Ontogenetic differences in heterostylous plants and implications for development from a herkogamous ancestor

Abstract.— Alternative ontogenetic pathways among heterostylous species of Rubiaceae may reflect differences in their evolutionary histories. In this study, measurements were taken at different developmental stages on a series of long-styled (LS) and short-styled (SS) buds of the heterostylous taxa Psychotria chiapensis, P. poeppigiana , and Bouvardia ternifolia (all Rubiaceae). Results indicated that modifications in growth rates of stamens relative to corollas in all three species led to differences in anther heights between LS and SS flowers. Distinct style heights for LS and SS flowers of P. chiapensis and P. poeppigiana originate in the earlier stages of bud development and are maintained as styles elongate at equal rates. This contrasts with B. ternifolia , which has differences in style heights resulting from unequal relative growth rates between floral morphs. The "approach herkogamous" floral morphology, defined by having stigmas positioned above anthers, has been proposed as a potential evolutionary precursor for heterostylous taxa. To examine this hypothesis, floral development of two species with approach herkogamous morphologies, Psychotria pittieri and P. brachiata , was compared to that of the three heterostylous taxa. Differences in the relative rates of style elongation for flowers of approach herkogamous versus heterostylous species predict additional steps in the original model for the evolution of heterostyly from an approach herkogamous ancestor. The diversity of heterostylous ontogenies found within Rubiaceae provides insight into potential evolutionary pathways for this sexual system in other angiosperm families.     

SMALL AND UGLY? PHYLOGENETIC ANALYSES OF THE “SELFING SYNDROME” REVEAL COMPLEX EVOLUTIONARY FATES OF MONOMORPHIC PRIMROSE FLOWERS

One of the most common trends in plant evolution, loss of self‐incompatibility and ensuing increases in selfing, is generally assumed to be associated with a suite of phenotypic changes, notably a reduction of floral size, termed the selfing syndrome. We investigate whether floral morphological traits indeed decrease in a deterministic fashion after losses of self‐incompatibility, as traditionally expected, using a phylogeny of 124 primrose species containing nine independent transitions from heterostyly (heteromorphic incompatibility) to homostyly (monomorphic self‐compatibility), a classic system for evolution of selfing. We find similar overall variability of homostylous and heterostylous species, except for diminished herkogamy in homostyles. Bayesian mixed models demonstrate differences between homostylous and heterostylous species in all traits, but net effects across species are small (except herkogamy) and directionality differs among traits. Strongly drift‐like evolutionary trajectories of corolla tube length and corolla diameter inferred by Ornstein–Uhlenbeck models contrast with expected deterministic trajectories toward small floral size. Lineage‐specific population genetic effects associated with evolution of selfing may explain that reductions of floral size represent one of several possible outcomes of floral evolution after loss of heterostyly in primroses. Contrary to the traditional paradigm, selfing syndromes may, but do not necessarily evolve in response to increased selfing.     

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.     

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.     

Flower Development and the Evolution of Self-fertilization in <Emphasis Type="Italic">Amsinckia</Emphasis>: The Role of Heterochrony

We studied the development of 26 flower traits under natural conditions in three clades of the genus Amsinckia (Boraginaceae). Each clade contained both a derived highly self-fertilizing taxon and an ancestral more highly outcrossing taxon. The more outcrossing taxa contained two flower morphs—pins and thrums—with opposite positioning of the sex organs (heterostyly). The highly selfing taxa had smaller flowers with sex organs in close proximity (homostyly). Growth trajectories were quantified over the entire or nearly the entire period from primordium initiation to flower opening. These trajectories were compared in the heterochronic framework and, in contrast with previous studies, character size was tracked over time rather than relative to another character. We focused on three hypotheses: (1) The distinct developmental trajectories leading to pins and thrums should be similar in all clades, while the trajectories leading to homostylous flowers might differ among clades. This was supported. Specifically, contrasting growth rates of stamen and pistil heights in heterostylous flowers caused pin and thrum flowers to have the reciprocal arrangement of anther and stigma heights. From the viewpoint of heterochrony, the decreased size (paedomorphosis) of the homostylous morph, compared to pins and thrums, resulted from decreased growth rate (neoteny) and earlier offset (progenesis) in all clades. Nevertheless, multiple heterochronic processes were involved in the mosaic development and evolution of homostylous flowers. (2) We tested the hypothesis that small, self-fertilizing flowers have reduced development times, one of the proposed selective advantages of increased self-fertilization rates. We found in contrast that developmental duration of homostylous flowers was either the same (two clades) or longer (one clade) compared to duration of pins and thrums. (3) Finally, we tested von Baer’s Law, which proposes that developmental differences among closely related taxa should arise later in development than differences among more distantly related taxa. Von Baer’s Law was supported strongly among homostyles, moderately among thrums and weakly among pins.     

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.     

ON THE DARWINIAN HYPOTHESIS OF THE ADAPTIVE SIGNIFICANCE OF TRISTYLY

Darwin proposed that the function of the stamen-style polymorphism in heterostylous plants is to increase the probability of legitimate (compatible) pollinations among the floral morphs. Conspicuous pollen trimorphism in tristylous Pontederia cordata enables a test of the hypothesis. Comparison of the composition of pollen loads in naturally pollinated stigmas of intact and emasculated flowers were made at a population in Paugh Lake, Ontario, which was visited primarily by bumblebees. The magnitude of legitimate pollination was analyzed by ANOVA. In intact flowers, significant legitimate pollination was detected in the long-styled morph only. Following emasculation legitimate pollination was evident in the long- and short-styled morphs, with the mid-styled morph just short of displaying significant legitimate pollination. Similar results were obtained by chi-square analysis. It has been suggested that heterostyly may reduce mutual interference between maternal and paternal reproductive function. Two aspects of pollen-stigma interference were investigated in P. cordata. The potential importance of stigmatic or stylar clogging by incompatible pollen was examined by controlled field pollinations and measurements of seed set. The results indicate that prior application of large amounts of incompatible pollen has no significant effect on the seed set of open-pollinated inflorescences. Comparison of legitimate pollen capture in intact and emasculated flowers provided no evidence that the presence of stamens within flowers of the floral morphs interferes with the receipt of legitimate pollen. Pollen-stigma interference remains to be demonstrated in heterostylous plants.     

Development of distylous flowers and investment of biomass in male and female function in Palicourea padifolia (Rubiaceae)

Knowledge of developmental pathways for achieving differences in style and anther heights, in concert with those of ancillary features accompanied with data in regard to biomass investment to male and female function, provide an excellent opportunity for examining the developmental correlations between primary and ancillary floral traits so as to understand the evolution of heterostyly. The ontogenetic relationships between bud length and anther height and between bud length and style height, and between bud length versus bud width, anther length, and number of pollen grains per anther for long-styled (LS) and short-styled (SS) morphs of P. PADIFOLIA are described. We also described the ontogenetic biomass allocation to male and female function and to corolla with elongation of buds harvested at regular intervals. We observed an early termination of stylar growth in SS buds, whereas LS styles steadily increased in size. Morph differences for relative growth rates were significant for anther height, anther length, and pollen number but not for bud width. Bud width and anther length had a negative allometric relationship with bud elongation. The relationship between bud length and number of pollen grains per anther was positive and morph differences in pollen number were detected at later stages of development. An increase in corolla mass involved a disproportionate allocation to the female function in SS flowers and male allocation was similar for the two morphs over the course of development. Our results are consistent with theoretical and empirical data for distylous species with an approach herkogamous ancestor, and with the more general hypothesis of ontogenetic lability of heterostyly, in which morph differences in style and anther heights are achieved in various ways. Variations observed in sexual investment between floral morphs suggest differences in sex expression during flower development.     

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.     

Patterns of Floral Variation between Dimorphic and Monomorphic Populations in Distylous Luculia pinceana (Rubiaceae)

Flower morphology plays an important role in the evolution and maintenance of plant mating systems, including disassortative mating of heterostylous species. The transition of mating patterns may be associated with the remodification of intraspecific flower morphology. To determine the functional relationship between floral variation and transition of mating patterns, we conducted a series of morphometric analyses in a distylous species Luculia pinceana, which possesses dimorphic and monomorphic populations. Our results indicate that floral variation was higher between different types of populations than between populations of the same type. Compared to dimorphic populations, some floral characters, reduced stigma anther separation within flowers and increased overlap of stigmas and anthers (illegitimate spatial matching of sexual organs) among individuals in populations containing only the long styled morph may have been modified to increase both selfing and intra morph crossing. The observed patterns of floral variation between dimorphic and monomorphic populations coincide with the transition of mating patterns from disassortative mating to selfing and/or intra morph crossing.     

Inversostyly: a new stylar polymorphism in an oil-secreting plant, Hemimeris racemosa (Scrophulariaceae)

A new kind of stylar polymorphism, provisionally called inversostyly, is described. The polymorphism occurs in Hemimeris racemosa (Scrophulariaceae), a common annual herb of the Cape region of South Africa. Most populations are dimorphic for style orientation: the style alternates with the two stamens and is deflected either upwards or downwards. Thus, there is reciprocal placement of the style and stamens in a vertical plane in zygomorphic flowers. The flowers are symmetrical, and the floral parts do not vary in length. All flowers on a given plant are of the same stylar orientation. Pollination is by specialized oil-collecting bees (Rediviva spp.), which carry the pollen of the two morphs separately in discrete anterior or posterior locations on the underside of the body. Most inversostylous populations have a slightly higher proportion of the style-down morph, and this bias increases with decreasing pollinator abundance. In contrast with inversostylous populations, all individuals in homostylous populations of H. racemosa have the style and the stamens clustered together in the down position and high levels of autogamous seed set. Homostylous populations of H. racemosa, as well as the homostylous species Hemimeris sabulosa, occur where oil-collecting bees are less abundant.     

被子植物异型花柱及其进化意义

异型花柱(heterostyly)是被子植物中一种特殊的花多态现象和雌雄异位形式,包括二型花柱(distyly)和三型花柱(tristyly)2种类型.据报道,在被子植物的约31个科中有异型花柱植物.该类型植物的花部特征在避免自交、促进准确的异交传粉以及通过降低雌雄功能干扰以提高亲本适合度等方面具有重要的进化意义.该文从以下3个方面总结和分析了异型花柱及其进化意义:(1)异型花柱植物的类型和花部特征、附属多态性和种群结构:(2)异型花柱植物在被子植物中的分布、起源和演化:(3)异型花柱植物的进化适应意义.结合目前作者开展的有关工作,对异型花柱植物研究中存在的一些问题进行讨论和展望,希望能为国内工作者开展该领域的研究提供一些参考.     

被子植物异型花柱及其进化意义

异型花柱(heterostyly)是被子植物中一种特殊的花多态现象和雌雄异位形式, 包括二型花柱(distyly)和三型花柱(tristyly) 2种类型。据报道, 在被子植物的约31个科中有异型花柱植物。该类型植物的花部特征在避免自交、促进准确的异交传粉以及通过降低雌雄功能干扰以提高亲本适合度等方面具有重要的进化意义。该文从以下3个方面总结和分析了异型花柱及其进化意义: (1)异型花柱植物的类型和花部特征、附属多态性和种群结构; (2)异型花柱植物在被子植物中的分布、起源和演化; (3) 异型花柱植物的进化适应意义。结合目前作者开展的有关工作, 对异型花柱植物研究中存在的一些问题进行讨论和展望, 希望能为国内工作者开展该领域的研究提供一些参考。     

Reproductive trait variation in the functionally dioecious and morphologically heterostylous island endemic Chassalia corallioides (Rubiaceae)

Documenting the floral biology and breeding system of species throughout the Rubiaceae family provides data on the number of times heterostyly and dioecy may have evolved in this large family. The objectives of this paper are to quantify (a) whether Chassalia corallioides , a small tree endemic to La Reunion Island in the Indian Ocean, is another example of the evolution of dioecy from distyly and (b) whether reproductive traits linked to male and female function vary over the ecological distribution of this species. Quantification of pollen production and fruit set following controlled and natural pollinations demonstrate that this species is dioecious. Male flowers have longer corolla tubes than female flowers. Female flowers have long styles with stigmas placed above the anthers whereas males have short styles with stigmas placed below the anthers. Stigmas and anthers are reciprocally placed in each morph, illustrating that the species is morphologically heterostylous. Both fecundity and flower size are negatively correlated with altitude. In male plants, corollas are shorter and wider and anthers are placed closer to the mouth of the corolla tube with increasing altitude. Male plants flowered more often than female plants, the likely cause of the male biased sex ratio in each of the two years studied. The evolution of dioecy in relation to the island biogeography of the region and the diversification of the genus Chassalia is discussed.     

The development of enantiostyly

Enantiostyly, the deflection of the style either to the left (left-styled) or right (right-styled) side of the floral axis, has evolved in at least ten angiosperm families. Two types of enantiostyly occur: monomorphic enantiostyly, in which individuals exhibit both stylar orientations, and dimorphic enantiostyly, in which the two stylar orientations occur on separate plants. To evaluate architectural or developmental constraints on the evolution of both forms of enantiostyly, we examined inflorescence structure and floral development among unrelated enantiostylous species. We investigated relations between the position of left- and right-styled flowers and inflorescence architecture in four monomorphic enantiostylous species, and we examined the development of enantiostyly in nine monomorphic and dimorphic enantiostylous species from five unrelated lineages. The location of left- and right-styled flowers within inflorescences ranged from highly predictable (in Solanum rostratum) to random (in Heteranthera mexicana). There were striking differences among taxa in the timing of stylar bending. In Wachendorfia paniculata, Dilatris corymbosa, and Philydrum lanuginosum, the style deflected in the bud, whereas in Heteranthera spp., Monochoria australasica, Cyanella lutea, and Solanum rostratum, stylar bending occurred at the beginning of anthesis. Comparisons of organ initiation and development indicated that asymmetries along the left-right axis were expressed very late in development, despite the early initiation of a dorsiventral asymmetry. We suggest that the evolution of dimorphic enantiostyly from monomorphic enantiostyly may be constrained by a lack of left-right positional information in the bud.     

Three-dimensional reciprocity of floral morphs in wild flax (Linum suffruticosum): a new twist on heterostyly

Here, we studied the floral morphology and pollination of the distylous plant Linum suffruticosum (Linaceae) in southern Spain.We observed a previously unreported form of distyly that involved twisting and bending of styles and stamens during floral development to achieve three-dimensional reciprocity of anthers and stigmas in the long-styled (pin) and short-styled (thrum) morphs. This developmental pattern causes pin pollen to be placed on the underside of pollinating Usia flies (Bombyliidae), and thrum pollen to be placed on the top of the thorax and abdomen. The pin stigmas contact the flies on the dorsum, apparently picking up predominantly thrum pollen, and the thrum stigmas contact the flies on the ventral surface, apparently picking up predominantly pin pollen.This form of heterostyly would appear on morphological grounds to be far more efficient in dispersing pollen between compatible morphs than the typical pin-thrum system. If so, this plant fits Darwin's prediction of efficient pollen flow between heterostylous morphs more closely than anything Darwin himself reported.Molecular phylogenetic analyses indicate that this form of heterostyly evolved in a lineage that already had typical heterostyly. The analyses also indicate that there have been several independent origins of heterostyly in Linum and at least one reversal to stylar monomorphism.     

Distyly and heteromorphic incompatibility in oceanic island species ofErythroxylum (Erythroxylaceae)

Surveys of oceanic island floras have shown that heterostyly is usually absent in such regions, probably because this floral polymorphism is often associated with a self-incompatibility system. In this context we describe the floral biology of three species ofErythroxylum on La Réunion island and examine the compatibility relationships of one of these species,E. laurifolium. All three species are distylous but differ in relative stigma-anther separation in the different morphs. In general, short-styled flowers have greater stigma-anther separation than long-styled flowers, which are often homostylous in appearance. This lack of stigma-anther separation in long-styled flowers is due to style twisting which improves reciprocity at the high organ level. The reduced stigma-anther separation does not appear to be associated with the evolution of selfing asErythroxylum laurifolium shows heteromorphic self-incompatibility. The presence of heteromorphic incompatibility in a group of species that have colonized an oceanic island is discussed.     

Stigma�Canther reciprocity, pollinators, and pollen transfer efficiency in populations of heterostylous species of Lithodora and Glandora (Boraginaceae)

According to Darwin, the reciprocal position of sexual whorls in heterostylous plants enhances disassortative pollen transfer between different floral morphs. It is believed that greater reciprocity between morphs will promote more efficient transfer of pollen. Additionally, efficient pollination will act as a selective force in achieving greater reciprocity between floral morphs. In this study we test whether variation in reciprocity of sexual organs between morphs is related to the efficiency of pollinators in transferring pollen between them. To do this, we first describe the pollinator??s array in several populations of species of the genus formerly known as Lithodora, which have different types of stylar polymorphism and degrees of reciprocity, and determine their abundance, plant visitation rate, number of flowers visited per plant and handling time in the population. We estimate the efficiency of the pollinator arrays by use of an approximation based on qualitative (location of pollen loads on different areas of insect bodies) and quantitative (plant visitation rate) measurements. Our results show a correlation between the degree of reciprocity and the efficiency of pollinators associated with the populations. These observations suggest that pollinators are a possible selective force driving the evolution of heterostyly.     

Evolutionary transitions of style polymorphisms in Lithodora (Boraginaceae)

Floral polymorphisms provide suitable model systems to test hypotheses concerning the evolution of outbreeding in plants. Although heterostyly has evolved in more than 28 angiosperm families, the evolutionary pathways involving related floral conditions have not yet been fully resolved. In this study, the reconstruction of ancestral states of style polymorphism, with both parsimony and maximum likelihood methods, was carried out for Boraginaceae species in the tribe Lithospermeae, particularly in the genus Lithodora sensu lato, where species present a wide variety of stylar conditions. Detailed floral morphometric analysis confirm different types of style polymorphism within Lithodora. They also reveal a novel style polymorphism (relaxed style dimorphism) in which anther height is variable within a flower (each anther being at a different height), which contrasts to regular distyly (constant anther height within flowers). Style monomorphism is likely to be the ancestral condition in Lithospermeae where the evolution of distyly has occurred several times. Style dimorphism is probably ancestral to distyly, as predicted by certain evolutionary models proposed for heterostyly. However, a reversion from distyly to style dimorphism also appears to occur in this tribe. This is the first documented occurrence of such a transition. This secondary style dimorphism is of the relaxed type and demonstrates the labile nature of floral polymorphisms, which are not necessarily a transition towards heterostyly. We discuss the selective forces involved in the evolution, maintainance and loss of style polymorphisms.     

Floral development and vasculature in Eriocaulon (Eriocaulaceae) provide insights into the evolution of Poales

Background and AimsFloral developmental studies are crucial for understanding the evolution of floral structures and sexual systems in angiosperms. Within the monocot order Poales, both subfamilies of Eriocaulaceae have unisexual flowers bearing unusual nectaries. Few previous studies have investigated floral development in subfamily Eriocauloideae, which includes the large, diverse and widespread genus Eriocaulon. To understand floral variation and the evolution of the androecium, gynoecium and floral nectaries of Eriocaulaceae, we analysed floral development and vasculature in Eriocaulon and compared it with that of subfamily Paepalanthoideae and the related family Xyridaceae in a phylogenetic context.MethodsThirteen species of Eriocaulon were studied. Developmental analysis was carried out using scanning electron microscopy, and vasculature analysis was carried out using light microscopy. Fresh material was also analysed using scanning electron microscopy with a cryo function. Character evolution was reconstructed over well-resolved phylogenies.Key ResultsPerianth reductions can occur due to delayed development that can also result in loss of the vascular bundles of the median sepals. Nectariferous petal glands cease development and remain vestigial in some species. In staminate flowers, the inner stamens can emerge before the outer ones, and carpels are transformed into nectariferous carpellodes. In pistillate flowers, stamens are reduced to staminodes and the gynoecium has dorsal stigmas.ConclusionsFloral morphology is highly diverse in Eriocaulon, as a result of fusion, reduction or loss of perianth parts. The nectariferous carpellodes of staminate flowers originated first in the ancestor of Eriocaulaceae; petal glands and nectariferous branches of pistillate flowers originated independently in Eriocaulaceae through transfer of function. We present a hypothesis of floral evolution for the family, illustrating a shift from bisexuality to unisexuality and the evolution of nectaries in a complex monocot family, which can contribute to future studies on reproductive biology and floral evolution in other groups.