Inflorescence dimorphism,heterodichogamy and thrips pollination in Platycarya strobilacea (Juglandaceae)

Background and Aims

Unlike other taxa in Juglandaceae or in closely related families, which are anemophilous, Platycarya strobilacea has been suggested to be entomophilous. In Juglandaceae, Juglans and Carya show heterodichogamy, a reproductive strategy in which two morphs coexist in a population and undergo synchronous reciprocal sex changes. However, there has been no study focusing on heterodichogamy in the other six or seven genera, including Platycarya.

Methods

Inflorescence architecture, sexual expression and pollination biology were examined in a P. strobilacea population in Japan. Flowering phenology was monitored daily for 24 trees in 2008 and 27 in 2009. Flower visitors and inhabitants were recorded or collected from different sexes and stages.

Key results

The population of P. strobilacea showed heterodichogamous phenology with protogynous and duodichogamous–protandrous morphs. This dimorphism in dichogamy was associated with distinct inflorescence morphologies. Thrips pollination was suggested by the frequent presence of thrips with attached pollen grains, the scarcity of other insect visitors, the synchronicity of thrips number in male spikes with the maturation of female flowers, and morphological characters shared with previously reported thrips-pollinated plants. Male spikes went through two consecutive stages: bright yellow and strong-scented M1 stage, and brownish and little-scented M2 stage. The latter contained more thrips, synchronized better with the receptive stage of female flowers of the reciprocal morph and is probably the main period of pollen export.

Conclusions Platycarya strobilacea

is heterodichogamous and thrips-pollinated, both of which are relatively rare conditions in angiosperms. In male spikes of P. strobilacea, there is probably a temporal decoupling of pollinator attraction and pollen export.     

Heterodichogamy in Kingdonia (Circaeasteraceae, Ranunculales)

Background and Aims

Preliminary field observations in 2001 and 2002 suggested that Kingdonia uniflora (Circaeasteraceae, Ranunculales) exhibits heterodichogamy, an unusual kind of reproductive heteromorphy, hitherto unreported in Ranunculales and known from only one other genus in basal eudicots.

Methods

During several subsequent years flowers were observed in the field. Flowers were fixed in FAA and studied with microtome sections series and with the scanning electron microscope.

Key Results

The flowers proved to be heterodichogamous, with protandrous and protogynous morphs, which have a 1 : 1 ratio. Both morphs equally set fruit. Each year a single flower is formed at the tip of a rhizome or more rarely two flowers. The flowers are already open when they appear at the soil surface, before they are receptive and before pollen is dispersed. In both floral morphs the styles elongate early and the stigmas are positioned above the anthers before anthesis begins. In protogynous flowers the stigmas become receptive in this position; later the styles become reflexed and then the anthers dehisce. In contrast, in protandrous flowers the stamen filaments elongate during early anthesis such that the dehiscing anthers come to lie above the (still unreceptive) stigmas; after dehiscence of all anthers in a flower the styles begin to elongate and become receptive.

Conclusions

This is the first record of heterodichogamy in a representative of Ranunculales, in an herbaceous eudicot, and in a plant with uniflorous ramets. The occurrence of heterodichogamy in Kingdonia in which clonal reproduction appears to be dominant might be an adaptation to avoid mating between the ramets from a common mother individual (genet).Key words: Kingdonia, Circaeasteraceae, Ranunculales, heterodichogamy, reproductive heteromorphy     

Gender specialization and factors affecting fruit set of the wind-pollinated heterodichogamous Juglans regia

Gender expression, flowering phenology, reproductive performance and factors affecting fruit set (i.e., flowering synchrony, size and distance to the nearest pollen donor) were investigated in a cultivated population of a wind-pollinated self-compatible heterodichogamous Juglans regia (Juglandaceae). Four flowering morphs, (i.e., protandrous, protogynous, male and female) were observed. The sexual functions of the protandrous and protogynous morphs were almost synchronous; however, they were not reciprocal, and the separation of male and female flowering within most monoecious individuals was not complete. Thus, within-morph mating and geitonogamous pollination may be common. The ratio of protandrous versus protogynous morphs was biased towards the protandrous morph, but the fruit set did not differ between the morphs, suggesting that the fruit set of the protandrous morph could be partly compensated by within-morph pollination. The ratio of the female flower number or fruit number to the total male catkin length was higher in the protogynous morph than in the protandrous morph and did not vary with plant size, suggesting that gender variation was not size dependent and that the sexual function of protandrous morphs was more male biased. Fruit set depended on plant size only for protandrous morphs. The fruit set of individual plants decreased with increasing distance to the nearest pollen donor regardless of morph, possibly because of pollen limitation. The fruit set of individual plants increased with flowering synchrony, indicating that flowering synchrony could affect reproductive success.     

Acer pseudoplatanus (Sapindaceae): Heterodichogamy and thrips pollination

One of the pathways to dioecy is via heterodichogamy, a system including protandrous (flowering male first) and protogynous (female first) plants. Using a research crane the reproductive ecology of the heterodichogamous Acer pseudoplatanus was studied in 74 mature trees over 2 years. The synchronized flowering phenology of the trees resulted in reciprocal pollination between the two morphs. Protandrous trees were more numerous (3:1), had more female flowers (2–3:1), had much less pollen on their stigmas (1:15) and had a much lower seed to fruit ratio (1:3–4). The pollinators were probably breeding thrips. The heterodichogamy of A. pseudoplatanus is confirmed and underlined as a functioning ecological system. Depending on the way pollination efficiency changes in time, either of the morphs can be interpreted as “more female” or “more male”. The evolution of heterodichogamy towards dioecy thus depends on more components of the reproductive ecology than have been assumed.     

Gender specialization in heterodichogamous Grayia brandegei (Chenopodiaceae): evidence for an alternative pathway to dioecy

We examined components of male and female reproductive success in protogynous and protandrous sexual morphs of the heterodichogamous and largely monoecious chenopod shrub Grayia brandegei. Percentage femaleness of flowering stalks ranged from 0 to 37.6% female ( = 15.5%) for protandrous plants and from 14 to 100% female ( = 55.8%) for protogynous plants. Functional gender estimates based on ovule production at two locations ranged from 23.0 to 31.8% female for the protandrous morph, and from 65.3 to 77.0% female for the protogynous morph. Realized gender estimates based on total seed production ranged in value from 3.6 to 16.8% female for the protandrous morph and from 76.5 to 96.4% for the protogynous morph, depending on location and year. Differences in reproductive success of the two morphs were largely due to a reduction in the female function of protandrous plants. Protogynous plants produced more female flowers per stalk and had a higher percentage of seed-filled fruits than did protandrous plants. Differences between sexual morphs were more pronounced in dry areas or years in which overall seed production was minimal. Differential seed production between morphs likely reflects temporal patchiness in environmental conditions, particularly in water availability. The significance of these findings in support of heterodichogamy as an evolutionary pathway to dioecy is discussed.     

Flowering phenology and mating success of the heterodichogamous tree Machilus thunbergii Sieb. et Zucc (Lauraceae)

Heterodichogamy is defined as the presence of two flower morphs that exhibit male and female functions at different times among individuals within a population, and is regarded as an adaptation to promote outbreeding through enhanced intermorph pollination. In highly fragmented populations in which the morph frequency is biased, heterodichogamy may hamper population growth by reducing seed sets of the more numerous morph, and enhancing seed sets of the less numerous morph. In such situations, we hypothesize that individual plants experience greater seed sets if the opposite sexual morphs are nearby, and that individuals of a less numerous sexual morph have greater seed sets. After confirming heterodichogamy by observing flowering behavior and phenology, we tested these two hypotheses in a highly fragmented population of Machilus thunbergii, a putative heterodichogamous evergreen laurel tree. Our observations confirmed that M. thunbergii is heterodichogamous, consisting of two types of protogynous and bisexual flowers: a morning female (MF)–afternoon male morph and a morning male (MM)–afternoon female morph at the individual level. Sexual expression of the two morphs was highly synchronized and reciprocal. Investigation of seed‐set rates revealed greater rates of both morphs if the opposite morph was nearby. The less numerous sexual morph (MF) showed a greater seed‐set rate than the more numerous sexual morph (MM).     

Disassortative mating, sexual specialization, and the evolution of gender dimorphism in heterodichogamous Acer opalus

In sexually polymorphic species, the morphs are maintained by frequency-dependent selection through disassortative mating. In heterodichogamous populations in which disassortative mating occurs between the protandrous and protogynous morphs, a decrease in female fitness in one morph is hypothesized to drive sexual specialization in the other morph, resulting in dimorphic populations. We test these ideas in a population of the heterodichogamous species, Acer opalus . We assessed both prospective gender of individuals in terms of their allocations and actual parentage using microsatellites; we found that most matings in A. opalus occur disassortatively. We demonstrate that the protogynous morph is maintained by frequency-dependent selection, but that maintenance of males versus protandrous individuals depends on their relative siring success, which changes yearly. Seeds produced later in the reproductive season were smaller than those produced earlier; this should compromise reproduction through ovules in protandrous individuals, rendering them male biased in gender. Time-dependent gender and paternity analyses indicate that the sexual morphs are specialized in their earlier sexual functions, mediated by the seasonal decrease in seed size. Our results confirm that mating patterns are context-dependent and change seasonally, suggesting that sexual specialization can be driven by seasonal effects on fitness gained through one of the two sexual functions.     

Phenological match drives pollen‐mediated gene flow in a temporally dimorphic tree

• Variation in flowering phenology is common in natural populations, and is expected to be, together with inter‐mate distance, an important driver of effective pollen dispersal. In populations composed of plants with temporally separated sexual phases (i.e. dichogamous or heterodichogamous populations), pollen‐mediated gene flow is assumed to reflect phenological overlap between complementary sexual phases. In this study, we conducted paternity analyses to test this hypothesis in the temporally dimorphic tree Acer opalus.
• We performed spatially explicit analyses based on categorical and fractional paternity assignment, and included tree size, pair‐wise genetic relatedness and morph type as additional predictors. Because differences between morphs in flowering phenology may also influence pollination distances, we modelled separate pollen dispersal kernels for the two morphs.
• Extended phenological overlap between male and female phases (mainly associated with inter‐morph crosses) resulted in higher siring success after accounting for the effects of genetic relatedness, morph type and tree size, while reduced phenological overlap (mainly associated with intra‐morph crosses) resulted in longer pollination distances achieved. Siring success also increased in larger trees.
• Mating patterns could not be predicted by phenology alone. However, as heterogeneity in flowering phenology was the single morph‐specific predictor of siring success, it is expected to be key in maintaining the temporal dimorphism in A. opalus, by promoting not only a prevalent pattern of inter‐morph mating, but also long‐distance pollination resulting from intra‐morph mating events.

     

Fluctuating selection by water level on gynoecium colour polymorphism in an aquatic plant

Background and Aims

It has been proposed that variation in pollinator preferences or a fluctuating environment can act to maintain flower colour polymorphism. These two hypotheses were tested in an aquatic monocot Butomus umbellatus (Butomaceae) with a pink or white gynoecium in the field population.

Methods

Pollinator visitation was compared in experimental arrays of equivalent flowering cymes from both colour morphs. Seed set was compared between inter- and intramorph pollination under different water levels to test the effect of fluctuating environment on seed fertility.

Key Results

Overall, the major pollinator groups did not discriminate between colour morphs. Compared with the white morph, seed production in the pink morph under intermorph, intramorph and open pollination treatments was significantly higher when the water level was low but not when it was high. Precipitation in July was correlated with yearly seed production in the pink morph but not in the white morph.

Conclusions

The results indicated that the two colour morphs differed in their tolerance to water level. Our study on this aquatic plant provides additional evidence to support the hypothesis that flower colour polymorphism can be preserved by environmental heterogeneity.     

Effects of population density, sex morph, and tree size on reproduction in a heterodichogamous maple, Acer mono, in a temperate forest of Japan

The effects of local population density, sex morph [protogynous (PG) or protandrous (PA)], and individual tree size on the demographic processes of seed production were investigated in a heterodichogamous maple, Acer mono Maxim. var. Marmoratum (Nichols.) Hara f. dissectum, in a temperate forest of Japan. As the distance from conspecific reproductive adults increased, the percentage of immature seed fall and empty seeds increased significantly, indicating higher pollination success along with local population density. Although the difference was not distinct, pollination success was affected by the local population density of the reciprocal sex morph rather than that of both sex morphs. The trees at higher local population density sites suffered higher seed mortality due to predation and decay, and tended to produce smaller seeds. Thus, the impacts of local population density operated both positively and negatively on reproduction. As a factor of individual traits, tree size scarcely affected any demographic processes. On the other hand, sex morph did affect pollination success. Trees of PG type had lower immature seed fall than those of PA type, suggesting that the former has higher efficiency of pollen acceptance than the latter. The results on seed demography presented here partly support previous suggestions that heterodichogamous plants exhibit reciprocal cross-pollination and gender specialization as reproductive traits.     

HETERODICHOGAMY IN GRAYIA BRANDEGEI (CHENOPODIACEAE): REPORT FROM A NEW FAMILY

The reproductive biology of Grayia brandegei was examined. Although monoecious, Grayia brandegei exhibits a phenotypic dimorphism of protogynous and protandrous mating types known as heterodichogamy. For individual plants, the temporal separation of staminate and pistillate flowering phases appears to be complete. No self-fertilization is possible in protandrous plants, but may be possible in protogynous plants provided pistillate flowers remain unfertilized. Flowering phases of protogynous and protandrous mating types are synchronized and reciprocal, ensuring cross-fertilization between mating types. Less than 15% overlap of sexual functions occurred between plants of the same mating type. Protogynous and protandrous plants were randomly dispersed in the environment and in relation to each other. Mating type frequencies did not differ significantly from 1:1. We discuss the possibility of a heterodichogamous pathway to dioecy.     

SEXUAL TETRAMORPHISM IN THYMELAEA HIRSUTA (THYMELAEACEAE): EVIDENCE OF THE PATHWAY FROM HETERODICHOGAMY TO DIOECY AT THE INFRASPECIFIC LEVEL

After more than 5 years of study on natural populations of Thymelaea hirsuta (L.) Endl. (Thymelaeaceae), four distinct sexual phenotypes are shown to occur: protogynous, protandrous, subandroecious (male), and subgynoecious (female) individuals. The four populations studied differ significantly in the relative abundance of sexual types. Plant size and sexual phenotype are independent in three populations. Each sexual form produces viable seeds. Growth and morphogenesis of the aerial vegetative and sexual parts are described and found not to be related to sexual phenotype. The system, called sexual tetramorphism, combines characteristics of subdioecy (subandroecious and subgynoecious individuals) and heterodichogamy (protogynous and protandrous individuals). High fruit production in females and protandrous plants vs. low fruit production in males and protogynous plants provides evidence of a strong tendency toward functional dimorphism. The probable specialization of protogynous types toward the male function and the obvious specialization of protandrous types toward the female function suggest an evolution from heterodichogamy to dioecy, sexual tetramorphism being the intermediate stage. Tetramorphism in T. hirsuta provides further support for the hypothesis of an evolutionary pathway from heterodichogamy to dioecy.     

Clonal growth is enhanced in the absence of a mating morph: a comparative study of fertile stylar polymorphic and sterile monomorphic populations of Nymphoides montana (Menyanthaceae)

Background and Aims

Many aquatic species with stylar polymorphisms have the capacity for clonal and sexual reproduction and are sensitive to the balance of the two reproductive modes when there are a limited number of mating morphs within a population. This study asked how the clonal and sexual reproductive modes perform in populations that contain only a single morph and where fitness gain through sexual reproduction is rare. In clonal aquatic Nymphoides montana, polymorphic populations normally contain two mating morphs in equal frequencies. Populations are sexually fertile and appear to be maintained by pollen transfer between the two partners. However, in a monomorphic population of N. montana where mating opportunities are unavailable, female and male function is impaired and clonality maintains the population. Here, the consequences of intraspecific variation in sexuality were explored between monomorphic and polymorphic N. montana populations in eastern Australia.

Methods

Comparative measurements of male and female fertility, total dry mass and genotypic diversity using ISSR markers were made between populations with variable sexuality.

Key Results and Conclusions

Very few seeds were produced in the monomorphic population under natural and glasshouse conditions due to dysfunctional pollen and ovules. Stigma–anther separation was minimal in the monomorphic population, which may be a consequence of the relaxed selective pressures that regulate the maintenance of sexual function. However, clonal reproduction was favoured at the expense of sexual reproduction in the monomorphic population; this may facilitate the establishment of sterility throughout the population via resource reallocation or pleiotropic effects. The ISSR results showed that the monomorphic population was one large, single genotype, unlike the multi-genotypic fertile polymorphic populations. Evolutionary loss of sex in a clonal population in which a mating morph is absent was evident; under these conditions clonal growth may assure reproduction and expand the population via spreading stolons.     

Heterodichogamy and nitidulid beetle pollination in Anaxagorea prinoides, an early divergent Annonaceae

Heterodichogamy in a natural population of an Annonaceae species from the rainforests of French Guiana is described for the first time. Anaxagorea prinoides had bisexual flowers and two floral morphs within the studied population were protogynous. The population under study comprised 7 mature trees belonging to one morph and 12 to the other. Statistical analyses showed that the two morphs were in a 50:50% ratio, and therefore the temporal sexual pattern of heterodichogamy is given. When anthesis of flowers in the male stage ended in one morph, anthesis started with flowers in the female stage in the complementary morph. Approximately 1?h before the end of anthesis in one morph, flowers of the reciprocal morph started to emit a fruit-like scent. The temporal separation of the female and the male stages of the two different morphs lasted only approximately 1?h. Six of the seven identified compounds in the banana-like floral scent were esters and one was an alcohol. The main compounds examined are known to be components of fruit scents. Nitidulidae beetles of the genus Colopterus were the pollinators of A. prinoides and during flowering were maintained within the population of this species. This was not only due to the fact that the beetles remained sheltered in the pollination chamber of the flowers, but also because upon release from individuals of male-stage flowers at the end of flower anthesis they were attracted by the odoriferous female-stage flowers of other individuals of the same population. Heterodichogamy of A. prinoides appears to be a means by which reproductive success is augmented. Attraction of beetle pollinators by ??fruit-imitating?? floral scent is not restricted to species of Anaxagorea, but occurs in many representatives of the Annonaceae.     

Flowering phenology and wind-pollination efficacy of heterodichogamous Juglans mandshurica (Juglandaceae)

BACKGROUND AND AIMS: Heterodichogamy differs from normal dichogamy, in that it involves two mating types (protogyny and protandry) that occur at a 1 : 1 ratio in a population. Flowering phases of the two mating types are synchronized and reciprocal, which was considered to ensure between-type outcrossing. This study aims to quantify the flowering pattern and pollination efficacy in Juglans mandshurica, a wind-pollinated heterodichogamous tree. METHODS: The pattern of flowering phenology was monitored within individual trees and pollen traps were used to measure air-borne pollen loads during the spring in 2003 and 2004. Pollen longevity was determined by staining technique. Also a pollen supplementation experiment was performed in 2004 to assess pollen limitation of fruit production. KEY RESULTS: There was no overlap between sexual functions within individual trees. Flowering periods of the two mating types were reciprocal and synchronous in both 2003 and 2004. Air-borne pollen loads were large, and protogynous and protandrous individuals each produced a high pollination peak, consistent with the two blooming periods. Maximum pollen longevity was about 4 h for protandrous individuals, and 3 h for protogynous individuals. Pollen supplementation did not increase fruit production in either protogynous or protandrous individuals. CONCLUSIONS: Heterodichogamous flowering in Juglans mandshurica effectively avoids selfing, promotes between-type outcrossing, and leads to efficient pollination in a natural population.     

Floral polymorphism and the fitness implications of attracting pollinating and florivorous insects

Background and Aims

Floral polymorphism is frequently attributed to pollinator-mediated selection. Multiple studies, however, have revealed the importance of non-pollinating visitors in floral evolution. Using the polymorphic annual daisy Ursinia calenduliflora, this study investigated the importance of different insect visitors, and their effects on fitness, in the maintenance of floral polymorphism.

Methods

The spatial structure of a discrete floral polymorphism was characterized based on the presence/absence of anthocyanin floret spots in U. calenduliflora. A 3-year observational study was then conducted in polymorphic populations to investigate differences in visitation rates of dominant visitors to floral morphs. Experiments were performed to explore the floral preference of male and female Megapalpus capensis (the dominant insect visitor) and their effectiveness as pollinators. Next, floral damage by antagonistic florivores and the reproductive success of the two floral morphs were surveyed in multiple populations and years.

Key Results

Floral polymorphism in U. calenduliflora was structured spatially, as were insect visitation patterns. Megapalpus capensis males were the dominant visitors and exhibited strong preference for the spotted morph in natural and experimental observations. While this may indicate potential fitness benefits for the spotted morph, female fitness did not differ between floral morphs. However, as M. capensis males are very efficient at exporting U. calenduliflora pollen, their preference may likely increase the reproductive fitness of the spotted morph through male fitness components. The spotted morph, however, also suffered significantly greater costs due to ovule predation by florivores than the spotless morph.

Conclusions

The results suggest that pollinators and florivores may potentially exert antagonistic selection that could contribute to the maintenance of floral polymorphism across the range of U. calenduliflora. The relative strength of selection imposed by each agent is potentially determined by insect community composition and abundance at each site, highlighting the importance of community context in the evolution of floral phenotypes.     

Floral polymorphism in Chamaecrista flexuosa (Fabaceae-Caesalpinioideae): a possible case of atypical enantiostyly?

Background and Aims

Reciprocal herkogamy, including enantiostyly and heterostyly, involves reciprocity in the relative positions of the sexual elements within the flower. Such systems result in morphologically and, since pollen is deposited on and captured from different parts of the pollinator, functionally distinct floral forms. Deviations from the basic pattern may modify the functionality of these mechanisms. For heterostylous species, such deviations are generally related to environmental disturbances, pollination services and/or reduced numbers of one floral morph. Deviations for enantiostylous species have not yet been reported. This study aims to investigate enantiostyly in Chamaecrista flexuosa, in particular the presence of deviations from the standard form, in an area of coastal vegetation in north-east Brazil.

Methods

Observations and investigations of floral biology, the reproductive system, pollinator behaviour, floral morphology and morphometry were performed.

Key Results

In C. flexuosa flowers, anthers of different size but similar function are grouped. The flowers were self-compatible and set fruits after every treatment, except in the spontaneous self-pollination experiment, thereby indicating their dependence on pollen vectors. The flowers were pollinated by bees, especially Xylocopa cearensis and X. grisencens. Pollen is deposited and captured from the ventral portion of the pollinator''s body. Variations in the spatial arrangement of floral elements allowed for the identification of floral morphs based on both morphological and functional criteria. Using morphological criteria, morphologically right (MR) and morphologically left (ML) floral morphs were identified. Three floral morphs were identified using functional criteria: functionally right (FR), functionally central (FC) and functionally left (FL). Combinations of morphologically and functionally defined morphs did not occur in equal proportions. There was a reduced frequency of the MR–FR combination.

Conclusions

The results indicate the occurrence of an atypical enantiostyly in C. flexuosa. This seems to improve reproductive success by increasing the efficiency of pollen deposition and capture.     

Adaptive significance of flexistyly in Alpinia blepharocalyx (Zingiberaceae): a hand-pollination experiment

BACKGROUND AND AIMS: Flexistyly is a sexual dimorphism where there are two morphs that differ in the temporal expression of sexual function and also involve reciprocal movement of the stigmatic surface through a vertical axis during the flowering period. The adaptive significance of flexistyly has been interpreted as a floral mechanism for outcrossing, but it may also function to reduce sexual interference in which styles and stigmas impede the pollen export. Here these two explanations of flexistyly were tested in Alpinia blepharocalyx through a hand-pollination experiment. METHODS: Hand-pollinations were performed in two temporal morphs and consisted of two sequential pollination treatments, namely self-pollination in the morning and inter-morph pollination in the afternoon (treatment 1) or conversely inter-morph pollination in the morning and self-pollination in the afternoon (treatment 2), and two simultaneous self- and inter-morph cross-pollination treatments either in the morning (treatment 3) or in the afternoon (treatment 4). Seed paternity was then determined to assess relative success of self- versus cross-pollen using allozyme markers. KEY RESULTS: In the sequential pollination treatments, whether the stigmas of recipients are receptive in the morning is crucial to the success of the pollen deposited. When the cataflexistylous (protandrous) morph served as pollen recipient, early-arriving pollen in the morning can sire only a very small proportion (<15%) of seeds because the stigmas were then unreceptive. However, when the anaflexistylous (protogynous) morph served as pollen recipient, early pollen did gain a large competitive advantage over the late pollen, particularly when cross-pollen arrived first. Simultaneous self- and inter-morph cross-pollination indicated that outcross-pollen is more competitive than self-pollen on receptive stigmas. CONCLUSIONS: Differential maturing of male and female organs in Alpinia blepharocalyx is sufficient for selfing avoidance, obviating the need for style movements. Instead, the upward style curvature of the cataflexistylous morph in the morning and the anaflexistylous morph in the afternoon most likely represents a means of reducing interference with pollen export.     

Patterns of style polymorphism in five species of the South African genus Nivenia (Iridaceae)

Background and Aims

Heterostylous plants have been characterized by the presence of two or three discrete morphs that differ in their sex organ position within populations. This polymorphism is widely distributed among the angiosperms, but detailed studies are limited to few taxonomic groups. Although a small representation, evolutionary meaningful variations of the heterostylous syndrome have been reported when precise measurements of the sexual whorls were taken. A thorough exploration of groups where heterostyly has been reported should offer new opportunities to further testing the evolutionary hypotheses explaining heterostyly. Here, the traits defining heterostyly were explored in half of the species in Nivenia, the only genus of Iridiaceae where heterostyly has been reported.

Methods

Detailed morphometric analysis of the flower sexual whorls and some traits considered as ancillary are supplied to determine for each population (a) the kind of stylar polymorphism, (b) the morph ratio and (c) the degree of reciprocity between sexual whorls. Also the rates of assortative (within morph) versus disassortative (between morphs) pollen transfer were estimated by analysing pollen loads on stigmas. The association between floral phenotypic integration and the reciprocity between sexual whorls was estimated; both characteristics have been quoted as dependent on the accuracy of the fit between pollinators and flowers and therefore related to the efficiency of pollen transfer.

Key Results

Different types of polymorphism, differing in their degree of reciprocity, were found in Nivenia. Effective disassortative mating appears to be common, since (a) all dimorphic populations show equal morph-ratios (isoplethy), and (b) the pollen placed on the stigmas of each morph is likely to be coming from the other (complementary) morph. The most reciprocal populations of the heterostylous species have also the highest values of phenotypical integration.

Conclusions

Stigma height dimorphism, as opposed to distyly, is proven for the first time in Nivenia. The presence of different types of polymorphism within the genus is consistent with hypotheses of the evolution of heterostyly. The role of the pollinators as the leading force of the transition seems to be apparent, since floral integration is related to reciprocity.     

Morph-specific mating patterns in a low-density population of a heterodichogamous tree, Juglans ailantifolia

Heterodichogamy, including protandrous (PA) and protogynous (PG) morphs, is considered a mechanism to avoid selfing and promote disassortative mating. Although morphotypes are usually present in a population at a 1:1 ratio, this ratio may be biased in a low-density population by demographic stochasticity, resulting in a deficiency of mating partners in a neighbourhood dominated by a single morph. In this study, we determined morph ratio of the heterodichogamous tree species, Juglans ailantifolia by observing flowering in a low-density population during 2?years. The morph ratio (PG: PA) of 2.56:1 deviated significantly deviated from 1:1. We genotyped 59 reproductive trees and 405 offspring derived from eight PG-mother and three PA-mother trees with 11 microsatellite markers. Paternity analysis was conducted to clarify the effects of mother morph on the proportion of intra-morph mating. Then, we applied the Bayesian mixed effect mating model (MEMM) to clarify mating system, pollen dispersal, and individual fecundity of PG- and PA-mother trees. We found that the selfing rate and the distance of pollen dispersal were not clearly different between PG- and PA-mother trees. In contrast, the proportion of intra-morph mating was higher in the majority-morph (PG) mother trees than in the minor-morph (PA) mother trees. The MEMM indicated that mean dispersal distance of PG-mother trees was larger than that of PA-mother trees with large variance. Furthermore, we observed individuals with unusually high intra-morph fecundity for majority-morph (PG) trees. These findings indicate that intra-morph mating may occur when majority-morph mothers suffer a deficiency of potential inter-morph mates.