gelsemium, representing an "intermediate" stage in the evolution of distyly. Genome-level analysis and functional studies in more species with "typical" and "anomalous" distyly would further decipher this "most complex marriage arrangement" in angiosperms and improve our knowledge of floral evolution.

Bee-Mediated Selection Favors Floral Sex Specialization in a Heterantherous Species: Strategies to Solve the Pollen Dilemma.

Animal-pollinated plants show a broad variation in floral morphology traits and gametophyte production within populations. Thus, floral traits related to plant reproduction and sexuality are usually exposed to pollinator-mediated selection. Such selective pressures may be even stronger in heterantherous and pollen flowers, in which pollen contributes to both bee feeding and pollination, overcoming the “pollen dilemma” or the inability to perform both functions simultaneously. We describe the phenotypic gender and sexual organ morphology of flowers in two populations of Macairea radula (Melastomataceae), a heterantherous and buzz-pollinated species with pollen flowers. We estimated selection gradients on these traits through female and male fitness components. Both populations showed sizeable phenotypic gender variation, from strict hermaphrodites to increased femaleness or maleness. We found a continuous variation in style and stamen size, and this variation was correlated with corresponding shape values of both sexual organs. We detected bee-mediated selection towards short and long styles through seed number and towards intermediate degrees of heteranthery through pollen removal in one population, and selection towards increased maleness through pollen dispersal in both populations. Our results suggest that bee-mediated selection favors floral sex specialization and stylar dimorphism in M. radula, optimizing reproductive success and solving the pollen dilemma.

Avoiding sexual interference: herkogamy and dichogamy in style dimorphic flowers of Narcissus broussonetii (Amaryllidaceae).

Spatial (herkogamy) or temporal (dichogamy) separation of sex organs are mechanisms considered to restrict self-pollination and promote outcrossing. Additionally, avoidance of self-interference is proposed to be the driving force for the evolution of these mechanisms, particularly in self-incompatible species. However, species with anthers and stigmas at different levels may increase the rate of imprecise pollen transfer, resulting in pollen discounting. Non-reciprocal stylar dimorphism has been considered a transitional, unstable stage towards the evolution of reciprocal style dimorphism (distyly), to simultaneously avoid interference and lack of precision. In this study we investigate the spatial and temporal separation of sex organs in a population of the style dimorphic and self-incompatible Narcissus broussonetii and their consequences in the reciprocity between the sex organs of morphs and their fecundity. First, we evaluated the relative growth of sex organs after anthesis. Then, we studied the stigma receptivity along the flower lifespan including its effect on seed production in both morphs. Finally, given the weak reciprocity between the sex organs of morphs of this species, we estimated population genetic diversity parameters in Long- and Short-styled plants to explore differences between them as a result of rates of inbreeding due to different mating strategies. We observed that Long-styled plants and Short-styled plants present different strategies to avoid sexual interference and both of them had negative consequences in the reciprocity between the sex organs of morphs. Long-styled plants exhibited a delay in stigma receptivity and a higher growth rate of the style after anthesis, while Short-styled plants presented higher herkogamy and no delay in stigma receptivity. These findings suggest that the avoidance of self-interference, in stylar dimorphic Narcissus species, seems to be more critical than improving of reciprocity between the sex organs of morphs. This might explain why reciprocal herkogamy (distyly) is rare in the genus.

Characterization of microsatellite markers for Narcissus dubius, N. cuatrecasasii, N. assoanus and N. rupicola (Amaryllidaceae)

AbstractThe genusNarcissusL. (Amaryllidaceae) provides a model system to study the evolution and maintenance of sexual polymorphisms. In this study, we characterized microsatellite markers forN. dubius,N. cuatrecasasii,N. assoanusandN. rupicolafor studies of genetic diversity and paternity analyses to investigate the stability of stylar dimorphism. We proved 40 new primer pairs from a genomic library ofN. papyraceusand 12 microsatellite markers characterized also forN. papyraceusin a previous study (52 primer pairs overall). Twenty markers amplified, but their transferability and variability were different among species. Polymorphism was tested at least on 74 individuals and one population per species. The number of polymorphic loci per species ranged from four to eight. The number of alleles per locus ranged from two to 19 and the observed heterozygosity and gene diversity, from 0.107 to 0.729 and 0.103 to 0.894, respectively. These markers can be used for studies of genetic diversity and paternity analyses among individuals ofN. dubius,N. cuatrecasasii,N. assoanusandN. rupicolato study the stability of stylar dimorphism.

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.

Late-acting self-incompatibility and a narrow floral tube as selective forces for stylar dimorphism in Narcissus (Amaryllidaceae): A response to Simón-Porcar

Simón-Porcar proposes an alternative origin in the evolutionary pathway to heterostyly as described in a model proposed by Lloyd and Webb. Below I summarize her key points and namely consider the implications of comparing her model to ideas within the Charlesworth and Charlesworth model. The hypothesis that she has chosen to propose is similar to ones that others have proposed for this same genus, thus I have tried to distinguish how Simón-Porcar’s hypothesis differs from theirs, as well as give credit to their contributions to these ideas. Finally, I have added a few more suggestions related to testing her hypothesis.

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.

Long‐tongued insects promote disassortative pollen transfer in style‐dimorphic Narcissus papyraceus (Amaryllidaceae)

Summary In hermaphroditic flowers, reciprocal herkogamy, for example heterostyly, enhances pollen transfer between floral morphs (disassortative pollination) while avoiding self‐interference between sexual organs. By contrast, disassortative pollination might be compromised in style‐dimorphic flowers, which lack perfect reciprocity between the two floral morphs. This suboptimal functioning has been considered to explain why stylar dimorphism is rare in nature. Some style‐polymorphic species receive a wide array of floral visitors, including long‐tongued insects that feed on nectar and short‐tongued insects that feed on pollen. Differences in the morphology and behaviour of these two insect types could be manifested as different pollination patterns in each floral morph. By observing individual pollinators and pollen deposition and delivery in small field‐based experimental arrays with emasculated and intact flowers, we studied pollination patterns mediated by different insect types (long‐ and short‐tongued) in the two floral morphs (long‐ and short‐styled) of the style‐dimorphic Narcissus papyraceus. We investigated the patterns of pollen transfer between and within style morphs in cross‐pollinations, as well as self‐pollination and pollen removal rates, for each floral morph mediated by each insect type. Long‐tongued insects were efficient pollinators of both floral morphs as they removed little pollen from the anthers but deposited comparatively large amounts on the stigmas. Although disassortative pollen transfer and assortative pollen transfer were equally high to the long‐styled morph, the former prevailed in the short‐styled morph. Short‐tongued insects removed large amounts of pollen from the anthers but deposited only a few pollen grains on the long‐styled stigmas and a negligible number of grains on short‐styled stigmas regardless of the morph of the donor. Synthesis. In this study, we provide empirical support for the hypothesis that, under the action of long‐tongued pollinators, pollen transfer patterns in the stylar‐dimorphic Narcissus papyraceus resemble in part those of heterostylous species. In addition, we found that short‐tongued insects act mostly as pollen thieves, thereby limiting the male fitness of both style morphs, besides depleting the female fitness of S‐morph plants. In view of these results, we propose that the differing pollination efficiencies of floral visitors, in addition to their frequency, are key in determining the morph ratio of populations in this Narcissus.

Geographic variation in floral morphology and style‐morph ratios in a sexually polymorphic daffodil

Narcissus triandrus is a tristylous daffodil from the Iberian Peninsula that has striking geographical variation in floral morphology and style-morph ratios. Here, we investigate the relation between this variation and ecological factors to understand mechanisms governing morph ratios. We estimated morph ratios in 124 populations throughout the range of N. triandrus and measured 13 morphological traits in 35-78 populations. Sampling of morph ratios conducted over 2-10-yr intervals demonstrated strong temporal stability. Variation in floral and vegetative traits enabled statistical prediction of morph-frequency variation among populations. Latitudinal gradients in precipitation and temperature were correlated with plant and flower size, with larger flowers in northern populations associated with bumblebee visitation and stylar dimorphism. Flowers of the L- and M-morphs differed significantly from the S-morph in several size-related characters, unlike other tristylous species. This pattern and the similarity in anther positions of the L- and M-morphs suggest that the M-morph of N. triandrus originated through genetic modifiers that shortened styles of the L-morph. Our findings support the hypothesis that geographical variation in style-morph ratios is largely governed by climatic gradients in the Iberian Peninsula, which influence the floral morphology and pollination biology of N. triandrus populations.

Convergent evolution of flower polymorphism in Narcissus (Amaryllidaceae)

Summary • We present new results on morphological variation for style polymorphism and perianth features in the seven species of Narcissus sect. Apodanthi and in N. pallidulus (sect. Cyclaminei), which exhibits a wide array of flower conditions: heterostyly, style dimorphism of variable reciprocity, and style monomorphism. • There is a significant association between perianth morphology and style polymorphism in the studied species. Among significant flower features, the two heterostylous species show pendulous flowers. Most style-dimorphic species have patent flowers, whereas monomorphic and one style-dimorphic species display erect flowers. • Interpretation of a chloroplast ( trn L- trn F) phylogeny of species representing most sections in the genus leads us to conclude that the two heterostylous species have independent origins, supporting previous hypothesis of convergence of heterostyly in Narcissus . Remarkable resemblance of flower features between both species indicates that pollinator activity might have driven flower convergence. •A second chloroplast ( trn T- trn L) phylogeny in sect. Apodanthi helps elucidate some evolutionary transitions in moulding heterostyly. Stylar dimorphism appears to precede distyly and style monomorphism. Population studies on style dimorphic and monomorphic species of Narcissus are congruent with a pollinator-mediated process also involved in a shift to monomorphism.

Evolution and maintenance of stigma-height dimorphism in Narcissus. I. Floral variation and style-morph ratios

An unusual stylar dimorphism occurs in Narcissus, a plant genus of insect-pollinated Mediterranean geophytes. To determine the characteristics of the sexual polymorphism, we investigated floral variation in 46 populations of N. assoanus (section Jonquillae) and 21 populations of N. dubius (section Tazettae) in SW France. Flowers possess two stamen levels in each morph that occupy slightly different positions within the floral tube. In long-styled plants (L-morph), the stigma is located within or slightly above the upper-level stamens, whereas in short-styled plants (S-morph) the stigma is placed well below the lower-level stamens. The stigma-height dimorphism is distinct from heterostyly because the reciprocity of stigma and anther positions in the two style morphs is only weakly developed and there are no differences between the style morphs in pollen size or production. In both species, mean stigma-anther separation is much greater in the S-morph than the L-morph. In N. assoanus, population style-morph ratios vary from isoplethy (1L:1S) to L-biased, whereas in N. dubius they are usually strongly L-biased or occasionally contain only the L-morph. Populations fixed for the S-morph, or with S-biased morph ratios, were not observed. In N. assoanus, style-morph ratios were associated with population size: large continuous populations always exhibited 1:1 morph ratios, whereas smaller, fragmented populations were often L-biased. This pattern was not evident in N. dubius. We argue that biased style-morph ratios largely result from morph-specific differences in assortative mating.