Andromonoecy in Solanum lycocarpum A. St. -Hil. (Solanaceae): Floral attributes,visitors and variation in sexual expression over time

Sexual expression in andromonoecious species—those in which a single individual can bear both staminate and hermaphroditic flowers—may vary among reproductive events in the same plant, among individuals and across populations. This variation influences, in turn, the individual contribution of hermaphroditic plants via male and female fitness functions (i.e., Lloydʼs phenotypic gender). However, temporal variation in sexual expression in andromonoecious species and its relationship with seasonal changes in climatic conditions remain poorly understood. Here we analyze floral attributes, visitors and variation in sexual expression in three populations of Solanum lycocarpum A. St. -Hil. Seasonality in the production of floral types, the mating system and floral visitors were also investigated. Hermaphroditic flowers produced more pollen grains, but the pollen of staminate flowers had higher viability. Only hermaphroditic flowers produced fruits, and ovules in staminate flowers were sterile. Solanum lycocarpum is mainly pollinated by large bees with the ability to vibrate flowers. Phenotypic gender varied throughout the year, and the seasonal production of staminate flowers is associated with the local climate. We suggest that the higher and seasonally variable relative abundance of staminate flowers compared to the low and uniform production of hermaphroditic flowers may be explained by (a) the very high energetic costs incurred in producing large fruits, which in turn make hermaphroditic flower production very costly, and (b) the potentially lower energy expenditure of the smaller staminate flowers with reduced pistils and non-viable ovules that allow them to rapidly respond to climate variability.     

Effects of cleistogamy and pollen source on seed production and offspring performance in three endangered violets

In rare plants that often occur in small or isolated populations the probability of selfing between close relatives is increased as a consequence of demographic stochasticity. The mode of pollination (selfing, outcrossing) may have considerable effects on seed traits and offspring performance and hence potential viability. Since current efforts aiming at the restoration of floodplain grasslands through the transfer of plant material from species-rich source stands may lead to the establishment of initially small populations consisting of founders from different populations, the present paper experimentally investigated the effects of pollen source and floral types (i.e. chasmogamous (CH) and cleistogamous (CL) flowers) on seed traits and offspring performance in three highly endangered violet species (Viola elatior, V. pumila, V. stagnina) of these grasslands. We estimated inbreeding depression and tested the performance of selfed and outcrossed offspring in two microbial environments, i.e. in soil inoculated with (i) non-sterile substrate from the same species (‘home’-conditions) and (ii) sterilised substrate.Plants produced more CL capsules than CH flowers. Pollinator exclusion had only small effects on CH seed production. CL seeds had a significantly lower mass per seed than CH seeds. This may be related to constraints in allocation or environmental conditions. Seedling growth was reduced in plants grown under ‘home’-conditions as compared to control soils. Under ‘home’-conditions, relative fitness of selfed seedlings of V. stagnina was significantly higher than that of crossed progeny. Our results suggest that high genetic differentiation among populations as a consequence of isolation may result in outbreeding depression, e.g., through biochemical or physiological incompatibilities between genes or the breaking of coadapted gene complexes. In V. stagnina, offspring fitness differed considerably between environments, but in general we found no indications for inbreeding depression in these rare species.     

Optimal nectar production in a hummingbird pollinated plant

It is hypothesized that the average rate of nectar production per flower for a population of plants is such than an individual plant which possesses this rate has maximum fitness (i.e., is optimal). This basic hypothesis is used to develop predictions concerning nectar production in scarlet gilia (Ipomopsis aggregata), a hummingbird pollinated plant. The optimal standing crop of nectar per flower is not significantly different from the observed.     

Floral symmetry is associated with flower size and pollen production but not insect visitation rates in Geranium robertianum (Geraniaceae)

The degree to which fine‐scaled variation in floral symmetry is associated with variation in plant fitness remains unresolved, as does the question of whether floral symmetry is in itself a target of pollinator‐mediated selection. Geranium robertianum (Geraniaceae) is a broadly distributed species whose five‐petaled flowers vary widely with respect to their degree of rotational asymmetry. In this study, we used a naturally occurring population of plants to investigate whether floral rotational asymmetry and leaf bilateral symmetry were phenotypically correlated with a series of fitness‐related traits, and also used an experimental array with model flowers to investigate the preference of insect visitors for varying degrees of floral size and symmetry. We found that leaf asymmetry was not associated with any of the phenotypic traits measured, and that the degree of floral rotational asymmetry was strongly associated with decreased flower size and decreased pollen production. Our experimental arrays showed that insect visitors did not discriminate among model flowers on the basis of size or symmetry alone; however, insect visitors preferentially visited smaller, symmetric model flowers over larger, severely asymmetric model flowers. Taken together, our results suggest that floral and leaf symmetry in G. robertianum are not likely strong indicators of phenotypic quality, and that floral symmetry is unlikely to be a target of pollinator‐mediated selection. However, the relationship between floral asymmetry and pollen production may provide a role for fecundity selection on symmetry in this species. These data importantly add to the growing literature on the adaptive nature of floral symmetry in the wild.     

Selective fruit abortion in Prunus mahaleb (Rosaceae)

In Prunus mahaleb (Rosaceae), only a small proportion of those flowers that initially develop give rise to fruits, and the probability of producing fruit differs between flowers. The results of this study indicate that the probability of producing fruit is about 60% for the first (i.e., most proximal) flower to develop in each inflorescence, but very low for the last (i.e., most distal). This pattern is maintained regardless of between-inflorescence differences in flowering phenology, suggesting that each inflorescence can be viewed as an independent subunit with respect to allocation of resources for reproduction. These results are compatible with the ovary reserve and selective abortion hypotheses for the adaptive value of low fruit set in P. mahaleb.     

Effects of low-efficiency pollinators on plant fitness and floral trait evolution in <Emphasis Type="Italic">Campanula americana</Emphasis> (Campanulaceae)

Floral visitors vary in their pollination efficiency and their preferences for floral traits. If low-efficiency pollinators decrease the amount of pollen available to higher efficiency visitors, then low-efficiency visitors may actually have negative fitness consequences for the plants that they visit. We used experimental arrays in two populations to determine the floral preferences and the fitness effects of low-efficiency (or ugly) pollinators on Campanula americana. These ugly pollinators (halictid bees) preferentially visited flowers with pollen over flowers that had had their pollen removed. C. americana pollen color varies quantitatively from light tan to dark purple, and we found that natural variation in pollen color influenced the magnitude of halictid preferences for flowers with pollen. In general, preferences for flowers with pollen were stronger when the ugly pollinators foraged in arrays of flowers with tan-colored pollen than in arrays with purple-colored pollen. When plants received few visits by efficient Bombus pollinators, visits by ugly pollinators significantly decreased siring success relative to plants where visits by ugly pollinators were prevented. In contrast, ugly pollinators did not influence siring success when higher efficiency pollinators were more abundant. Thus, the relationship between low-efficiency pollinators and the plants that they visit varies from commensalistic to antagonistic depending on the presence of other pollinators in the community. Our findings suggest that the negative fitness effects and floral preferences of low-efficiency or ugly pollinators may contribute to the maintenance of a pollen color polymorphism in C. americana.     

THE EVOLUTION OF FLORAL LONGEVITY: RESOURCE ALLOCATION TO MAINTENANCE VERSUS CONSTRUCTION OF REPEATED PARTS IN MODULAR ORGANISMS

The component parts of modular organisms often show interspecific variation in their longevity. In plants, the flower is an example of such a structure. Models are developed in this paper to predict optimal floral longevity (the optimal length of time that flowers should remain open and functional) under a variety of conditions. A tradeoff involving allocation of resources to floral construction versus floral maintenance is assumed. The main model variables are the rate at which pollen and seed fitness accrue over time (fitness-accrual rates) and the daily cost of maintaining an existing flower relative to the cost of constructing a new one (floral maintenance cost). Long-lived flowers are selected when fitness-accrual rates and floral maintenance costs are low, whereas short-lived flowers are selected when fitness-accrual rates and floral maintenance costs are high. Dichogamy favors longer-lived flowers relative to homogamy, whereas nonindependence among flowers in their attractiveness to pollinators (attraction to flower clusters) selects for shorter-lived flowers. Reduction in floral maintenance costs later on in the flower's life favors longer-lived flowers. Observations on the dissemination and receipt of pollen in individual flowers over time, together with measurements of corolla respiration and nectar sugar production rate are required to test the model quantitatively. The parameters important to the evolution of optimal floral longevity (i.e., maintenance and construction costs, and fitness-accrual rates) may be general features of evolution of optimal longevities of other repeated structures.     

ADVANTAGES OF BEING A SPECIALIST FEMALE IN NODIOECIOUS SILENE VULGARIS S.L. (CARYOPHYLLACEAE)

According to sex allocation theory, to maintain a mutant male-sterile plant in a population of hermaphrodites such a plant must compensate its loss of fitness caused by inhibition of pollen production with a higher reproductive success through its female function. In the present study of a gynodioecious population of Silene vulgaris (Caryophyllaceae) I show that hermaphrodites not only benefit from outcrossing, in that progeny from outcrossed flowers are more vigorous than those from selfed flowers within an individual plant, but they also suffer heavily from self-pollination between different flowers of the same individuals, which could be demonstrated in experimentally made male-sterile (emasculated) individuals. Seeds from the emasculation period were heavier and germinated better than when the same individual was an intact hermaphrodite. Naturally male-sterile (female) individuals produced more fruits due to flowers staying open longer for pollen to arrive via some vector. However, the higher seed number alone could not provide the fitness advantage needed for females to be maintained in the population, but females also produced heavier seeds as compared to the hermaphrodites. Differences in seed survival and seedling establishment in the field are expected to add the advantages necessary for female plants to be selectively plausible.     

Floral traits variation, legitimate pollination, and nectar robbing in Polygala vayredae (Polygalaceae)

The variation of floral morphology and its effect on the flower visitors of Polygala vayredae Costa (Polygalaceae), a narrow endemic species from the Oriental pre-Pyrenees, were examined. First, to account for the main floral reward (i.e., nectar), the relationship between the dimensions of the nectar gland and nectar production was investigated. Second, floral traits variation was assessed within and between the three most representative populations of the species. Finally, the role of several floral traits in the female fitness was evaluated. Furthermore, as nectar robbing was highly frequent, preferences of robbers for specific floral traits and their impact on legitimate pollinations were also evaluated. The flowers of this species are characterized by significant variations in floral characteristics and nectar rewards. A significant and positive correlation between the nectar gland dimensions and nectar production per flower was observed, with the gland dimensions being a good measure to infer the rewards offered by the flowers of P. vayredae. In general, corolla traits were significant and positively correlated with each other. Nectar was revealed to be an important trait in flower–visitor interactions, with legitimate pollinations being primarily influenced by this floral reward. Negative correlations between robbing frequency and legitimate pollinations were observed in two of the studied populations, and positive correlations between flower size and robbing frequency were observed in one population. An indirect negative selection over phenotypic floral traits mediated by nectar robbers is proposed.     

Predation Success By A Plant-Ant Indirectly Favours The Growth And Fitness Of Its Host Myrmecophyte

Mutualisms, or interactions between species that lead to net fitness benefits for each species involved, are stable and ubiquitous in nature mostly due to “byproduct benefits” stemming from the intrinsic traits of one partner that generate an indirect and positive outcome for the other. Here we verify if myrmecotrophy (where plants obtain nutrients from the refuse of their associated ants) can explain the stability of the tripartite association between the myrmecophyte Hirtella physophora, the ant Allomerus decemarticulatus and an Ascomycota fungus. The plant shelters and provides the ants with extrafloral nectar. The ants protect the plant from herbivores and integrate the fungus into the construction of a trap that they use to capture prey; they also provide the fungus and their host plant with nutrients. During a 9-month field study, we over-provisioned experimental ant colonies with insects, enhancing colony fitness (i.e., more winged females were produced). The rate of partial castration of the host plant, previously demonstrated, was not influenced by the experiment. Experimental plants showed higher δ¹⁵N values (confirming myrmecotrophy), plus enhanced vegetative growth (e.g., more leaves produced increased the possibility of lodging ants in leaf pouches) and fitness (i.e., more fruits produced and more flowers that matured into fruit). This study highlights the importance of myrmecotrophy on host plant fitness and the stability of ant-myrmecophyte mutualisms.     

Multiple herbivores and coevolutionary interactions in an Ipomopsis hybrid swarm

Studies focusing on pairwise interactions between plants and herbivores may not give an accurate picture of the overall selective effect of herbivory, given that plants are often eaten by a diverse array of herbivore species. The outcome of such interactions may be further complicated by the effects of plant hybridization. Hybridization can lead to changes in morphological, phenological and chemical traits that could in turn alter plant–herbivore interactions. Here we present results from manipulative field experiments investigating the interactive effects of multiple herbivores and plant hybridization on the reproductive success of Ipomopsis aggregata formosissima X I. tenuituba. Results showed that ungulate herbivores alone had a net positive effect on plant relative fitness, increasing seed production approximately 2-fold. Caterpillars had no effect on plant relative fitness when acting alone, with caterpillar-attacked plants producing the same number of flowers, fruits and seeds as the uneaten controls. Caterpillars, however, significantly reduced flower production of ungulate browsed plants. Flower production in these plants, however, was still significantly greater (approximately 1.7-fold greater) than uneaten controls, likely leading to an increase in reproductive success through the paternal component of fitness given that fruit and seed production was not significantly different from that of herbivore-free controls. Although results suggest that herbivore imposed selection is pairwise, ungulates likely have a large influence on the abundance of, and hence the amount of damage caused by, caterpillar herbivores. Thus, because of the ecological interactions between ungulates and caterpillars, selection on Ipomopsis may be diffuse rather than pairwise, assuming such interactions translate into differential effects on plant fitness as herbivore densities vary. Plant hybridization had no significant effect on patterns of ungulate or caterpillar herbivory; i.e., no significant interactions were detected between herbivory and plant hybridization for any of the fitness traits measured in this study nor did plant hybridization have any significant effect on host preference. These results may be due to patterns of introgression or the lack of species-specific differences between I. aggregate formosissima and I. tenuituba. Plant hybridization per se resulted in lowered reproductive success of white colored morphs due in part to the effects of pollination. Although it appears that there would be strong directional selection favoring darker flower colors due to the lower reproductive success of the white colored morphs in the short run, the natural distribution of hybrids suggest that over the long run selection either tends to average out or there are no fitness differences among morphs in most years due to the additive fitness effects of hawkmoth and hummingbird pollinators.     

Re-evaluating the role of selective abscission in moth/yucca mutualisms

Conflicts of interest are common to mutualisms, particularly those derived from exploitative interactions. Conflicts of interest are particularly pronounced in pollination/seed predation mutualisms, such as moth/yucca interactions, where consumption of seeds by larvae of a plant's pollinator will raise the fitness of the pollinator but lower the fitness of the plant. A central question in these mutualisms is, therefore, “what limits seed predation?” If plants with excess flowers selectively abscise flowers containing many eggs, they may reduce seed predation and overall increase their fecundity. If eggs in abscised flowers die, selective abscission may additionally contribute to the limitation or regulation of pollinator populations, thereby decreasing the probability of future overexploitation. We examined the effect of selective abscission in the mutualism between Yucca kanabensis and one of its pollinating moths, Tegeticula altiplanella. Per capita mortality of moth eggs due to abscission was high (95.5%), but did not increase on inflorescences with more ovipositions per flower. Overall mortality was partitioned into two components based upon the proportion of visited flowers abscised (i.e. resource‐limitation) and additional mortality (=selective abscission). Resource‐limitation per se inflicted 93.9% egg mortality, or most of the mortality due to abscission. But, the average number of eggs in fruit was lower than the average number of eggs in flowers, indicating that there was some selectivity of abscission. However, neither source of mortality increased on inflorescences with more ovipositions per visited flower. Egg mortality resulting from selective abscission was not as high as possible, because the yuccas appeared to use oviposition‐damaged ovules as a cue for selective abscission, and there was considerable variation in the relationship between oviposition number and damaged ovules. However, even if yuccas had retained the flowers containing the fewest eggs, selective abscission still would not have been higher on inflorescences with more ovipositions per flower. Considering also that, 1) number of ovipositions is a poor predictor of the number of larvae that hatch and feed on the developing seeds in a fruit and that, 2) there are several moth/yucca interactions in which selective abscission does not occur, we conclude that abscission, and particular selective abscission, may have density‐limiting effects on moth populations, but will fail as general explanations for regulating the dynamics of moth populations.     

Analysing hierarchically-structured fitness and modular dynamics in plants: integration of concepts from population dynamics

This paper analyses the reproductive strategies of a modular organism by means of demographic analysis of its reproductive units. The buds of the reproductive structure of Retama sphaerocarpa (L.) Boiss., a perennial shrub with a simple modular structure, were considered as ‘individuals’ of a population of modules within each individual plant. The development of the buds leads to the production of subpopulations of new units (inflorescences, flowers, fruits) of a lower hierarchical level. Fitness and hierarchical fitness (defined by the integration of developmental hierarchical levels in the plant) were analysed at the shoot module level, starting from the analogy provided by classical population dynamics and taking into account the different subpopulations of modules that take part in the reproductive process. Consideration of the dynamics of such subpopulations reveals the way in which demographic strategies partitioning the reproductive effort are exerted throughout the plant, and their consequences for fitness. The analysis of the reproductive process in R. sphaerocarpa shows a critical developmental transition from flowering to fruiting buds, which is a consequence of a low survival rate of the subpopulation of flowers. Despite the specificity of the empirical information used in this investigation, the fitness analyses proposed in this paper are fully applicable to any hierarchically-structured biological system, either modular or unitary.     

FLORAL BIOLOGY OF MAGNOLIA

The floral biology of eight species of Magnolia native to the United States is described. The flowers are protogynous. They are pollinated by several species of beetles that enter buds as well as closed and open flowers to feed on nectar, stigmas, pollen, and secretions of the petals. Individual flowers persist from two to four days and undergo a series of petal, stigma, and stamen movements that assure pollination by beetles. It is suggested that the flowers of Magnolia are highly specialized for exclusive pollination by beetles. These specialization mechanisms produce large quantities of food for the beetles and deny other types of insects (bees, moths, etc.) access to the flowers at critical stages in the pollination process, i.e., when stigmas are mature and pollen is shed.     

Trade-off mitigation: a conceptual framework for understanding floral adaptation in multispecies interactions

Explanations of floral adaptation to diverse pollinator faunas have often invoked visitor-mediated trade-offs in which no intermediate, generalized floral phenotype is optimal for pollination success, i.e. fitness valleys are created. In such cases, plant species are expected to specialize on particular groups of flower visitors. Contrary to this expectation, it is commonly observed that flowers interact with various groups of visitors, while at the same time maintaining distinct phenotypes among ecotypes, subspecies, or congeners. This apparent paradox may be due to a gap in our understanding of how visitor-mediated trade-offs could affect floral adaptation. Here we provide a conceptual framework for analysing visitor-mediated trade-offs with the hope of stimulating empirical and theoretical studies to fill this gap. We propose two types of visitor-mediated trade-offs to address negative correlations among fitness contributions of different visitors: visitor-mediated phenotypic trade-offs (phenotypic trade-offs) and visitor-mediated opportunity trade-offs (opportunity trade-offs). Phenotypic trade-offs occur when different groups of visitors impose conflicting selection pressures on a floral trait. By contrast, opportunity trade-offs emerge only when some visitors’ actions (e.g. pollen collection) remove opportunities for fitness contribution by more beneficial visitors. Previous studies have observed disruptive selection due to phenotypic trade-offs less often than expected. In addition to existing explanations, we propose that some flowers have achieved ‘adaptive generalization’ by evolving features to avoid or eliminate the fitness valleys that phenotypic trade-offs tend to produce. The literature suggests a variety of pathways to such ‘trade-off mitigation’. Trade-off mitigation may also evolve as an adaptation to opportunity trade-offs. We argue that active exclusion, or floral specialization, can be viewed as a trade-off mitigation, occurring only when flowers cannot otherwise avoid strong opportunity trade-offs. These considerations suggest that an evolutionary strategy for trade-off mitigation is achieved often by acquiring novel combinations of traits. Thus, phenotypic diversification of flowers through convergent evolution of certain trait combinations may have been enhanced not only through adaptive specialization for particular visitors, but also through adaptive generalization for particular visitor communities. Explorations of how visitor-mediated trade-offs explain the recurrent patterns of floral phenotypes may help reconcile the long-lasting controversy on the validity of pollination syndromes.     

FLORAL COLOR CHANGE IN LUPINUS ARGENTEUS (FABACEAE): WHY SHOULD PLANTS ADVERTISE THE LOCATION OF UNREWARDING FLOWERS TO POLLINATORS?

I examined the adaptive significance of two floral traits in the perennial herb, Lupinus argenteus: 1) the retention of corollas on “spent” flowers, i.e., flowers containing inviable pollen, unreceptive stigmas, and negligible pollinator rewards and 2) a change in corolla color of retained “spent” flowers, which is restricted to a spot on the banner petal. At anthesis, this spot is yellow, and approximately four days later, it changes to purple. After the change, purple flowers remain on plants an additional 5–7 days before corolla abscission occurs; purple flowers were avoided by pollinators, presumably because they contained less pollen (rewards) than yellow ones. I experimentally tested the hypothesis that purple flowers contribute to the floral display of the plant by removing varying numbers of spent flowers and assessing the effect on pollination visitation. Pollinators preferentially approached and foraged on plants with greater numbers of flowers per inflorescence; they did not discriminate between yellow (rewarding) and purple (nonrewarding) flowers at interplant distances greater than 0.4 meters but would preferentially forage on plants with more total flowers, even if these individuals contained fewer rewarding flowers. Thus, spent flowers increased the overall attractiveness of plants to pollinators. In theory, color change may benefit plants in two ways. First, by directing pollinators to rewarding flowers, the change may increase pollinator foraging efficiency, with the result that pollinators visit more flowers before leaving plants (pollinator-tenure mechanism). Second, by directing pollinators to receptive flowers, the color change may prevent incoming pollen from being wasted on unreceptive stigmas and may prevent collection of inviable pollen (pollination-efficiency mechanism). I tested the pollinator-tenure mechanism experimentally by removing pollen from yellow flowers, thereby reducing the reliability of the color-reward signal. Pollinators visited fewer total flowers on experimental plants than on controls, resulting in reduced seed production in one year.     

Dracula orchids exploit guilds of fungus visiting flies: new perspectives on a mushroom mimic

1. Dracula orchids mimic mushrooms in appearance and scent, and are pollinated by mushroom flies. This study examined the guilds of insect visitors to Dracula species and to co‐occurring mushrooms in an Ecuadorean cloud forest. Significant visitor overlap was documented (mostly in the genera Zygothrica and Hirtodrosophila; Drosophilidae). To test the brood‐site mimicry hypothesis, behavioural observations were also performed and rearing success was examined. 2. Many fly species that visit Dracula are also found on mushrooms. Furthermore, the presence of pollinia enabled us to document the fact that particular individuals visiting mushrooms also visit Dracula. Roughly two‐thirds of the visitors to these unusual flowers are fungal‐associated, and the rest appear to be Dracula specialists. 3. A variety of behaviours common on both host groups were observed, including courtship semaphoring, feeding, sheltering, defending territory, and mating. Given this suite of shared reproductive behaviour, it was hypothesised that flies may also oviposit in both substrates. Flies were reared from mushrooms, Dracula spp., and other flowers (controls) to determine which substrate led to the highest fitness for the flies. Very few flies emerged from the Dracula flowers and there was no species overlap between the emergent insects and the known pollinators. 4. Despite the fact that there is evidence for brood‐site mimicry, the Dracula situation is complex. All the flies could derive many of the same fitness benefits from the flowers as they do from mushrooms (shelter, food, mating), but they are clearly not gaining the same fitness benefit, in terms of progeny, on flowers compared with mushrooms.     

Competition between cyanogenic and acyanogenic morphs of Trifolium repens

Summary To investigate the cost of the dimorphic cyanogenic system in Trifolium repens L., genotypes of the cyanogenic (T_c) and acyanogenic (T_a) morphs were grown in mixtures over a range of ratios and in pure stands at two levels of total density (low in a first experiment, high in a second experiment). The principles of the competition analysis employed were those related to the inverse linear model response. The morphs were compared using two interaction indices, the substitution rate and the relative resource total (RRT). The relative fitness of the two morphs, i.e. biomass and number of flowers per plant, suggests that the T_a morph has a competitive advantage over the T_c morph.     

Nocturnal Versus Diurnal Pollination Success in Isertia laevis (Rubiaceae): A Sphingophilous Plant Visited by Hummingbirds

Abstract: Isertia laevis (Rubiaceae) possesses flowers with traits typical for the pollination syndrome of sphingophily. Diurnal flower observation showed that nine different hummingbirds (Trochilidae) and one flower piercer (Coerebidae) were frequent visitors. Their activity on the flowers peaked in the morning hours. Very low nectar volumes were found in the morning (8.00 h) in unbagged flowers. Nectar volumes, however, reached their peaks (27 μl) at night (2.00 h) in bagged, as well as in unbagged flowers. At night few individuals of sphingids were observed. Pollination experiments showed that flowers presented to nocturnal pollinators from 18.00 h to 6.00 h had low fruit set (14 %) but high seed set (59 %). Flowers accessible from 6.00 h to 18.00 h for diurnal flower visitors showed high fruit set of 63 % but low seed set of 14 %. This suggests that pollination of individual flowers is less effective during daytime. Regarding relative reproductive success, i.e., efficiency of pollination defined as fruit set x seed set, both diurnal and nocturnal pollinators, however, are equally successful. We conclude that frequently occurring, but not very effective pollinators contribute substantially to seed production, when the expected pollinators are scarce.     

EVOLUTION OF TRADE-OFFS BETWEEN SEXUAL AND ASEXUAL PHASES AND THE ROLE OF REPRODUCTIVE PLASTICITY IN THE GENETIC ARCHITECTURE OF APHID LIFE HISTORIES

Life‐history theory postulates that evolution is constrained by trade‐offs (i.e., negative genetic correlations) among traits that contribute to fitness. However, in organisms with complex life cycles, trade‐offs may drastically differ between phases, putatively leading to different evolutionary trajectories. Here, we tested this possibility by examining changes in life‐history traits in an aphid species that alternates asexual and sexual reproduction in its life cycle. The quantitative genetics of reproductive and dispersal traits was studied in 23 lineages (genotypes) of the bird cherry‐oat aphid Rhopalosiphum padi, during both the sexual and asexual phases, which were induced experimentally under specific environmental conditions. We found large and significant heritabilities (broad‐sense) for all traits and several negative genetic correlations between traits (trade‐offs), which are related to reproduction (i.e., numbers of the various sexual or asexual morphs) or dispersal (i.e., numbers of winged or wingless morphs). These results suggest that R. padi exhibits lineage specialization both in reproductive and dispersal strategies. In addition, we found important differences in the structure of genetic variance–covariance matrices ( G ) between phases. These differences were due to two large, negative genetic correlations detected during the asexual phase only: (1) between fecundity and age at maturity and (2) between the production of wingless and winged parthenogenetic females. We propose that this differential expression in genetic architecture results from a reallocation scheme during the asexual phase, when sexual morphs are not produced. We also found significant G × E interaction and nonsignificant genetic correlations across phases, indicating that genotypes could respond independently to selection in each phase. Our results reveal a rather unique situation in which the same population and even the same genotypes express different genetic (co)variation under different environmental conditions, driven by optimal resource allocation criteria.