Recent work concerning the regulation of pollen and pollen tube development at the biochemical level in angiosperms has been reviewed, commencing with the microspore immediately after meiosis and terminating with the entry of the pollen tube into the embryo sac. Some of the topics that have been considered are, (1) the role of the tapetum in pollen development, (2) gene activity in the microspore and pollen grain prior to anthesis, (3) DNA synthesis in the microspore and pollen tube, (4) the typ
es of RNA and protein synthesized in the pollen tube including the biochemistry of the vegetative and generative nuclei and finally, (5) the interactions between pollen and pistil after pollination.
Sexually dimorphic seed plants have partial or complete separation of ovule and pollen functions into two separate sexes, here uniformly called male and female. Secondary sex characters (differences between the sexes in structures other than the androecia and gynoecia) of such populations are reviewed. In a number of perennial species, males exceed females in vigor, growth rate or vegetative reproduction. In several monocarpic or short-lived polycarpic species, including hemp, spinach an
d species of Silene, females are larger than males. In asparagus, males exceed females in total growth, but individual shoots of females are larger. In some long-lived species, the greater survival rate of males than of females contributes to a predominance of males, but in species of Silene and Rumex acetosa males have a higher mortality rate than females. Males and females sometimes have different microdistributions, and in Mercurialisperennis and Rumexacetosella this appears to be associated with different environmental optima of the sexes. Differences between males and females in habit, leaf characters and minor morphological features are described. In a considerable range of species, male inflorescences emerge or bolt earlier and males begin flowering sooner than females. Several authors have suggested that females spend proportionately more of their resources on sexual reproduction (have a higher reproductive effort) than do males. The only available data, those for Rumex acetosella, support this. There are more reports of males flowering earlier in life and more frequently, having more numerous inflorescences and more flowers per inflorescence, than of females exceeding males in these features. Differences between individual male and female flowers are reviewed. Secondary sex characters may be incidental consequences of the developmental or genetic basis of sex determination and not adaptive themselves, or they may be selected as optimal strategies for the sexes, or both kinds of explanation may simultaneously be applied. We propose that many of the observed differences between the sexes are of direct selective value in relation to the distinct roles of males and females in sexual reproduction. In particular, the benefits to males of increasing pollen production and to females of increasing ovule production differ in such a way that the optimal reproductive effort will often be less for males than for females; this appears to explain a number of secondary sex characters.
Cleistogamy-the production of open (chasmogamous-CH) and closed (cleistogamous-CL) floral forms by a species-is widespread among the angiosperms. While the CL flower is autogamous, the CH flower may provide a means for outcrossing. The term "cleistogamy" has also been used to describe other phenomena. A classification of types of cleistogamy is proposed. In this review, a restricted definition of cleistogamy is used to refer to species which show real floral dimorphisms, with divergent developme
ntal pathways leading to CL and CH as well as intermediate floral forms. Reductions in the androecium and corolla are the most common feature of the CL flowers. The structural, developmental, and functional aspects of cleistogamy are reviewed. Evidence is presented to show that the CL flowers have modifications in their development which ensure self pollination. A proposal is made for using this phenomenon of dimorphic flower production as a system for the study of floral morphogenesis, function and evolution
A number of morphologic features have evolved in evolutionarily divergent plant groups that appear to increase the efficiency of wind pollination. Among these features are the appearance of low density pollen grains, female ovulate organs that direct air currents carrying pollen toward stigmas or micropyles, and population structures with a high density of con-specifics. This paper reviews the aerodynamic theory, and the experimental and field data that are relevant to an understanding of the ad
aptive significance of these and other features of anemophily. Emphasis is placed on the mathematical description of the behavior of airflow patterns around ovulate organs. The efficiency of wind pollination is shown to be dictated principally by the vectoral properties of air currents created by and around ovulate organs and the physical properties of pollen that dictate their behavior as airborne particles.
A number of theoretical and methodological problems plague studies of sex allocation in hermaphroditic plants. Most models assume that consumable resources limit reproduction, yet one or both sexes may be mate-limited. Most models also assume that resource limitation causes trade-offs between the allocations to male and female functions, but the sexes may be limited by different critical resources, or their resource needs may not overlap in time. Problems arise in deciding which plant parts are
reproductive, when to harvest them, how to apportion them to male and female roles, and what are the appropriate measures of their cost. When energy directly limits reproduction, or other needs can be translated into energetic costs, the great variation in composition of reproductive parts makes construction costs or respiration a more potentially accurate measure of investment than energy content or especially biomass. Some simple predictions from theoretical models include a 1:1 allocation ratio to male and female function in outcrossing hermaphrodites, a female-biassed ratio in proportion to the level of selfing, and lower average allocations to male function in animal-pollinated than in windpollinated plants. These predictions have not received much support from existing studies, thus emphasizing the need for better measures of allocation as well as better accounting of the many other possible factors that may determine individual (and population) allocation ratios.
A review is given of the literature concerning palm pollination. Results of this review indicate great diversity in pollination, but three basic syndromes are common in the family, cantharophily, mellitophily, and myophily. Anemophily appears uncommon and derived. Evidence of a close association between certain beetles and palms may be indicative of ancestral cantharophily.
In most self-incompatible (SI) plants, pollen tube growth in self-pollinated flowers is inhibited on the stigma or in the style. SI systems that operate in the ovary have been assumed to be extremely rare. Evidence from many plant species is presented to show that the SI barriers in the ovary, described here as late-acting SI systems, are quite common. The late-acting SI systems are divided into four categories: (1) ovarian inhibition of incompatible pollen tubes before the ovule is reached; (2)
prefertilization inhibition in the ovule; (3) post-zygotic rejection of the embryo, and (4) ovular inhibition for which the cytological details have not been established. Whether or not post-zygotic incompatibility systems can be distinguished from inbreeding depression depends upon the assumptions underlying the genetic models of self-incompatibility. However, four approaches are outlined that could distinguish between active uniform rejections that are presumably evolved responses to inbreeding depression and the passive, variable failures that are commonly understood to be expressions of typical inbreeding depression. Possible advantages of late-acting SI include an extended period of time over which pollen genotypes may be evaluated by the maternal parent and greater flexibility in the choice of male parents. Due to a paucity of data regarding the genetics and physiology of lateacting SI systems, little can be said at this time about the possible diversity of such systems of their evolutionary relationships with classical gametophytic and sporophytic SI. An hypothesis for the operation of post-zygotic SI is described whereby maternal resources to developing embryos are terminated if the embryo (and/or endosperm) fall below a threshold level of heterosis. This hypothesis is a modification of one first proposed by Westoby and Rice in 1982 to explain variable maternal resource allocation to developing embryos.
In vivo and in vitro pollen germination and pollen tube growth are affected by air pollutants. The stimulation and inhibition of these pollen characteristics depend on the pollen species as well as on the pollutant and its concentration, the exposure time, the relative humidity during exposure and some other factors. Mitosis, gas exchange, size and chemistry of pollen can also be influenced by air contaminants. Pollen germination and tube elongation differ in their response to air pollution stre
ss, and both are more susceptible in vitro compared to the in vivo situation. Some possible mechanisms of action are reported in this review study. The effects of air contaminants on pollen are discussed in relation to sexual reproduction, gametophytic selection and adaptation to stress. Pollen may be used as sensitive biological indicators of pollution. /// Pollenkeimung und das Wachstum des Pollenschlauches werden in vivo und in vitro von Luftverunreinigungen beeinflusst. Sowohl die Stimulation, als auch die Hemmung dieser Vitalitätsmerkmale des Pollens sind abhängig von der Pollenart, der Art der Verunreinigung, deren Konzentration, der Expositionszeit, der relativen Luftfeuchtigkeit und Temperatur während der Exposition, sowie chemischen Faktoren. Die Mitose, der Gasaustausch und die Stoffwechselprozesse der Pollenkörner werden von Luftverschmutzungen gestört. Die Keimung des Pollens und das Wachstum des Pollenschlauches reagieren auf den Luftverunreinigungen verschieden. In vitro reagiert die Pollenkeimung stets empfindlicher als in vivo. In dieser Literaturübersicht wird über einige, möglichen Reaktionsmechanismen berichtet. Die Effekte der Luftverunreinigungen werden diskutiert hinsichtlich der sexuellen Reproduktion, der gametophytischen Selektion und der Stressadaptation. Pollen kann als empfindlicher Bioindikator der Luftverschmutzung benutzt werden.
Discussion focuses on various terms for "nectary," "nectar," and related phenomena (e.g., "nectar-holder") and on the morphological circumscription of nectaries. "Nectarial" is the general term pertaining to nectaries. The very explicit "nectar-secreting" should be used instead of "nectariferous" to stress clearly the secretory function of cells (not all nectarial cells are nectar-secreting). "Nectary" is defined anatomically as a more or less localized, often multicellular glandular structure (
some nectaries are unicellular hairs) that occurs on vegetative or reproductive organs and that regularly secretes nectar, a sweet solution containing mainly sugars and generally serving as a reward for pollinators or for protectors (e.g., ants) against herbivores, or, in carnivorous plants, as a lure for animal prey. It seems preferable for general use to adopt Caspary's (1848) "floral/extrafloral" terminology of nectaries based on structure rather than Delpino's (1873) "nuptial/extranuptial" terminology based on function, unless there is clear evidence for the latter. All nectaries in the flower should be regarded as "floral," a designation counter to the "extrafloral" one commonly used for many nectaries abaxially located on sepals and petals. Furthermore, from a topographical viewpoint, the term "reproductive nectaries" is proposed for those nectaries present on any type of reproductive structure, whether inflorescence part, peduncle/pedicel, bract/bracteole, fruit, ovule (in gymnosperms), and, of course, any floral part. In contrast, "extra-reproductive nectaries" is proposed for nectaries occurring on strictly vegetative organs-stems, leaves, cotyledons, and their parts.
Reversion from floral to vegetative growth is under environmental control in many plant species. However the factors regulating floral reversion, and the events at the shoot apex that take place when it occurs, have received less attention than those associated with the transition to flowering. Reversions may be categorized as flower reversion, in which the flower meristem resumes leaf production, or inflorescence reversions, in which the meristem ceases to initiate bracts with flowers in their
axils and begins instead to make leaves with vegetative branches in their axils. Related to these two types of reversion, but distinct from them, are examples of partial flowering, when non-floral meristems grow out so that the plant begins to grow vegetatively again. Anomalous or proliferous flowers may form as a result of unfavourable growth conditions or viral infection, but these do not necessarily involve flower reversions. There are many examples of inflorescence reversion but fewer clearly defined cases of flower reversion. In flower reversion the meristem of the flower itself reverts to vegetative growth so that flowers with basal floral organs and distal leaves on the same axis are formed successively by the apical meristem. In Pharbitis nil, Anagallis arvensis, and Impatiens balsamina flower reversions have been caused by defined environmental conditions. However, only in Impatiens has detailed study of the changes in growth and development at the shoot apex during reversion been carried out. These studies show that changes in apical growth and phyllotaxis that typically accompany flowering can be separated from the development of floral organs and suggest that the floral stimulus plays a role throughout flower morphogenesis. The occurrence of reverting organs intermediate between leaves and petals is of particular interest in allowing experiments to be done on the progress of determination at the cell, tissue and organ levels. Reversion indicates that the flowering process must be regarded as a continuum, with physiological stages such as commitment to flower, and even morphological stages such as different floral organ types, being to varying extents artificial. Further study of the regulation of floral morphogenesis, and of the events associated with reversion, may provide important information on the nature of the factors that bring about the onset of flowering itself.
The first reports of Carex hybrids in North America appeared in the middle and late 1800's. By the early 1900's and up to the middle 1900's there was disagreement among the influential North American botanists regarding the extent to which hybrids existed. Those who recognized hybridization in Carex have been largely supported by recent studies, but only a small proportion of the reported hybrids are well documented. An understanding of hybridization in the genus is very important to systematic
research and routine identification, as well as to the study of evolution and ecology. A total of 253 Carex hybrids have been reported from North America, the greatest number of hybrids occurring in the northeast, where recent glaciation and overlapping flowering periods are probably among the causal factors. Based on current reports, hybrids are much more prevalent in subgenus Carex than in subgenus Vignea. Field studies and cultivation have proven useful in determining hybrid parentage and delimiting morphological variation, respectively. Both reproductive and vegetative characters as well as anatomical and micromorphological characters have proven taxonomically useful. Cytological studies have documented meiotic irregularities in Carex hybrids and intermediate chromosome numbers. Variation in the meiotic anomalies is apparently associated with the relationship of the parents and with differences in chromosome number. Carex hybrids are largely sterile but some restoration of fertility is evident in backcrosses. Experimental hybridization, flavonoid chromatography, enzyme electrophoresis, phenological characteristics, developmental anomalies and host-parasite relationships have all been underexploited in the characterization of hybrids. A cross-referenced list of North American Carex hybrids is included, along with a list of hybrids not yet reported for North America involving circumpolar species.
We obtained information on dichogamy and other aspects of the biology of over 4200 species of angiosperms from several hundred published and unpublished sources. We used this information to describe patterns of occurrence of dichogamy and to test specific hypotheses relating dichogamy to other characteristics of plants or their environments. Protandry was more common than protogyny at the intrafloralevel, but the reverse was true at the interfloral level. Patterns of dichogamy varied significant
ly among major taxa, with protogyny more common among monocotyledons and primitive dicotyledons, and protandry expecially common in the Asteridae. Arctic species tended to be less dichogamous and more protogynous than temperate and tropical species. Aquatic and alpine species were especially protogynous. Patterns of dichogamy varied among sexual systems, with gynomonoecious and gynodioecious species especially protandrous, and monoecious species highly protogynous. Autogamous and self-compatible species were disproportionately protogynous. Flowers of intraflorally dichogamous species were slightly larger than those of adichogamous species, owing to the presence of many autogamous species in the latter group. Species with interfloral protogyny bore much smaller flowers than did species with interfloral protandry. Early-blooming species in north-temperate and polar regions were disproportionately protogynous. Sexual structures that abscised, shriveled or moved after completion of their function tended to be presented first, and those that facilitated the other sexual function were presented second. A negative association existed between type of intrafloral and interfloral dichogamy in diclinous species. Most animal-pollinated flowers were protandrous, except beetle-pollinated and refuge and trap blossoms. Wind pollination was markedly associated with protogyny. Vertical inflorescences visited by upwardly-moving vectors were protandrous. Our results suggest that three primary factors may be involved in promoting dichogamy: selection for avoidance of pollen-pistil interference, selection for avoidance of self-fertilization, and selection for synchrony of pollen discharge and stigma receptivity in the different flower types of diclinous species. In contrast to many earlier workers we reject the thesis that avoidance of self-fertilization is the universal or even the most important force in the evolution of most forms of dichogamy. We attribute the prevalence of intrafloral protandry to selection for avoiding interference between pollen export and pollen receipt. Intrafloral protogyny was associated with imprecise pollen transfer, where other means of avoiding pollen-pistil interference (e.g., herkogamy) are likely to be of limited value. The prevalence of interfloral protogyny seems to reflect the smaller size of unisexual flowers than bisexu al flowers, the absence of intrafloral pollen-pistil interference in diclinous species, and selection for synchrony of pollen discharge from one flower type with stigma receptivity in the other.
Bateman’s principle states that male fitness is usually limited by the number of matings achieved, while female fitness is usually limited by the resources available for reproduction. When applied to flowering plants this principle leads to the expectation that pollen limitation of fruit and seed set will be uncommon. However, if male searching for mates (including pollen dissemination via external agents) is not sufficiently successful, then the reproductive success of both sexes (or both sex f
unctions in hermaphroditic plants) will be limited by number of matings rather than by resources, and Bateman’s principle cannot be expected to apply. Limitation of female success due to inadequate pollen receipt appears to be a common phenomenon in plants. Using published data on 258 species in which fecundity was reported for natural pollination and hand pollination with outcross pollen, I found significant pollen limitation at some times or in some sites in 159 of the 258 species (62%). When experiments were performed multiple times within a growing season, or in multiple sites or years, the statistical significance of pollen limitation commonly varied among times, sites or years, indicating that the pollination environment is not constant. There is some indication that, across species, supplemental pollen leads to increased fruit set more often than increased seed set within fruits, pointing to the importance of gamete packaging strategies in plant reproduction. Species that are highly self-incompatible obtain a greater benefit relative to natural pollination from artificial application of excess outcross pollen than do self-compatible species. This suggests that inadequate pollen receipt is a primary cause of low fecundity rates in perennial plants, which are often self-incompatible. Because flowering plants often allocate considerable resources to pollinator attraction, both export and receipt of pollen could be limited primarily by resource investment in floral advertisement and rewards. But whatever investment is made is attraction, pollinator behavioral stochasticity usually produces wide variation among flowers in reproductive success through both male and female functions. In such circumstances the optimal deployment of resources among megaspores, microspores, and pollinator attraction may often require more flowers or more ovules per flower than will usually be fertilized, in order to benefit from chance fluctuations that bring in large number of pollen grains. Maximizing seed set for the entire plant in a stochastic pollination environment might thus entail a packaging strategy for flower number or ovule number per flower that makes pollen limitation of fruit or seed set likely. Pollen availability may limit female success in individual flowers, entire plants (in a season or over a lifetime), or populations. The appropriate level must be distinguished depending on the nature of the question being addressed.
The flowers of the Scrophulariaceae show a great diversity in form, especially of the corolla. The most common pollinators are bees collecting nectar, pollen, or oil; other pollinators are moths and butterflies, hummingbirds,yrphid flies, and (in one case) ants. The occurrence of bell-shaped corollas in most tribes of the Scrophulariaceae and in related families indicates that this is the basic (ancestral) flower form. Derived from it are narrow tubular corollas, wide flaring ones, corollas clos
ed to unsuitable visitors by a palate (an upcurving of the tube), corollas forming a keel around the style and anthers either on the upper or lower side of the flower, corollas inflated to form a balloon, and corollas with one or two spurs. Convergences due to selection by the same or similar pollinators limit the usefulness of most of these floral features in analyzing the systematic relationships of the tribes of the Scrophulariaceae. Nevertheless, their diversity of forms exemplifies the evolutionary potential of fused perianth parts.
The interpretation that all floral events on the capitulum take place in an acropetal or centripetal sequence is widely accepted, but many contradictory examples have been found. A non-acropetal sequence has long been recognized for species with secondarily or tertiarily condensed heads, but species with uncondensed heterogamous heads also display some degree of departure from the strictly acropetal plan. Ray, or peripheral, flowers occasionally are initiated after and below/outside of the first
series of actinomorphic (disk) flowers. In addition, ray flowers almost always lag behind in both size and developmental stage, only catching up much later, prior to anthesis. Flower primordia occasionally arise as common primordia together with the subtending receptacular or involucral bract. The pappus is quite plastic with regard to the timing of its initiation and to the number and type of primordia produced, although it is seldom the first organ to be initiated.
In earlier work (Grimes, 1992) on inflorescence morphology ir. the mimosoid tribes Ingeae and Acacieae I proposed that differences in inflorescence morphology result from three properties: the organization of components of the inflorescence and their relative positions; the hierarchical arrangement of the axes of the inflorescence and the positiorn they assume in total tree architecture; and the heterochronic development of components of the inflorescence. Further work shows that the first two p
roperties are better stated in terms of heterochrony; namely, that the organization of components of the inflorescence differs due to differences in timing of the development of organ systems and that the hierarchy of axes likewise differs due to heterochronic changes. Neither de novo origin of organs or organ systems nor suppression or loss of organs or organ systems accounts for the diversity in form. Observed heterochronic differences in the inflorescence structure may be divided into three types: spatial differences in the relationship between the unit inflorescence and the subtending leaf (hysteranthy); differences in the time of formation and/or the duration of whole axes; and changes in development pathways, leading to shoot dimorphism. These heterochronies are used as characters in a cladistic analysis, and it is shown that although some are homoplasious, many provide synapomorphies ofclades of exemplars representing genera in the Ingeae and Acacieae.
We are characterizing a suite of Pisum sativum mutants that alter inflorescence architecture to construct a model for the genetic regulation of inflorescence development in a plant with a compound raceme. Such a model, when compared with those created for Antirrhinum majus and Arabidopsis thaliana, both of which have simple racemes, should provide insight into the evolution of the development of inflorescence architecture. The highly conserved nature of cloned genes that regulate reproductive de
velopment in plants and the morphological similarities among our mutants and those identified in A. majus and A. thaliana enhance the probability that a developmental genetics approach will be fruitful. Here we describe six P. sativum mutants that affect morphologically and architecturally distinct aspects of the inflorescence, and we analyze interactions among these genes. Both vegetative and inflorescence growth of the primary axis is affected by UNIFOLIA TA, which is necessary for the function of DETERMINATE (DE7). DET maintains indeterminacy in the first-order axis. In its absence, the meristem differentiates as a stub covered with epidermal hairs. DETinteracts with VEGETA TIVEJ (VEGI). VEG1 appears essential for second-order inflorescence (12) development. vegi mutants fail to flower or differentiate the 12 meristem into a rudimentary stub. det vegi double mutants produce true terminal flowers with no stubs, indicating that two genes must be eliminated for terminal flower formation in P. sativum, whereas elimination of a single gene accomplishes this in A. thaliana and A. majus. NEPTUNE also affects 12 development by limiting to two the number of flowers produced prior to stub formation. Its role is independent of DET, as indicated by the additive nature of the double mutant det nep. UNI, BROC, and PIM all play roles in assigning floral meristem identity to the third-order branch. pim mutants continue to produce inflorescence branches, resulting in a highly complex architecture and aberrant flowers. uni mutants initiate a whorl of sepals, but floral organogenesis is aberrant beyond that developmental point, and the double mutant uni pim lacks identifiable floral organs. A wild-type phenotype is observed in broc plants, but broc enhances the pim phenotype in the double mutant, producing inflorescences that resemble broccoli. Collectively these genes ensure that only the third-order meristem, not higher- or lower-order meristems, generates floral organs, thus precisely regulating the overall architecture of the plant.
The presence of capitula, the head-type of inflorescences, is widespread in the Asterideae. Several families, predominantly terminal in the clade, display the tendency of maximizing reproductive output by condensing indeterminate inflorescences to the point of capitulum formation. This is accomplished by the process of halting or suppressing development of the intemodes, an example of paedomorphosis ofthe progenesis type. This tendency is either infrequent or absent in the basal members of the A
steridae. When inflorescence condensation is present, closely related taxa often demonstrate the progression of the paedomorphosis. More examples of capitulum formation are found in the more advanced families, culminating with the Asteraceae, almost all of which display fully condensed capitula of some sort. Other phenomena are also apparent besides the basic inflorescence condensation. Edge effects are often seen, ranging from a mere crowding of the outermost flowers to the formation of additional flower types. In some taxa, inflorescence condensation continues beyond the basic capitulum form, yielding even more condensed inflorescences that then become determinate. More highly condensed inflorescences have independently evolved several times in the Asteraceae, and some tertiarily condensed inflorescences have evolved as well.
Models of atmospheric dispersal of anemophilous pollen are important tools in Quaternary plant ecology for determining pollen-source areas and for applying distance-weightings to vegetation data in formal pollen-vegetation calibrations. The most widely applied model is Prentice's model, which uses a modified form of Sutton's equation for atmospheric diffusion to predict pollen-source areas from size of the depositional basin and a set of depositional parameters (deposition velocity of the pollen
grains and mean wind speed) and atmospheric parameters (turbulence parameter, vertical diffusion coefficient). We review the physical theory underlying Sutton's equation and Prentice's model, explore the effects of different values of the depositional and atmospheric parameters on model predictions, and provide prescriptions for model application, parameter specification, and further research on pollen dispersal. Most applications of the models to pollen dispersal have assumed neutral atmospheric conditions. We argue that most pollen dispersal takes place in unstable atmospheric conditions, and prescribe appropriate values for the atmospheric parameters for unstable conditions. Our simulations using these parameters indicate more widespread pollen dispersal from a source than under neutral conditions. We review available data sets for sedimentation velocity of pollen grains, and compare the measured estimates with sedimentation velocities predicted from Stokes's Law to assess validity of the data. Substantial variability exists among data sets, but several are suitable for application to pollen-dispersal models. Finally, we discuss aspects of release, dispersal, and deposition of anemophilous pollen that are in need of further theoretical and empirical study. Such studies will contribute not only to Quatemary plant ecology but also to understanding of pollination biology, population genetics, and functional morphology of pollen grains and pollen-bearing organs.
The evolution of plant morphology is the result of changes in developmental processes. Heterochrony, the evolutionary change in developmental rate or timing, is a major cause of ontogenetic modification during evolution. It is responsible for both interspecific and intraspecific morphological differences. Other causes include heterotopy, the change of structural position, and homeosis, the replacement of a structure by another. This paper discusses and reviews the role of heterochrony in plant e
volution at the organismal, organ, tissue, cellular, and molecular levels, as well as the relationships among heterochrony, heterotopy, and homeosis. An attempt has been made to include all published studies through late 1999. It is likely that most heterochronic change involves more than one of the six classic pure heterochronic processes. Of these processes, we found neoteny (decreased developmental rate in descendant), progenesis (earlier offset), and acceleration (increased rate) to be more commonly reported than hypermorphosis (delayed offset) or predisplacement (earlier onset). We found no reports of postdisplacement (delayed onset). Therefore, although rate changes are common (both neoteny and acceleration), shifts in timing most commonly involve earlier termination in the descendant (progenesis). These relative frequencies may change as more kinds of structures are analyzed. Phenotypic effects of evolutionary changes in onset or offset timing can be exaggerated, suppressed, or reversed by changes in rate. Because not all developmental changes responsible for evolution result from heterochrony, however, we propose that plant evolution be studied from a viewpoint that integrates these different developmental mechanisms.
In the anther of angiosperms, all types of plastids are found in the course of pollen development. They are located in the different cell layers of the microsporangium and have various functions that contribute to the formation of the functional male gametophyte. This includes photosynthesis, stomata opening, sugar storage and/or mobilization, lipid synthesis and secretion for pollenkitt formation, as well as serving as a physiological buffer under stress conditions. They are also involved in pl
astid inheritance, but to different extents, according to the species. The plastid is a semi-autonomous organelle. Plastid division in the anther is synchronous with cell division, except in the vegetative cell during pollen maturation. Furthermore, recent data seem to show that plastids are affected by programmed cell death and DNA degradation, which occur in the whole anther throughout pollen development. However, the timing of plastid disappearance fluctuates in the different cell layers and also depending on species. In vitro, following androgenesis, plastids that originate in the microspore are responsible for the occurrence of albino plantlets in Poaceae. This trait reflects the relative independence of the plastid genome when compared with that of the nucleus. In this family, microspore plastids may become so involved in programmed cell death that they are unable to follow the alternative sporopohytic program. The different pathways of plastid differentiation in neighboring anther cell layers require an accurate regulation of cell development that remains widely unknown in the anther.
The evolution of secondary sexual floral traits may be driven by selection through male or female reproductive success. Even so, the gender-biased function of a floral trait is often unapparent because secondary sexual traits and primary sexual organs of both genders co-occur within most bisexual flowers. Within dichogamous plants, however, secondary sexual traits may be unambiguously expressed in association with the primary sexual organs of one gender, making these species uniquely suited to s
tudies of natural and sexual selection on floral traits. The objectives of this article are to summarize patterns of gender-biased nectar production and to critically explore theories relevant to its evolution. We list 41 species with gender-biased nectar production and provide two sets of adaptive hypotheses for the trait: sexual selection hypotheses and inbreeding avoidance hypotheses. We formulate these hypotheses using sexual selection theory in plants and the literature that relates pollinator foraging to plant inbreeding. We also consider explanations based on resource trade-offs, enemies, and genetic correlations. Support for the sexual selection and inbreeding avoidance hypotheses is provided by only a few well-studied species. We outline a series of experiments that should facilitate sorting among hypotheses. Plants with gender-biased nectar production are likely to provide unique insights into the roles of natural and sexual selection in the evolution of floral traits.
This study examines how the latitude of cultivation of Ginkgo biloba affects the timing of all phases of its sexual reproductive cycle, from pollination through germination. Seeds produced by trees growing in warm-temperate climates germinate earlier in the year than seeds produced in cold-temperate climates, and they have a longer period of time available for seedling establishment. The embryos of G. biloba seeds possess a temperature-dependent developmental-delay mechanism that allows seeds to
survive winter by preventing premature germination in the fall. This and other cold-climate adaptations appear to have evolved within the genus Ginkgo during the early Cretaceous, when the Northern Hemisphere was undergoing dramatic cooling after a long period of stable, warm conditions. Ginkgo biloba seeds possess an odoriferous sarcotesta that attracts mammalian scavengers in Asia—most notably members of the Carnivora— presumably by mimicking the smell of carrion. Seeds cleaned of their sarcotesta germinated faster and at higher percentages than those with their sarcotesta intact, suggesting that animal dispersal plays an important role in promoting seedling establishment. During the Cretaceous, potential dispersal agents included mammals, birds, and carnivorous dinosaurs.
Cleistogamy, a breeding system in which permanently closed, self-pollinated flowers are produced, has received increasing attention in recent years, but the last comprehensive review of this system was over 20 years ago. The goal of this paper is to clarify the different types of cleistogamy, quantify the number of families, genera, and species in which cleistogamy occurs, and estimate the number of times and potential reasons why cleistogamy has evolved within angiosperms. Cleistogamous species
were identified through a literature survey using 13 online databases with references dating back to 1914; only those species well-supported by floral descriptions or empirical data were included in the data set. On the basis of this survey, we suggest the use of three different categories of cleistogamy in future studies: dimorphic, complete, and induced. Based on these categories, cleistogamy in general is present in 693 angiosperm species, distributed over 228 genera and 50 families. When analyzed on a family level across the angiosperms, the breeding system has evolved approximately 34 to 41 times. Theoretical investigations indicate that the evolution of cleistogamy in taxa may be influenced by the presence of heterogeneous environments, inbreeding depression and geitonogamy, and differential seed dispersal, as well as by various ecological factors and plant size. Cleistogamy will undoubtedly be discovered in additional species as the reproductive biology of more taxa is examined in the future. Such information will be invaluable for understanding the selective pressures and factors favoring the evolution of cleistogamy as well as the evolutionary loss of this breeding system, a subject that has received little attention to date.
Recent molecular studies have elucidated the phylogeny of Compositae tribe Arctotideae, and found it to contain two, well supported, monophyletic subtribes, Arctotidineae and Gorteriinae, as well as some polyphyletic and problematic genera. On the basis of this new information, it may now be possible to identify diagnostic characters and synapomorphies to support the groupings defined within Arctotideae. Pollen characters have been shown to be particularly variable in Composi
tae. This paper aims to investigate the utility of those characters in the context of recent molecular phylogenies, in order to determine synapomorphic and diagnostic characters in Arctotideae. The pollen of each genus is described, illustrated with scanning electron micrographs, and optimised on a phylogeny of the tribe. Many pollen characters were found to be very informative when considered in the context of the current best estimate of phylogenetic relationships. Pollen morphology provides synapomorphies for clades at a number of hierarchical levels within Arctotideae, including the two subtribes, Arctotidinae and Gorteriinae, the grouping of Eremothamnus and Hoplophyllum, and smaller clades. It also supports the exclusion of Platycarpha from the tribe. The plesiomorphic palynological state for the tribe is discussed. Particular attention is paid to the evolution of different patterns of lophae (surface ridges). A single origin for the lophate condition is proposed as the most parsimonious mode of evolution in Arctotideae.
The grass family is one of the largest families of angiosperms. It includes approximately 10 000 species and 600–700 genera (Clayton & Renvoize, 1986; Watson & Dallwitz, 1992). Its taxa have been morphologically distinguished by differences in inflorescence and spikelet morphology, among other characters (Clayton & Renvoize, 1986). The grass inflorescence may be composed of one, a few or many spikelets arranged in different ways: panicles, racemes or spikes of spikelets. These types of infloresc
ences are not adequate for the characterization of the multiple variations of the grass inflorescences (Gould & Shaw, 1983; Clayton & Renvoize, 1986). The term “panicle” is too imprecise to describe the diversity of the branching system in these inflorescences (Kellogg, 2006). Many works have raised this problem (Vegetti, 1991, 2000; Cámara Hernández & Rua, 1992; Vegetti & Weberling, 1996; Rua & Weberling, 1998; Vegetti & Anton, 2000; Reinheimer & Vegetti, 2008) and have also highlighted the real complexity of the grass inflorescence (Kellogg, 2006). These works show that grasses exhibit an extensive variation in adult inflorescence forms; in addition, other works show that grasses have diverse inflorescence developmental patterns even when they have similar mature inflorescence morphologies (Doust & Kellogg, 2002; Kellogg, 2006; Reinheimer et al., 2009). In view of this, this work aims at: a) stating the basics for interpreting inflorescence structures in Poaceae and their variations; and b) correlating such information with the various developmental processes that give origin to those structures. It must be noted that this work does not intend to comprise all of the variations occurring in the inflorescence structure in the Poaceae, but it includes the most relevant types.
The structure of the synflorescence and the flowering units in Amaranthaceae are characterized. The synflorescence is polytelic. In the flowering unit we recognize the main florescence and the enrichment zone. The florescences may consist of: (1) Fully developed partial florescences bearing three or more flowers; (2) Partial florescences reduced to one or a few fertile flowers having prophylls with more or less modified axillary productions; or (3) No partial florescences but solitary flowers ha
ving prophylls with no axillary productions. We described the flowering unit in species with florescences bearing a solitary flower and the flowering unit in species with florescences bearing partial florescences. Hypothesized developmental processes are described, with a view to finding relationships among different models characterized in the family as well as defining characters for cladistic studies, which may be useful to depict all the variations observed.
Orchid pollinators have highly varied life histories with complex biotic resource requirements, about which we have limited knowledge. Among the specialist orchid pollinators are insect predators and parasitoids with specific prey types such as aphids and subterranean scarab larvae; oligolectic bees that collect pollen from limited sources such as bellflowers; euglossine bees and butterflies that collect particular plant chemicals for reproduction and self defense; oil-collecting bees that provi
sion their brood cells with floral oils from a few plants; bees that collect rare floral resins to construct their nest and brood cells; moths and butterflies that require specific larval host plants; mosquitos and horse flies that need blood; and fungus gnats and carrion flies tied to fungi and dead animals. Loss of critical biotic resources and relationships can reduce the abundance of orchid pollinators and/or their effectiveness. Protection of large, plant rich, pesticide-free orchid habitats is key to conserving essential pollinator resources.
The Mediterranean flora is spectacularly rich in orchid species that have evolved remarkable adaptations to their environment. Orchids have complex and delicate interactions with their pollinators, which makes them particularly prone to local extinction. Conservation actions should be encouraged for a range of endangered Mediterranean orchid species, but the current taxonomic confusion in several genera and the apparent disagreement among orchid taxonomists make the situation particularly confus
ing from a conservation perspective. In this review, we document how the different pollination syndromes of Mediterranean orchids (nectar reward, shelter offering, food deception and sexual deception) can have a profound impact on the type of reproductive barriers among species, on floral phenotypic variation as we perceive it, on potentially related processes of species sorting and extinction and, consequently, should have a strong influence on the related conservation management programs. We also highlight that the majority of Mediterranean orchids are pollinated by specialised bees often occupying otherwise narrow ecological niches (e.g. pollen specialisation, brood cell parasites, specific nesting site). This condition makes the orchid-pollinator interactions very fragile and several orchid species prone to local extinction. We illustrate this phenomenon by a selection of case studies that show how the adequate integration of the ecological requirements/traits of the orchids and their associated pollinators into conservation actions could help protect endangered species and ensure the sustainability of the often complex local pollination web.
While Darwin (1862, 1877) showed that reproductive success in orchid populations depended on adaptive floral morphology coupled with pollinator visitation a more recent review of the literature (Tremblay et al., 2005) confirmed that many out-breeding species are pollinator-limited because most orchid species showing low fecundity also lack rewards. The absence of rewards depresses both pollinator fidelity and the frequency of pollinator visits to an orchid population even though
orchid flowers that lack rewards retain the same interlocking floral structures for precise pollinia removal and deposition found in related species that offer rewards. Using the genus, Cypripedium, as a model lineage of non-rewarding flowers this study also shows that the correlation between low fruit set in a Cypripedium sp. and its specific pollinator(s) is insufficient to predict specific frequencies of low fecundity. Annual rates of fruit set often vary broadly between populations of the same species and within the same population over several seasons. We speculate that fruit-set rates also decline when orchid demography and additional biotic and abiotic factors interrupt rates of pollinator activity (pre-zygotic) and fertilization/fruit maturation (post-zygotic). We suggest that that traditional field studies on pollination ecology and breeding systems be combined with data sets recording genetic variation and orchid flower demography in relation to seasonal variation in climate. We also propose that the same information be collected in regard to genetic variation, demography and phenology of populations of known orchid pollinators and co-blooming angiosperm species native to orchid habitats.
Different strategies of petal senescence and some important events associated with it have been discussed. On the basis of sensitivity to ethylene and associated symptoms of senescence, petal senescence has been classified into five different classes; besides changes in membrane permeability, autophagy and involvement of VPEs (Vacuolar processing enzymes), degradation of nucleic acids, protein turn over and remobilization of essential nutrients during petal senescence have been discussed. Nucleu
s appears to play a central role in administrating the execution of the events associated with petal senescence. Protein turn over appears to be an important factor governing petal senescence in both ethylene-sensitive and ethylene-insensitive flower systems and that the loss of membrane integrity, vacuolar autophagy and remobilization of essential nutrients being its important consequences. Autophagy seems to be a main process responsible for cell dismantling and remobilization of macromolecules besides final disintegration of nucleus. A large number of senescence-associated genes have been found to be differentially expressed during petal senescence. On the basis of the available literature, a schematic model representing some important events associated with petal senescence has been constructed. The review recommends that more elaborate work is required at cellular and organelle level to understand the ethylene-independent pathway and its execution in both ethylene-sensitive and ethylene-insensitive flower systems. It also recommends that ethylene sensitivity should not be generally assigned to plants at the family level on the basis of response of a few species in a family
Floral monosymmetry and asymmetry are traced through the angiosperm orders and families. Both are diverse and widespread in angiosperms. The systematic distribution of the different forms of monosymmetry and asymmetry indicates that both evolved numerous times. Elaborate forms occur in highly synorganized flowers. Less elaborate forms occur by curvature of organs and by simplicity with minimal organ numbers. Elaborate forms of asymmetry evolved from elaborate monosymmetry. Less elaborate form co
me about by curvature or torsion of organs, by imbricate aestivation of perianth organs, or also by simplicity. Floral monosymmetry appears to be a key innovation in some groups (e.g., Orchidaceae, Fabaceae, Lamiales), but not in others. Floral asymmetry appears as a key innovation in Phaseoleae (Fabaceae). Simple patterns of monosymmetry appear easily “reverted” to polysymmetry, whereas elaborate monosymmetry is difficult to lose without disruption of floral function (e.g., Orchidaceae). Monosymmetry and asymmetry can be expressed at different stages of floral (and fruit) development and may be transient in some taxa. The two symmetries are most common in bee-pollinated flowers, and appear to be especially prone to develop in some specialized biological situations: monosymmetry, e.g., with buzz-pollinated flowers or with oil flowers, and asymmetry also with buzz-pollinated flowers, both based on the particular collection mechanisms by the pollinating bees. Floral monosymmetry has developed into a model trait in evo-devo studies, whereas floral asymmetry to date has not been tackled in molecular genetic studies.
The species-rich Gesneriaceae are typically a tropical family in Asia, showing a monophyletic radiation and high endemism in Southwest China and Hainan Island. The Chinese species have diverse floral traits and pollination systems, but the detailed patterns of diversity and possible differentiations between Hainan Island and the Chinese mainland remain unclear. Here, we enumerate the species diversity and proportion of endemics of Gesneriaceae for Hainan Island and the nearby Chinese provinces o
f Yunnan, Guangxi, Guizhou, and Guangdong. We then investigate the species evolutionary histories and main floral traits to evaluate speciation and pollination syndromes for the island and mainland flora. We recognize 24 (eight endemic) species on Hainan Island, 236 (106 endemic) in Yunnan, 211 (120 endemic) in Guangxi, 97 (28 endemic) in Guizhou, and 60 (20 endemic) in Guangdong. Although Hainan Island harbors fewer species than nearby regions, probably due to its smaller area, its endemic ratio is high, only slightly lower than Guangxi. A phylogenetic tree based on nuclear ITS 1/2 and chloroplast Trn L-F indicated most Hainan-endemic species were genetically close to species from Southeast Asia, rather than mainland China. For most floral traits, e.g., growth form, corolla type and color, Hainan Island species show no differences from other regions. Hainan Island and Yunnan Province have equal proportions of species with four or two stamens, whereas most species in other regions have two stamens. Hainan Island has a much higher percentage (21%) of actinomorphic species than other regions (< 10%). Most species in Hainan Island have exserted stamens, while stamens are included in most species in other regions. We did not find any increased tendency in floral traits that promote selfing for the island flora. Generalist pollination mechanisms, supported by characters such as actinomorphic corolla, appear to provide sufficient reproductive assurance in pollinator-depauperate Hainan Island.