The evolution of ovule number and flower size in wind-pollinated plants

In angiosperms, ovules are "packaged" within individual flowers, and an optimal strategy should occur depending on pollination and resource conditions. In animal-pollinated species, wide variation in ovule number per flower occurs, and this contrasts with wind-pollinated plants, where most species possess uniovulate flowers. This pattern is usually explained as an adaptive response to low pollen receipt in wind-pollinated species. Here, we develop a phenotypic model for the evolution of ovule number per flower that incorporates the aerodynamics of pollen capture and a fixed resource pool for provisioning of flowers, ovules, and seeds. Our results challenge the prevailing explanation for the association between uniovulate flowers and wind pollination. We demonstrate that when flowers are small and inexpensive, as they are in wind-pollinated species, ovule number should be minimized and lower than the average number of pollen tubes per style, even under stochastic pollination and fertilization regimes. The model predicts that plants benefit from producing many small inexpensive flowers, even though some flowers capture too few pollen grains to fertilize their ovules. Wind-pollinated plants with numerous flowers distributed throughout the inflorescence, each with a single ovule or a few ovules, sample more of the airstream, and this should maximize pollen capture and seed production.     

OVULE PACKAGING IN STOCHASTIC POLLINATION AND FERTILIZATION ENVIRONMENTS

The modular morphology of plants has important consequences for reproductive strategies. Ovules are packaged in discrete structures (flowers) that usually vary stochastically in pollen capture and ovule fertilization, because of the vagaries of pollen transfer by external agents. Different ovule packaging schemes may use limited reproductive resources more or less effectively, so that some number of ovules per flower may be optimal, given the prevailing probabilities of ovule fertilization. I derive a phenotypic model for ovule number per flower that maximizes the expected total ovule fertilizations on a plant when pollination and fertilization vary randomly among individual flowers. This model predicts that, except for small or inexpensive flowers, ovules should be “oversupplied” relative to the mean receipt of pollen tubes, so that pollen limitation of seed set should be common. Published data are congruent with this prediction. Additional hypotheses on the relation of ovule packaging to floral cost, plant size, and variance in pollen receipt are suggested by the model, but few data exist to evaluate these hypotheses.     

Bateman’s principle and plant reproduction: The role of pollen limitation in fruit and seed set

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 functions 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.     

毛翠雀花花序内的性分配和繁殖成功

两性花植物花序内不同位置的性分配和繁殖成功一般存在差异,通常认为资源竞争、结构效应和交配环境是形成这种差异的主要原因。为了研究雄性和雌性繁殖资源在花序内不同位置间的最优分配问题,该文以青藏高原高寒草甸典型高山植物毛翠雀花为材料,通过比较花序内不同位置的花部特征和种子性状,对其花序内的性分配模式和雌性繁殖成功进行研究,并通过观察传粉者运动特点以及人工去花和补授花粉实验,探讨花序内资源竞争和交配环境对繁殖资源分配的影响。结果表明:(1)不同位置间的雄蕊数、雄蕊鲜重/雌蕊鲜重、花粉数及花粉胚珠比从花序基部到上部显著增加,而雌蕊鲜重和胚珠数逐渐减少,表现出上部花偏雄的性分配;上部花的结籽率显著低于基部花和中部花,不同位置间的发育种子数/果实和发育种子重/果实随着花位置的升高而显著降低,说明基部花具有更佳的雌性繁殖成效。(2)去花处理后,剩余果实的单个种子重/果实显著增加,但发育种子数/果实没有显著增加;而给上部花人工补授异花花粉后,位置间结籽率的差异消失,说明传粉限制而非资源竞争导致了花序内位置依赖的种子生产模式。(3)毛翠雀花雄性先熟的开花特征,以及传粉者苏氏熊蜂从花序基部到上部的定向访花行为,导致了花序内交配环境的变化。综上结果表明,毛翠雀花花序内的性分配和繁殖成功差异是对交配环境适应的结果,对其在高山环境中实现雌雄适合度最优化具有重要意义。     

云南草寇花序内不同部位的性分配

两性花植物花序内的性分配常存在差异,资源竞争、结构效应、交配环境(雌雄异熟、传粉者定向访花行为等)或授粉不均匀等几种假说可以解释这种现象。为验证上述假说,该研究以云南草寇两种表型(雄先熟型和雌先熟型)为材料,分析了其花序内不同部位(基部、中部和顶部)的每花花粉数、胚珠数、花粉/胚珠比、结实率和结籽率,花序内传粉者的定向访花行为,以及人工辅助授粉和去花处理对结实率和结籽率的影响。结果表明:两种表型花序内每花花粉数不随部位而变化,每花胚珠数、结实率和结籽率由基部到顶部依次降低,每花花粉/胚珠比由基部到顶部依次增加,表明顶部花存在偏雄的性分配。人工辅助授粉后,结实率、结籽率仍由基部到顶部依次降低,表明授粉不均匀假说不能解释云南草寇花序内不同部位结实率、结籽率的差异。去除基部和中部花后,顶部花人工辅助授粉条件下的结实率、结籽率与基部花人工辅助授粉条件下的结实率、结籽率无差异,表明云南草寇花序内不同部位结实率、结籽率的差异主要由资源竞争引起。雌先熟表型每花花粉数、花粉/胚珠比高于雄先熟表型,表明两种表型存在性分配差异。传粉者主要先访问云南草寇基部的花,然后向顶部移动。云南草寇花序内顶部偏雄的性分配可能是由资源竞争和传粉者定向访花造成的。     

Among-species differences in pollen quality and quantity limitation: implications for endemics in biodiverse hotspots

Background and Aims

Insufficient pollination is a function of quantity and quality of pollen receipt, and the relative contribution of each to pollen limitation may vary with intrinsic plant traits and extrinsic ecological properties. Community-level studies are essential to evaluate variation across species in quality limitation under common ecological conditions. This study examined whether endemic species are more limited by pollen quantity or quality than non-endemic co-flowering species in three endemic-rich plant communities located in biodiversity hotspots of different continents (Andalusia, California and Yucatan).

Methods

Natural variations in pollen receipt and pollen tube formation were analysed for 20 insect-pollinated plants. Endemic and non-endemic species that co-flowered were paired in order to estimate and compare the quantity and quality components of pre-zygotic pollination success, obtained through piecewise regression analysis of the relationship between pollen grains and pollen tubes of naturally pollinated wilted flowers.

Key Results

Pollen tubes did not frequently exceed the number of ovules per flower. Only the combination of abundant and good quality pollen and a low number of ovules per flower conferred relief from pre-zygotic pollen limitation in the three stochastic pollination environments studied. Quality of pollen receipt was found to be as variable as quantity among study species. The relative pollination success of endemic and non-endemic species, and its quantity and quality components, was community dependent.

Conclusions

Assessing both quality and quantity of pollen receipt is key to determining the ovule fertilization potential of both endemic and widespread plants in biodiverse hotspot regions. Large natural variation among flowers of the same species in the two components and pollen tube formation deserves further analysis in order to estimate the environmental, phenotypic and intraindividual sources of variation that may affect how plants evolve to overcome this limitation in different communities worldwide.     

Effects of floral sexual investment and dichogamy on floral longevity

Aims Floral longevity, the duration that a flower remains open and functional, varies greatly among species. Variation in floral longevity has been considered to be optimal strategy for resource allocation under different ecological conditions, mainly determined by the rates of pollination and cost of flower maintenance. However, it is unclear whether an intrinsic factor, floral sexual investment, constrains evolution of floral longevity. The theoretical model also predicts that dichogamy favors long-lived flowers, but empirical studies to test this prediction remain unexplored.Methods To examine the effect of floral sexual investment on floral longevity, we measured flower size together with pollen and ovule production in 37 sympatric flowering plants in a natural community. The duration of the female and male phase in 21 protandrous species and floral longevity of the other 16 adichogamous species were documented in the field.Important findings Floral longevity varied from 1 day to 15 days, while pollen number per flower varied from 643 to 710880 and ovule number per flower from 1 to 426 in the 37 species. Flower size was correlated with pollen production as well as ovule production. Floral longevity was positively related to pollen production but not to ovule production. Consistent with the prediction that dichogamy favors long-lived flowers, we found the floral longevity of protandrous species was significantly longer than that of adichogamous species. In the protandrous species, pollen production per flower was observed to be positively related to male duration, while ovule production was not related to female duration. Our analyses of variation in floral longevity and sexual investment among different species suggest that the floral sexual investment could be an intrinsic factor contributing to the selected floral longevity, particularly the male phase, and that high pollen production could potentially increase pollen removal, i.e. male productive success.     

Temporal variation in floral display size and individual floral sex allocation in racemes of Narthecium asiaticum (Liliaceae)

We analyzed the effects of temporal variation in floral display size (number of flowers open at one time on a plant) on the rate of pollen removed and receipt of individual flowers for the sequential blooming plant Narthecium asiaticum (Liliaceae). Because of the acropetal blooming of this species, the display sizes when upper flowers opened was much greater than the display sizes when lower flowers opened. Our experiments revealed that large displays lead to a high rate of pollen removal from individual flowers, though they do not lead to a high rate of ovule fertilization. Consequently, the rate of pollen grains removed per flower by pollinators was greater in upper flowers than in lower flowers. The pattern of sex allocation in individual flowers within a raceme was consistent with such variation. Namely, both maleness [stamen mass/(stamen mass + pistil mass)] and pollen?:?ovule ratio were larger in upper flowers. We suggest that the temporal variation in display size, in addition to such factors as dichogamy and pollinator directionality, also produces variation in the probability of successful pollen transfer from individual flowers that may cause the variation in the sex allocation of individual flowers.     

Flower biology of the cactus Coryphantha elephantidens in the tropical dry forest of Central Mexico

Floral traits and sexual systems in angiosperms are strategies that enhance outcrossing within hermaphrodite flowers and among individuals in a population. Sexual systems with unisexual flowers have also evolved among angiosperms, resulting in sex specialization. Furthermore, the interaction of floral traits and floral visitors determines successful plant reproduction. Globose cacti are bee pollinated, and variation in the diversity of their pollinator assemblages is strongly associated with floral phenotype. Our objective was to describe the floral biology of the cactus Coryphantha elephantidens and to determine its relationship with pollinators. Floral traits were studied by direct observations in live and fixed flowers. The breeding system was determined using two estimators based on floral morphology: pollen grains to ovules per flower (P/O) ratio and outcrossing index. Pollination treatments were conducted to determine the mating system. Floral visitors were recorded using direct observation. Flowers of C. elephantidens are variable in color, protandric, herkogamous and nectarless. Estimators of the breeding system indicated xenogamy, which is consistent with the obligate outcrossing revealed by the pollination experiment. Thirty-seven percent of the plants have female flowers that do not produce pollen, making this population functionally gynodioecious. Both fruit and seed set were high compared to other globose cacti. Pollinators included eight species of native bees, a more diverse pollinator assemblage than other globose cacti. Given the high pressure on pollen due to functional gynodioecy, nectarless flowers, an outcrossing mating system, and the necessity of pollinators to set seeds, we concluded that native bees are highly efficient pollinators that play a crucial role in the sexual reproduction of C. elephantidens.     

The relative importance of architecture and resource competition in allocation to pollen and ovule number within inflorescences of Hosta ventricosa varies with the resource pools

Background and Aims

Allocation of resources to floral traits often declines distally within inflorescences in flowering plants. Architecture and resource competition have been proposed as underlying mechanisms. The aim of the present study is to assess the relative importance of resource competition and architectural effects in pollen and ovule production on racemes of Hosta ventricosa, an apomictic perennial herb.

Methods

Combinations of two defoliation treatments (intact and defoliated) and two fruit-set treatments (no-fruit and fruit) were created, and the roles of architecture and resource competition at each resource level were assessed.

Key Results

Pollen and ovule number per flower increased after defoliation, but pollen to ovule ratio per flower did not change. Pollen, ovules and the pollen to ovule ratio per flower declined distally on racemes at each resource level. In the intact treatment, fruit development of early flowers did not affect either pollen or ovule number of late flowers. In the defoliated treatment, fruit development of early flowers reduced both pollen and ovule numbers of late flowers due to over-compensation caused by defoliation. Late flowers on defoliated fruit racemes produced less pollen than intact fruit racemes but the same number of ovules; therefore, the reduction in pollen number was not caused by over-compensation. In addition, the fruit-set rate of early flowers during flowering was higher in intact racemes than in defoliated racemes.

Conclusions

In flowering plants, the relative importance of architecture and resource competition in allocation to pollen and ovules may vary with the resource pools or the overall resource availability of maternal plants.     

Selecting for floral character associations in wild radish,Raphanus sativus L.

ESS models of reproductive allocation have been used extensively to explain patterns of floral diversity in angiosperms. These theoretical explorations assume that proportional allocation to pollen, ovules, and seeds, as well as to secondary features such as showy petals and nectar rewards, can evolve independently within the limits set by total resource availability. In populations of California wild radish, we have shown previously that petal size, a strong determinant of visitation by honey bee pollinators, is positively correlated with both pollen and nectar production, but not with ovule or seed number per flower. These phenotypic associations may reflect selection, environmental correlation, and/or genetic constraint. By exerting selection on the petal size : pollen number ratio over two generations, we eliminated the positive correlation between petal size and pollen production, with both characters showing significant change after a single selection episode. Once these two floral traits became uncoupled, nectar sugar production was significantly correlated only with petal size. Our results suggest that natural selection could readily alter reproductive allocation in these flowers, and that the phenotypic correlations observed in nature may be maintained by selection for effective reproductive phenotypes.     

COMPONENTS OF SEED AND POLLEN YIELD OF LOBELIA CARDINALIS: VARIATION AND CORRELATIONS

Plants from three Lobelia cardinalis populations were grown under common garden conditions to assess intra- and interplant variation in seed and pollen production. Seed number per flower and mean seed weight varied systematically with floral position on the inflorescence (lowest values were from terminal flowers) but pollen grain number per flower did not vary systematically with floral position. Most of the remaining variance in seed and pollen grain number per flower and mean seed weight was distributed among plants; clones produced very similar amounts of pollen and seed. Seed yield was positively correlated with seed production per flower and with total flower production, but not with mean seed weight; pollen yield was also positively correlated with pollen grain production per flower and total flower production. Seed and pollen yield were simple linear functions of plant size but only pollen yield was a simple linear function of flower production; seed yield was a quadratic function in which the second order term was negative. This quadratic relationship resulted from a negative correlation between seed number per flower and total flower production. This correlation, in addition to the wide variation among plants in pollen number per flower, accounts for the weak correlation of seed and pollen yield. I conclude from these data that it is unlikely that plants in natural L. cardinalis populations transmit genes to the population's seed crop equally through pollen and ovules—emphasizing the importance of measuring both male and female components of reproductive success.     

Factors affecting pollinator movement and plant fitness in a specialized pollination system

The rate of pollen exchange within and among flowers may depend on pollinator attraction traits such as floral display size and flowering plant density. Variations in these traits may influence pollinator movements, pollen receipt, and seed number. To assess how floral display size and flowering plant density affect parameters of pollinator visitation rate, pollen receipt per flower, seed number per fruit and the between-plant pollinator movements, we studied the self-incompatible plant, Nierembergia linariifolia. Per-flower pollinator visitation rate and bout length increased linearly with increasing floral display size. Pollen receipt per flower increased linearly with increasing flowering plant density. For seed number per fruit, a polynomial model describing an increased seed number per fruit at low density and a decreased seed number per fruit at high density provided a significant fit. Per-flower pollinator visitation rate was not associated with pollen receipt per flower and seed number per fruit. Bees visited plants located near to the center of the population more frequently than plants located at the periphery. Increases in both floral display size and flowering plant density led to an increased chance of a plant being chosen as the center of the pollinator foraging area. These results suggest that even though large floral displays and high flowering plant density are traits that attract more pollinators, they may also reduce potential mate diversity by restricting pollen movement to conspecific mates that are closely located.     

Floral display, pollinator visitation and reproductive success in the dioecious perennial herb Wurmbea dioica (Liliaceae)

Floral traits that increase attractiveness to pollinators are predicted to evolve through selection on male function rather than on female function. To determine the importance of male-biased selection in dioecious Wurmbea dioica, we examined sexual dimorphism in flower size and number and the effects of these traits on pollinator visitation and reproductive success of male and female plants. Males produced more and larger flowers than did females. Bees and butterflies responded to this dimorphism and visited males more frequently than females, although flies did not differentiate between the sexes. Within sexes, insect pollinators made more visits to and visited more flowers on plants with many flowers. However, visits per flower did not vary with flower number, indicating that visitation was proportional to the number of flowers per plant. When flower number was experimentally held constant, visitation increased with flower size under sunny but not overcast conditions. Flower size but not number affected pollen removal per flower in males and deposition in females. In males, pollen removal increased with flower size 3 days after flowers opened, but not after 6 days when 98% of pollen was removed. Males with larger flowers therefore, may have higher fitness not because pollen removal is more complete, but because pollen is removed more rapidly providing opportunities to pre-empt ovules. In females, pollen deposition increased with flower size 3 days but not 6 days after flowers opened. At both times, deposition exceeded ovule production by four-fold or more, and for 2 years seed production was not limited by pollen. Flower size had no effect on seed production per plant and was negatively related to percent seed set, implying a tradeoff between allocation to attraction and reproductive success. This indicates that larger flower size in females is unlikely to increase fitness. In both sexes, gamete production was positively correlated with flower size. In males, greater pollen production would increase the advantage of large flowers, but in females more ovules may represent a resource cost. Selection to increase flower size and number in W. dioica has probably occurred through male rather than female function. Received: 15 June 1997 / Accepted: 12 February 1998     

Do pollen carryover and pollinator constancy mitigate effects of competition for pollination?

Pollinator constancy and pollen carryover are both thought to mitigate competitive effects that result when shared pollinators cause loss of pollen to heterospecific flowers. I present analytical and simulation models to investigate how pollinator constancy and pollen carryover interact with each other and with the relationship between pollen receipt and seed set to determine pollination success in competitive environments. With inconstant pollinators, increased pollen carryover reduces variance in pollen receipt without affecting average pollen receipt. Consequently, for flowers requiring at least a threshold quantity of pollen for success, rare flowers with inconstant pollinators benefit from reduced carryover, especially for high pollen receipt thresholds, whereas common flowers benefit from increased carryover, especially for low receipt thresholds. Pollinator constancy is predicted to increase pollen receipt, especially if pollen carryover rates are low. As a result, increased pollinator constancy reduces the range of pollen receipt thresholds for which carryover is beneficial. Similarly, for flowers whose pollination success is a convex function of pollen receipt, carryover is expected to increase fecundity if pollinators are inconstant, but with even a low degree of pollinator constancy, carryover reduces fecundity. These results predict that rare plants with many ovules per flower benefit from dispersing aggregations of pollen, especially if their pollinators exhibit constancy, whereas plants with inconstant pollinators and low thresholds of pollen receipt benefit from pollen grains dispersing individually to increase the number of flowers reached by the pollen.     

Differential self‐fertilization rates in response to variation in floral traits within inflorescences of Aquilegia buergeriana var. oxysepala (Ranunculaceae)

To assess variation in the proportion of self‐fertilized seeds among flowers within inflorescences and the relationship between floral traits and the rate of self‐fertilization, the proportion of self‐fertilized seeds among individual flowers was estimated using ten microsatellite markers in self‐compatible plants of Aquilegia buergeriana var. oxysepala. Within‐inflorescence variation in floral traits, such as the duration of the male and female phases, flower size, herkogamy and the number of pollen grains and ovules in two natural populations, were investigated. The first flower in an inflorescence produced more seeds and a higher proportion of self‐fertilized seeds than the second flower. The higher proportion of self‐fertilized seeds in the first flowers was accompanied by a higher number of pollen grains and ovules in the bud stage and the female phase. These results indicate that the high proportion of self‐fertilized seeds in the first flowers in an inflorescence may be due to the high number of remaining pollen grains in the female phase. This suggests that variation in floral traits within inflorescences affects seed quality and quantity among flowers within inflorescences.     

The structure of phenotypic variation in gender and floral traits within and among populations of Spergularia marina (Caryophyllaceae)

We sampled four wild populations of the highly autogamous Spergularia marina (Caryophyllaceae) in California to detect and to measure the magnitude of within- and among-population sources of phenotypic variation in gender and floral traits. From flowers and fruits collected from field and greenhouse-raised plants, we measured ovule number, seed number, mean seed mass, pollen production (greenhouse families only), mean pollen grain volume (greenhouse families only), anther number, anther/ovule ratio, pollen/ovule ratio (estimated using different flowers for pollen than for ovules; greenhouse families only), petal number, and petal size. Using greenhouse-raised genotypes, variation among maternal families nested within populations was evaluated for each trait to determine whether populations differ in the degree of maternally transmitted phenotypic variation. For each population, we used 15 greenhouse-raised maternal families to estimate the broad-sense heritability and genetic coefficient of variation of each floral trait. The magnitude and statistical significance of broad-sense heritability estimates were trait- and population-specific. Each population was characterized by a different combination of floral traits that expressed significant maternally transmitted (presumably genetic) variation under greenhouse conditions. Flowers representing two populations expressed low levels of maternally transmitted variation (three or fewer of nine measured traits exhibited significant maternal family effects on phenotype), while flowers representing the other two populations exhibited significant maternal family effects on phenotype for five or more traits. Our ability to detect statistically significant differences among the four populations depended upon the conditions under which plants were grown (field vs. greenhouse) and on the floral trait observed. Field-collected flowers exhibited significant differences among population means for all traits except anther number. Flowers sampled from greenhouse-raised maternal families differed among populations for all traits except ovule number, seed number, and petal size. We detected negligible evidence that genetic correlations constrain selection on floral traits in Spergularia marina.     

Phylogenetic evidence for a flower size and number trade-off

The size and number of flowers displayed together on an inflorescence (floral display) influences pollinator attraction and pollen transfer and receipt, and is integral to plant reproductive success and fitness. Life history theory predicts that the evolution of floral display is constrained by trade-offs between the size and number of flowers and inflorescences. Indeed, a trade-off between flower size and flower number is a key assumption of models of inflorescence architecture and the evolution of floral display. Surprisingly, however, empirical evidence for the trade-off is limited. In particular, there is a lack of phylogenetic evidence for a trade-off between flower size and number. Analyses of phylogenetic independent contrasts (PICs) of 251 angiosperm species spanning 63 families yielded a significant negative correlation between flower size and flower number. At smaller phylogenetic scales, analyses of individual genera did not always find evidence of a trade-off, a result consistent with previous studies that have examined the trade-off for a single species or genus. Ours is the first study to support an angiosperm-wide trade-off between flower size and number and supports the theory that life history constraints have influenced the evolution of floral display.     

A perfect flower from the Jurassic of China

Flower, enclosed ovule and tetrasporangiate anther are three major characters distinguishing angiosperms from other seed plants. Morphologically, typical flowers are characterised by an organisation with gynoecium and androecium surrounded by corolla and calyx. Theoretically, flowers are derived from their counterparts in ancient ancestral gymnosperms. However, as for when, how and from which groups, there is no consensus among botanists yet. Although angiosperm-like pollen and angiosperms have been claimed in the Triassic and Jurassic, typical flowers with the aforesaid three key characters are still missing in the pre-Cretaceous age, making many interpretations of flower evolution tentative. Thus searching for flower in the pre-Cretaceous has been a tantalising task for palaeobotanists for a long time. Here, we report a typical flower, Euanthus paniigen. et sp. nov., from the Middle–Late Jurassic of Liaoning, China. Euanthus has sepals, petals, androecium with tetrasporangiate dithecate anthers and gynoecium with enclosed ovules, organised just like in perfect flowers of extant angiosperms. The discovery of Euanthus implies that typical angiosperm flowers have already been in place in the Jurassic, and provides a new insight unavailable otherwise for the evolution of flowers.     

Floral sex allocation and reproductive success within inflorescences of Hosta ventricosa,a pseudogamous apomict

Aims Within inflorescences of sexually reproducing hermaphrodites, the production of ovules, fruits and seeds commonly declines from basal (early-opening) to distal (late-opening) flowers, while pollen production remains constant or only changes slightly, with the result that distal (late-opening) flowers become functionally male. However, few empirical studies have specifically examined whether or not changes in allocation to pollen production actually lead to changes in the number of seeds sired, a more direct measure of male fitness. In pseudogamous apomicts, fitness depends on the number of seeds produced; thus, a contrasting pattern of variation in the pollen-to-ovule (P/O) ratio within inflorescences might be expected.Methods We investigated floral sex allocation and reproductive success within racemes of Hosta ventricosa, a pseudogamous apomictic hermaphrodite possessing flowers that open acropetally. We quantified variations in pollen number, ovule number, the P/O ratio and fruit and seed production, from 2007 to 2011, among flowers within racemes of four populations of H. ventricosa in southwest China. Ecological causes for fruit and seed production were evaluated by observing patterns of pollen deposition, flower removal and supplemental pollination.Important findings Pollen number, ovule number and the P/O ratio declined from basal-to-distal positions in all sampled populations (years). Fruit and seed production decreased distally in most populations (years). Low fruit and seed set of distal flowers was not due to pollen limitation because pollen deposition never declined distally and the low fruit and seed set of distal flowers remained even after supplementary pollination was provided. The flower-removal experiment indicated that inter-fruit competition for resources among flowers was common. The low P/O ratio of distal flowers in H. ventricosa might be favored because they were unable to obtain fitness by donating pollen and siring seeds on other plants. Our study may help to understand the adaptive significance of sex allocation among flowers within inflorescences of sexually reproducing hermaphrodites.