Extra-gynoecial pollen-tube growth in apocarpous angiosperms is phylogenetically widespread and probably adaptive

? Fusion of floral carpels (syncarpy) in angiosperms is thought to have allowed for significant improvements in offspring quantity and quality in syncarpous species over gymnosperms and apocarpous (free-carpelled) angiosperms. Given the disadvantages of apocarpy, it remains an evolutionary puzzle why many angiosperm lineages with free carpels (apocarpy) have been so successful and why some lineages show reversals to apocarpy. ? To investigate whether some advantages of syncarpy may accrue in other ways to apocarpous species, we reviewed previous studies of pollen-tube growth in apocarpous species and also documented pollen-tube growth in nine additional apocarpous species in six families. ? Anatomical studies of a scattering of apocarpous paleodicots, monocots, and eudicots show that, after transiting the style, 'extra' pollen tubes exit fully fertilized carpels and grow to other carpels with unfertilized ovules. In many species this occurs via openings in the simple carpels, as we report here for Sagittaria potamogetifolia, Sagittaria pygmaea, Sedum lineare, and Schisandra sphenanthera. ? The finding that extra-gynoecial pollen-tube growth is widespread in apocarpous species eliminates the possibility of a major fitness cost of apocarpy relative to syncarpy and may help to explain the persistence of, and multiple reversals to, apocarpy in the evolutionary history of angiosperms.     

Convergent elaboration of apocarpous gynoecia in higher advanced dicotyledons (Sapindales, Malvales, Gentianales)

The apocarpous gynoecia of three separate groups of higher advanced dicotyledons show postgenital fusion of their apical parts. In this fused region the pollen tube transmitting tissue of the carpels is united into a compitum, which provides advantages of a syncarpous to the apocarpous gynoecium. It is supposed that in at least some of these groups the general evolutionary trend of the angiosperms from apocarpy towards syncarpy is reversed.     

FUNCTIONAL SYNCARPY BY INTERCARPELLARY GROWTH OF POLLEN TUBES IN A PRIMITIVE APOCARPOUS ANGIOSPERM,ILLICIUM FLORIDANUM (ILLICIACEAE)

Fluorescence microscopy and histological studies have been used to show that in Illicium floridanum Ellis (Illiciaceae), a primitive apocarpous angiosperm, functional syncarpy is achieved by intercarpellary growth of pollen tubes. After pollen germinates on the separate stigmatic crests of the carpellary whorl, tubes grow within the carpels obliquely down and inward toward the central floral axis which is modified as a stigmalike “apical residuum.” In a restricted shallow region around the base of the apical residuum, some pollen tubes grow out between the unfused margins of the carpels and circumferentially around the surface of the apical residuum from where they may enter neighboring carpels. Some pollen germination and tube growth also occur on the apical residuum itself. The apical residuum with its associated unfused carpel margins acts as an extragynoecial compitum for pollen tube transfer between carpels, and, as such, is believed to represent a mechanism for increasing the efficiency of seed set. The pollen tube pathway of Illicium appears to be a primitive expression of a line of evolutionary development leading to syncarpous gynoecia through stages possibly exemplified by certain members of the Trochodendraceae (lower Hamamelididae).     

The evolution of key functional floral traits in the early divergent angiosperm family Annonaceae

Potential key functional floral traits are assessed in the species‐rich early divergent angiosperm family Annonaceae. Pollinators (generally beetles) are attracted by various cues (particularly visual, olfactory, and thermogenic), with pollinators rewarded by nectar (generally as stigmatic exudate), heat, and protection within the partially enclosed floral chamber. Petals sometimes function as pollinator brood sites, although this could be deceptive. Annonaceae species are self‐compatible, with outcrossing promoted by a combination of protogyny, herkogamy, floral synchrony, and dicliny. Pollination efficiency is enhanced by pollen aggregation, changes in anthesis duration, and pollinator trapping involving a close alignment between petal movements and the circadian rhythms of pollinators. Most Annonaceae flowers are apocarpous, with syncarpy restricted to very few lineages; fertilization is therefore optimized by intercarpellary growth of pollen tubes, either by stigmatic exudate (suprastylar extragynoecial compitum) or possibly the floral receptacle (infrastylar extragynoecial compitum). Although Annonaceae lack a distinct style, the stigmas in several lineages are elongated to form “pseudostyles” that are hypothesized to function as sites for pollen competition. Flowers can be regarded as immature fruits in which the ovules are yet to be fertilized, with floral traits that may have little selective advantage during anthesis theoretically promoting fruit and seed dispersal. The plesiomorphic apocarpous trait may have been perpetuated in Annonaceae flowers as it promotes the independent dispersal of fruit monocarps (derived from separate carpels), thereby maximizing the spatial/temporal distance between seedlings. This might compensate for the lack of genetic diversity among seeds within fruits arising from the limited diversity of pollen donors.     

Intercarpellary growth of pollen tubes in the extragynoecial compitum and its contribution to fruit set in an apocarpous species, Schisandra sphenanthera (Schisandraceae)

? Premise of the study: Apocarpous plants possess carpels that are separated in the gynoecium. Extragynoecial compita, commonly occurring in basal angiosperms, have been proposed to have the potential to increase offspring quantity in apocarpous species through the intercarpellary growth of pollen tubes. To date, the impact of an extragynoecial compitum on fruit or seed set has not been studied in any species. This study investigated the pollen tube pathway between adjacent carpels and its contribution to fruit set in Schisandra sphenanthera. ? Methods: We investigated the fruit set ratio in the field and collected hundreds of gynoecia at their full flowering stage. Pollinated carpel ratio and pollen tube pathway observations were performed using fluorescence optics. ? Key results: Pollen grains germinated and tubes extended along the pseudostyle surface. Some of them turned and entered the ovules at the end of the stigmatic crest, whereas others subsequently grew into neighboring carpels through promontory connections located at the base of the unfused carpels. No tubes were found growing on the surface of the receptacle. More than 24 carpels could be fertilized by pollen tubes from one carpel through hand pollination. The pollinated carpel ratio was significantly lower than the fruit set ratio under natural conditions. ? Conclusions: Pollen tubes from one carpel can easily cross in the extragynoecial compitum between the adjacent carpels of S. sphenanthera, and this intercarpellary growth of pollen tubes can significantly increase the fruit set of apocarpous species, at least in S. sphenanthera.     

Patterns and sources of variation in pollen deposition and pollen tube formation in flowers of the endemic monoecious shrub Cnidoscolus souzae (Euphorbiaceae)

Pollen deposition and pollen tube formation are key components of angiosperm reproduction but intraspecific variation in these has rarely been quantified. Documenting and partitioning (populations, plants and flowers) natural variation in these two aspects of plant reproduction can help uncover spatial mosaics of reproductive success and underlying causes. In this study, we assess variation in pollen deposition and pollen tube formation for the endemic monoecious shrub Cnidoscolus souzae throughout its distribution range in Mexico, and determine how this variation is structured among populations, plants and flowers. We also infer the relative importance of pollen quantity and quality in determining pollination success in this species. While we found no evidence suggesting that pollen receipt limits C. souzae reproduction across 19 populations, we did find extensive variation in pollen load size and pollen tube number per flower. Total variation in pollen receipt and pollen tube number was mostly explained by intra‐individual and among‐population variance. Furthermore, pollen load size had a stronger effect on the number of pollen tubes at the base of the style than pollen germination rate, suggesting that pollen quantity may be more important than quality for pollen tube success in C. souzae. Our results suggest that both small within‐plant flower differences and broad‐scale differences in community attributes can play an important role in determining pollination success. We emphasise the need to evaluate patterns and sources of variation in pollen deposition and pollen tube formation as a first step in understanding the causes of variation in pollination success over broad spatial scales.     

Perfect syncarpy in apple (Malus x domestica 'Summerland McIntosh') and its implications for pollination, seed distribution and fruit production (Rosaceae: Maloideae)

BACKGROUND AND AIMS: The gynoecium of the domestic apple, Malus x domestica, has been assumed to be imperfectly syncarpic, whereby pollination of each stigmatic surface can result in fertilization within only one of the five carpels. Despite its implied effect on fruit quantity and quality, the resulting influence of flower form on seed set and distribution within the apple fruit has seldom been investigated. Instead, poor fruit quality is usually attributed to problems with pollination, such as low bee numbers and/or ineffective pollinators within apple agro-ecosystems. The objective of this study was to determine the true nature of gynoecial structure and its influence on fruit production in the apple cultivar 'Summerland McIntosh'. METHODS: A stigma-excision method was used to determine the effects of uneven pollination among the five stigmas on fruit quantity (as measured by fruit set), and quality (seed number and distribution). In addition, flowers were examined microscopically to determine pollen tube pathways. KEY RESULTS: Fruit set, seed number, seed distribution, and the microscopic examination of flower gynoecial structure reported in this study indicated that the gynoecium of the cultivar Summerland McIntosh is perfectly syncarpic and not imperfectly syncarpic as previously thought. CONCLUSIONS: Pollination levels among the five stigmas need not be uniform to obtain full seed development within Summerland McIntosh fruit; even if one stigmatic surface is adequately pollinated, a full complement of seeds is likely. The importance of perfect syncarpy in recognizing true causes of poor fruit quality in apple is discussed.     

Evaluating the effects of pollinator‐mediated interactions using pollen transfer networks: evidence of widespread facilitation in south Andean plant communities

Information about the relative importance of competitive or facilitative pollinator‐mediated interactions in a multi‐species context is limited. We studied interspecific pollen transfer (IPT) networks to evaluate quantity and quality effects of pollinator sharing among plant species on three high‐Andean communities at 1600, 1800 and 2000 m a.s.l. To estimate the sign of the effects (positive, neutral or negative), the relation between conspecific and heterospecific pollen deposited on stigmas was analysed with GLMMs. Network analyses showed that communities were characterised by the presence of pollen hub‐donors and receptors. We inferred that facilitative and neutral pollinator‐mediated interactions among plants prevailed over competition. Thus, the benefits from pollinator sharing seem to outweigh the costs (i.e. heterospecific deposition and conspecific pollen loss). The largest proportion of facilitated species was found at the highest elevation community, suggesting that under unfavourable conditions for the pollination service and at lower plant densities facilitation can be more common.     

Effects of inbreeding and pollen donor provenance and diversity on offspring performance under environmental stress in the rare plant Cochlearia bavarica

Habitat degradation and loss can reduce size and genetic variability of natural populations, increasing individual homozygosity and average relatedness between individuals. While the resulting inbreeding depression may be reduced by natural selection under prevailing environmental conditions, it may increase again under environmental stress. To investigate the effect of environmental stress on offspring performance and the expression of inbreeding depression, we hand-pollinated maternal plants in small (< 100, n=5) and large populations (> 400 flowering plants, n=5) of the rare plant Cochlearia bavarica (Brassicaceae) and raised the offspring under experimentally manipulated water and light regimes (normal or reduced supply). In addition to considering natural variation in inbreeding levels due to population size, we manipulated pollen donor provenance and diversity. Maternal plants were pollinated with nine donors from a different population or with one or nine donors from the same population. One further inflorescence of each maternal plant was exposed to free pollination. Offspring growth and survival were monitored over 300 days. Offspring performance varied significantly among populations and maternal plants. Environmental stress interacted significantly with these factors. However, there was no general indication that offspring from small populations were more negatively affected. In seven out of 10 populations, offspring derived from between-population pollination performed better than offspring derived from within-population pollination. Also, in five out of 10 populations, average offspring size was higher after within-population pollination with nine than after pollination with one pollen donor. These results suggest low genetic diversity within C. bavarica populations, both smaller and larger ones. Interactions between environmental stress and pollination treatment indicated that using pollen donors from outside a population or increasing the number of pollen donors can reduce inbreeding depression, but that this beneficial effect is impaired under stressful conditions.     

Pollen tube growth in association with a dry-type stigmatic transmitting tissue and extragynoecial compitum in the basal angiosperm Kadsura longipedunculata (Schisandraceae)

Spatial features of pollen tube growth and the composition of the extracellular matrix (ECM) of transmitting tissue in carpels of Kadsura longipedunculata, a member of the basal angiosperm taxon Schisandraceae, were characterized to identify features of transmitting tissue that might have been important for pollen-carpel interactions during the early history of angiosperms. In addition to growing extracellularly along epidermal cells that make up stigmatic crests of individual carpels, pollen tubes grow on abaxial carpel epidermal cells between unfused carpels along an extragynoecial compitum to subsequently enter an adjacent carpel, a feature important for enhancing seed set in apocarpous species. Histo- and immunochemical data indicated that transmitting tissue ECM is not freely flowing as previously hypothesized. Rather, the ECM is similar to that of a dry-type stigma whereby a cuticular boundary with associated esterase activity confines a matrix containing methyl-esterified homogalacturonans. The Schisandraceae joins an increasing number of basal angiosperm taxa that have a transmitting tissue ECM similar to a dry-type stigma, thereby challenging traditional views that the ancestral pollen tube pathway was similar to a wet-type stigma covered with a freely flowing exudate. Dry-type stigmas are posited to provide tighter control over pollen capture, retention, and germination than wet-type stigmas.     

Effects of natural and artificial pollination on fruit and offspring quality

Worldwide, many crops rely on insect pollination. Insufficient pollination can reduce fruit and seed set by directly reducing pollen deposition, and can also affect offspring quality, such as growth rate and resistance to herbivores, by limiting outcrossing opportunities. Both effects are important in fruit agroecosystems where fruit size and the quality of seeds for re-planting are dependent on sufficient pollination. We experimentally manipulated pollination of the cape gooseberry, Physalis peruviana L. (Solanaceae), to test the effects of honey and bumble bee pollination compared to manual outcrossing and autonomous self-pollination on fruit and offspring characteristics. Compared to manual and self-pollination, bee pollination increased fruit size, seed set and germination rates, supporting the hypothesis that sufficient pollination increases plant fitness. Interestingly, plant growth rate and herbivore resistance were significantly and marginally greater in manually outcrossed plants compared to self-pollinated offspring, suggesting that inbreeding reduces offspring quality. Herbivore resistance and plant growth did not differ between one honeybee visit and self-pollination suggesting that multiple pollinator visits are needed to prevent inbreeding events. Our data suggest that the quantity and quality of pollen deposited by bee visitation can significantly alter ecologically and economically relevant traits in this agroecosystem.     

Evolution of syncarpy and other morphological characters in African Annonaceae: a posterior mapping approach

The congenital fusion of carpels, or syncarpy, is considered a key innovation as it is found in more than 80% of angiosperms. Within the magnoliids however, syncarpy has rarely evolved. Two alternative evolutionary origins of syncarpy were suggested in order to explain the evolution of this feature: multiplication of a single carpel vs. fusion of a moderate number of carpels. The magnoliid family Annonaceae provides an ideal situation to test these hypotheses as two African genera, Isolona and Monodora, are syncarpous in an otherwise apocarpous family with multicarpellate and unicarpellate genera. In addition to syncarpy, the evolution of six other morphological characters was studied. Well-supported phylogenetic relationships of African Annonaceae and in particular those of Isolona and Monodora were reconstructed. Six plastid regions were sequenced and analyzed using maximum parsimony and Bayesian inference methods. The Bayesian posterior mapping approach to study character evolution was used as it accounts for both mapping and phylogenetic uncertainty, and also allows multiple state changes along the branches. Our phylogenetic analyses recovered a fully resolved clade comprising twelve genera endemic to Africa, including Isolona and Monodora, which was nested within the so-called long-branch clade. This is the largest and most species-rich clade of African genera identified to date within Annonaceae. The two syncarpous genera were inferred with maximum support to be sister to a clade characterized by genera with multicarpellate apocarpous gynoecia, supporting the hypothesis that syncarpy arose by fusion of a moderate number of carpels. This hypothesis was also favoured when studying the floral anatomy of both genera. Annonaceae provide the only case of a clear evolution of syncarpy within an otherwise apocarpous magnoliid family. The results presented here offer a better understanding of the evolution of syncarpy in Annonaceae and within angiosperms in general.     

The realized effect of postpollination sexual selection in a natural plant population

The ultimate importance of postpollination sexual selection has remained elusive, largely because of the difficulty of assigning paternity in the field. Here I use a powerful new molecular marker (AFLP) for paternity analysis in a natural population of the outcrossing angiosperm Persoonia mollis (Proteaceae) to assess male reproductive success following equal pollination of 15 pollen donors on each of 6310 pistils. These results were contrasted with male reproductive success of these same plants following natural mating. Following equal pollination, there was a significant departure from equal siring success, indicating a potential for postpollination sexual selection. The most successful pollen donor sired more than twice the expected number of seeds, and this was largely consistent across recipient plants. However, siring success following natural mating was significantly different from siring success following artificial pollination and showed that the reproductive gains to be made from superior pollen performance did not translate into increased reproductive success following natural mating. As the ecological context for post-pollination sexual selection is strong in P. mollis, I suggest that pollen competition may ultimately have only a weak effect on non-random male mating success under natural conditions because the realized opportunities for pollen competition within pistils are limited.     

EFFECTS OF POLLEN DONOR IDENTITY ON OFFSPRING QUALITY IN WILD RADISH,RAPHANUS SATIVUS

If pollen donor performance during mating correlates with differences in offspring growth and fitness, processes that sort among potential mates may directly improve offspring fitness. Here seeds sired by three pollen donors on ten maternal plants were grown for eight weeks in the greenhouse. The performance of the pollen donors during pollination and fertilization was known from a previous experiment. There were significant effects of paternity on two measures of early growth: leaf number and plant height. Paternal effects on three measures more closely related to fitness; final plant weight, day of first flower production, and total flower number were also significant. Under the conditions of this experiment, final plant weight was probably the best predictor of fitness. The pollen donor that sired the largest seeds in the previous experiment sired offspring that were largest after 8 weeks of growth. Half of the plants were grown under low-water conditions. Paternal effects on growth were not masked by the environmental effects. In fact, some paternal effects became stronger under stress. This suggests that paternal effects could also be important in the field. Plants sired by donor A bolted very early when water was limited and would probably have an advantage in a season that was very short due to an early and severe drought. During fertilization and seed filling, seeds sired by this donor were more frequent on water-stressed maternal plants than on control maternal plants (Marshall, 1988). The data from this experiment indicate a connection between pollen donor performance during mating and offspring growth. These results suggest that the processes that sort among potential fathers during pollination, fertilization, and seed filling, may improve offspring quality.     

Unique growth paths of heterospecific pollen tubes result in late entry into ovules in the gynoecium of Sagittaria (Alismataceae)

• Pollen‐pistil interactions are a fundamental process in the reproductive biology of angiosperms and play a particularly important role in maintaining incipient species that exist in sympatry. However, the majority of previous studies have focused on species with syncarpous gynoecia (fused carpels) and not those with apocarpous gynoecia (unfused carpels).
• In the present study, we investigated the growth of conspecific pollen tubes compared to heterospecific pollen tubes in Sagittaria species, which have apocarpous gynoecia. We conducted controlled pollinations between S. pygmaea and S. trifolia and observed the growth of conspecific and heterospecific pollen tubes under a fluorescence microscope.
• Heterospecific and conspecific pollen tubes arrived at locules within the ovaries near simultaneously. However, conspecific pollen tubes entered into the ovules directly, whereas heterospecific tubes passed through the carpel base and adjacent receptacle tissue, to ultimately fertilize other unfertilized ovules. This longer route taken by heterospecific pollen tubes therefore caused a delay in the time required to enter into the ovules. Furthermore, heterospecific pollen tubes displayed similar growth patterns at early and peak pollination. The growth pattern of heterospecific pollen tubes at late pollination was similar to that of conspecific pollen tubes at peak pollination.
• Heterospecific and conspecific pollen tubes took different routes to fertilize ovules. A delayed entry of heterospecific pollen into ovules may be a novel mechanism of conspecific pollen advantage (CPA) for apocarpous species.

     

Differences in female reproductive success between female and hermaphrodite individuals in the subdioecious shrub Eurya japonica (Theaceae)

Subdioecy is thought to occupy a transitional position in the gynodioecy–dioecy pathway, explaining one of the evolutionary routes from hermaphroditism to dioecy. Quantifying any female reproductive advantage of females versus hermaphrodites is fundamental to examining the spectrum between subdioecy and dioecy; however, this is challenging, as multiple interacting factors, such as pollen limitation and resource availability, affect plant reproduction. We compared the female reproductive success of females and hermaphrodites via a field experiment in which we hand‐pollinated individuals of the subdioecious shrub Eurya japonica of similar size growing under similar light conditions. Effects of pollen limitation and seed quality were also evaluated through comparing the results of hand‐ and natural‐pollination treatments and performing additional laboratory and greenhouse experiments. Overall, females had higher fruit set and produced heavier fruit and more seeds than hermaphrodites, and these results were more pronounced for hand‐pollinated than for natural‐pollinated plants of both sexes. We also found that seeds naturally produced by females had a higher mean germination rate. These results indicate that females had a pronounced advantage in female reproductive success under conditions of no pollen limitation. The sexual difference in the degree of pollen limitation suggests a pollinator‐mediated interaction, whereas the higher female reproductive success of females even under natural conditions implies that E. japonica is a good model species for elucidating the later stages of the gynodioecy–dioecy pathway.     

Gynoecium structure and extra-gynoecial pollen-tube growth in an apocarpous species, <Emphasis Type="Italic">Sagittaria trifolia</Emphasis> (Alismataceae)

Apocarpy is regarded as an original feature obtained during the evolution of angiosperms. Compared with syncarpous plants, apocarpous plants have some adaptive disadvantages in apocarpous plants, for example, the number of offspring is lower under conditions of uneven pollen-tube distribution. However, in some apocarpous species, extra-gynoecial pollen-tube growth (EGPG) may remedy this disadvantage. We conducted micro-observations and field studies of Sagittaria trifolia, to investigate the gynoecium structure and the pathway of pollen-tube growth in the entire gynoecium. In a single-carpel pollination experiment, we found that the extra-gynoecial pollen tubes from a carpel of S. trifolia were able to fertilize approximately 13 carpels. Simulated EGPG in the entire gynoecium of S. trifolia revealed that its effect on the seed set could be divided into two stages: stage of low/high-level stigmas pollination, in which the cutoff point was about 0.1. The seed set would be markedly improved during the low-level stigmas pollination stage by EGPG when the maximum distance of extra-gynoecial pollen tubes could span three carpels, as in the present experiment. Our simulation also showed that the high pollen load could enhance the effect of EGPG on the seed set, and if the number of germinating pollen is triple the carpel number in the gynoecium, a 100% seed set rate would be obtained when approximately 50% of the stigmas are pollinated.     

Variability in Pollen Load: Implications for Reproduction and Seedling Vigor in a Rare Plant, Silene douglasii var. Oraria

Habitat fragmentation can markedly influence the levels of pollen deposition and seed production in natural populations, and rare plants may be especially susceptible to any associated reductions in pollen quantity and quality. In order to ascertain the potential for pollen limitation of maternal fitness in a rare plant, Silene douglasii var. oraria, which is endemic to western coastal prairies, we counted ovules and measured conspecific and heterospecific pollen deposition on stigmas collected from open-pollinated plants. We further investigated the effect of increasing pollen intensity on fruit production, seed number and weight, as well as several measures of progeny vigor. Three levels of outcross pollen were added to plant stigmas for comparison with autogamous and open pollination in the largest naturally occurring population. Both seed and fruit production were significantly greater (P<0.05) for supplemented versus nonsupplemented stigmas, but flowers receiving different levels of pollen addition were statistically indistinguishable. Seed germination and seedling survival were also lowest for the offspring of nonsupplemented flowers; however, in natural populations, opportunities for pollen competition are very limited since open-pollinated flowers averaged fewer viable pollen grains than ovules. Seed production was equivalent for open- and autogamously pollinated flowers in 1996, indicating that natural pollen transfer may have involved mostly self pollen. Overall, the low reproductive success of var. oraria likely reflects both low pollen quantity and quality. Multiyear empirical studies of pollen intensity in field populations are needed so that we can better understand the fitness consequences of pollen limitation in rare perennials.     

Landscape Simplification Constrains Adult Size in a Native Ground-Nesting Bee

Bees provide critical pollination services to 87% of angiosperm plants; however, the reliability of these services may become threatened as bee populations decline. Agricultural intensification, resulting in the simplification of environments at the landscape scale, greatly changes the quality and quantity of resources available for female bees to provision their offspring. These changes may alter or constrain the tradeoffs in maternal investment allocation between offspring size, number and sex required to maximize fitness. Here we investigate the relationship between landscape scale agricultural intensification and the size and number of individuals within a wild ground nesting bee species, Andrena nasonii. We show that agricultural intensification at the landscape scale was associated with a reduction in the average size of field collected A. nasonii adults in highly agricultural landscapes but not with the number of individuals collected. Small females carried significantly smaller (40%) pollen loads than large females, which is likely to have consequences for subsequent offspring production and fitness. Thus, landscape simplification is likely to constrain allocation of resources to offspring through a reduction in the overall quantity, quality and distribution of resources.     

Within-carpel and among-carpel competition during seed development, and selection on carpel number, in the apocarpous perennial herb Helleborus foetidus L. (Ranunculaceae)

Apocarpous flowers share opportunities for post-fertilization ovule selection among more functional levels than syncarpous flowers, because the occurrence of a variable number of unfused carpels adds a new source of variation to the likelihood of successful female reproduction. The extent to which post-fertilization events might differ among these unfused carpels may promote variations in the reproductive strategies of plants. We report a study of the variation, within and among carpels and flowers, in seed production and mass in the apocarpous Helleborus foetidus (Ranunculaceae), in relation to the number of carpels per flower. Differences within and among carpels in female reproductive success were affected by carpel number and pollination environment. When analysing whole flowers as functional units we also found that the magnitude of the differences related to carpel number and pollination treatment actually depended on the “distribution” of pollen types within flowers. Thus, variable within-flower pollination environments, more likely to occur in apocarpous than in syncarpous flowers, may affect the strategies of resource allocation for fruit development at different stages of the reproductive process. Regarding seed production, we found that producing more flowers with four carpels was under directional; however, when mean diaspore mass was considered as a measure of fitness, directional selection was found on producing flowers with two and three carpels (the modal carpel number found in wild populations). We discuss ecological and developmental reasons which could explain the observed pattern, and conclude that selection on an optimum carpel number may be very variable across the species range, as the discussed reasons may impose constraints on eventual evolutionary response, thus contributing to the maintenance of the intra-individual variability in carpel number.