Pollination thoroughness and maternal yield regulation in wild radish,Raphanus raphanistrum (Brassicaceae)

Summary Under conditions where resources are limited, there are often negative correlations between components of maternal yield, or between fruit and flower production. Pollination, in turn, may vary among individuals and influence total maternal expenditure. We examined the impact of variation in pollination thoroughness upon yield components and overall plant growth in wild radish (R. raphanistrum) plants grown in the greenhouse. Plants received different pollination treatments in which 0% to 100% of all flowers produced were hand-pollinated. Fruit set was increased by hand-pollination, but rarely exceeded 30%, even when more than 50% of the flowers were pollinated. Plants receiving more thorough pollination or having greater proportion fruit set produced significantly smaller seeds. Seed number per fruit was not influenced by pollination treatment. Mean values of yield components and interactions between components often varied among plants from different maternal families. Increasing pollination thoroughness also resulted in dramatic decreases in flower production. If male fitness is related to flower number, there may be a tradeoff between maternal fecundity and successful pollen export operating at the whole-plant level in this species.     

Examination of the Biological Effects of High Anionic Peroxidase Production in Tobacco Plants Grown under Field Conditions. I. Insect Pest Damage

At least 25 wild type and high peroxidase tobacco Nicotiana tabacum L. plants were examined semiweekly over several weeks for pest insect distribution and damage in a 2 year field study. Incidence and/or severity of naturally occurring caterpillar damage (dingy cutworm (Feltia ducens Walker), black cutworm (Agrotis ipsilon (Hufnagel), tobacco hornworm (Manduca sexta L.), and false tobacco budworm (= corn earworm Helicoverpa zea (Boddie)) was significantly reduced at several sample dates for high peroxidase vs. wild type plants. These results parallel those of prior laboratory studies with caterpillars. The number of adult whiteflies (Trialeurodes vaporariorum (Westwood) per plant was significantly reduced on high peroxidase compared to wild type plants on most sample dates in both years. The number of plants with leaves containing >100 aphids (primarily Myzus persicae Sulzer) per leaf on high peroxidase plants was significantly lower that on wild type plants after an equivalent invasion period in both years. A significantly higher proportion of aphids were found dead on leaf five of high peroxidase compared to wild type plants at most sample dates in both years. These results indicate that high peroxidase plants have resistance to a wide range of insects, implicating this enzyme as a broad range resistance mechanism. The U.S. Government’s right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Competition alters life history and increases the relative fecundity of crop-wild radish hybrids (Raphanus spp.)

The evolutionary impact of crop-to-wild gene flow depends on the fitness of hybrids under natural, competitive conditions. Here, we measured the performance of third-generation (F3) radish hybrids (Raphanus raphanistrum x Raphanus sativus) and weedy R. raphanistrum to understand how competitive interactions affect life history and relative fecundity. Three wild and three F1 crop-wild hybrid radish populations were established in semi-natural, agricultural conditions in Michigan, USA. The effects of competition on life-history traits and fecundity of F3 progeny were measured 2 yr later in a common garden experiment. Third-generation hybrid plants generally produced fewer seeds per fruit and set fewer fruits per flower than wild plants, resulting in lower lifetime fecundity. With increasing competition, age at reproduction was delayed, the relative number of seeds per fruit was reduced in wild plants and differences between hybrid and wild fecundity diminished. Competition may enhance the fecundity of advanced-generation hybrids relative to wild plants by reducing differences in life history, potentially promoting the introgression of crop alleles into weed populations.     

Different-sized oak trees are equally protected by the aphid-tending ants

Theoretical models predict that the ecological impacts of ants on plants will vary with the size of the plant, but experimental evidence supporting this hypothesis is lacking. Focusing on aphid-tending ants, Lasius fuliginosus, and host oak trees, Quercus liaotungensis, we surveyed the density of aphids on the leaves of different-sized trees and the relationship between the densities of aphids and ants on the leaves. We then evaluated the impacts of ants on small (diameter at breast height (DBH) < 5 cm) and large trees (DBH > 25 cm). The field survey showed that the density of ants on leaves was determined by the density of aphids and the latter was similar among different-sized trees; through an ant-exclusion treatment, we found that the impacts of ants on plants, aphids, caterpillars, and galls did not differ by the size of the tree. L. fuliginosus had a significant protective effect for Q. liaotungensis, but this effect did not convert to enhanced fruit production during the 2-year study period. Considering that size can be used as an indicator of a plant’s ontogenetic stage, our results indicate that with similar densities of aphids and, in turn, ants on their canopy, trees at early and later ontogenetic stages are equally protected by aphid-tending ants.     

Evidence for the transfer of a soil‐borne contaminant from plants to ants via an aphid mediator

1. Uptake of environmental contaminants by lower trophic groups can have negative effects on higher trophic groups. This study tested the ability of selenium, an environmental contaminant found in high concentrations throughout the tissues of certain accumulating plants, to be transferred to ants via aphid tissue and honeydew. 2. Plants of the selenium accumulator, Raphanus sativus (wild radish), were watered with three different selenium treatments (0, 0.25 or 0.5 µg Se ml^?1). Aphids, Myzus persicae, and Argentine ant colonies, Linepithema humile, were added to each caged plant and allowed to interact freely. Ant colonies were supplemented with one of three different food options to encourage the consumption of aphids, aphid honeydew, or aphids and honeydew. 3. The accumulation of selenium by each trophic group and a trophic transfer factor (TTF) was calculated. The TTF for plants to aphids was > 1, indicating biomagnification, whereas the TTF for aphids to worker ants was < 1, indicating only biotransfer. Accumulated levels by worker ants did not statistically differ as a result of diet. 4. The amounts of selenium acquired by ants as a factor of diet and caste were compared. Plants, aphids and worker ants accumulated selenium in a dose‐dependent manner. Ant queens did not contain detectable amounts of selenium. Honeydew contained comparable amounts of selenium to plant selenium levels. 5. Access to toxic compounds via honeydew and insect protein may have negative effects on the range expansion of invasive species, such as the Argentine ant.     

THRIPS-MEDIATED SELF-POLLINATION OF TWO FACULTATIVELY XENOGAMOUS WETLAND SPECIES

Thrips and/or aphids played an important role in the self-pollination of two facultatively xenogamous herbs that inhabit wetlands in northwestern Iowa. In both Ranunculus sceleratus and Potentilla rivalis the fruit set and/or successful pollination of plants that were sprayed with malathion to kill thrips and aphids and caged to exclude typical flower visitors was substantially and significantly lower than that of open-pollinated and caged plants. We observed pollenbearing thrips and aphids on the flowers. The high fruit sets of emasculated flowers of R. sceleratus showed that insects moved pollen between flowers, and our observations of bees collecting pollen and moving between plants suggest that cross-pollination can occur in both species.     

NONRANDOM MATING IN WILD RADISH: VARIATION IN POLLEN DONOR SUCCESS AND EFFECTS OF MULTIPLE PATERNITY AMONG ONE- TO SIX-DONOR POLLINATIONS

Nonrandom mating in plants may involve differential success of self- vs. outcross pollen, of pollen from relatives vs. nonrelatives, and of pollen from several compatible donors. Of these, the last is least understood. Wild radish plants were crossed with six pollen donors in all possible combinations to investigate differences in pollen donor performance and to compare multiply sired fruits with different numbers and identities of fathers. Pollen donors differed in number of seeds sired, position of seeds sired within fruits, and mass of seeds sired. Performance in single pollination was not a good predictor of performance in mixed pollination. Performance in two-donor pollinations was a better predictor of the outcome of multidonor pollinations. Multiply sired fruits were selectively filled, but not all multiply sired fruits were equal. Both the number and identity of donors affected fruit filling. These results and others with wild radish suggest that variation in pollen donor ability to sire seeds is relatively common in wild radish. Since wild radish plants in the field typically receive pollen from several donors, there may be considerable opportunity for selection at this stage of the life cycle in wild radish.     

Effects of dietary amino acids on the production of sexual morph by the green peach aphid,Myzus persicae

In order to complete a holocycle of the green peach aphid, Myzus persicae, on a synthetic diet, the effects of dietary amino acid contents on development of sexuals were investigated. For this purpose, the aphids were reared on holidic diets for two to three generations at 20°C under a scotophase of 15 h per diem. Under these conditions, the production of males by the aphids on a synthetic diet was very much poorer than those reared on radish seedling. On reducing the amino acid concentration of the diet, the proportion of males produced was comparable to that produced by aphids fed on radish seedlings. The obtained males were smaller and survived longer than those reared on radish seedling. Under the long night photoperiod and on the synthetic diet with reduced amino acids, gynoparae and then oviparae were also obtained. The oviparae were mated with the males grown on the synthetic diet, and laid eggs. The eggs, however, did not turn dark in color, and perished. © 1995 Wiley-Liss, Inc.     

Food web structure of aphids and their parasitoids in Belgian fruit agroecosystems

Community structures of aphids and their parasitoids were studied in fruit crop habitats of eastern Belgium in 2014 and 2015. Quantitative food webs of these insects were constructed separately for each year, and divided into subwebs on three host‐plant categories, fruit crop plants, non‐crop woody and shrub plants and non‐crop herbaceous plants. The webs were analyzed using the standard food web statistics designed for binary data. During the whole study period, 78 plant species were recorded as host plants of 71 aphid species, from which 48 parasitoid species emerged. The community structure, aphid / parasitoid species‐richness ratio and trophic link number varied between the two years, whereas the realized connectance between parasitoids and aphids was relatively constant. A new plant–aphid–parasitoid association for Europe was recorded. Dominant parasitoid species in the study sites were Ephedrus persicae, Binodoxys angelicae and Praon volucre: the first species was frequently observed on non‐crop trees and shrubs, but the other two on non‐crop herbaceous plants. The potential influence, through indirect interactions, of parasitoids on aphid communities was assessed with quantitative parasitoid‐overlap diagrams. Symmetrical links were uncommon, and abundant aphid species seemed to have large indirect effects on less abundant species. These results show that trophic indirect interactions through parasitoids may govern aphid populations in fruit crop habitats with various non‐crop plants, implying the importance for landscape management and biological control of aphid pests in fruit agroecosystems.     

ECOLOGICAL CONSEQUENCES AND PHENOTYPIC CORRELATES OF PETAL SIZE VARIATION IN WILD RADISH,RAPHANUS SATIVUS (BRASSICACEAE)

In this paper we examine some ecological consequences and phenotypic correlates of flower size variation in wild radish, Raphanus sativus. Mean corolla diameter varied significantly among individuals within natural populations of R. sativus in California. On the average, almost 40% of flower biomass was allocated to corolla tissue. In field experiments, pollinator visitation increased significantly with corolla size. Large flowers also accumulated more nectar when pollinators were excluded from plants. In three populations, corolla size was positively correlated with allocation to pollen per flower (either anther weight or pollen grain number), but there was usually no phenotypic relationship between corolla size and several measures of female allocation (ovule number per flower, proportion fruit set, and total seed mass per fruit). Plants growing in the field produced fewer large flowers per unit of stem, and stem biomass was negatively related to corolla size for plants grown under controlled greenhouse conditions. Male and female fitness may covary differently with allocation to attractive floral features in species such as R. sativus, where seed production is often limited by resources rather than by pollen.     

Silencing the expression of the salivary sheath protein causes transgenerational feeding suppression in the aphid Sitobion avenae

Aphids produce gel saliva during feeding which forms a sheath around the stylet as it penetrates through the apoplast. The sheath is required for the sustained ingestion of phloem sap from sieve elements and is thought to form when the structural sheath protein (SHP) is cross‐linked by intermolecular disulphide bridges. We investigated the possibility of controlling aphid infestation by host‐induced gene silencing (HIGS) targeting shp expression in the grain aphid Sitobion avenae. When aphids were fed on transgenic barley expressing shp double‐stranded RNA (shp‐dsRNA), they produced significantly lower levels of shp mRNA compared to aphids feeding on wild‐type plants, suggesting that the transfer of inhibitory RNA from the plant to the insect was successful. shp expression remained low when aphids were transferred from transgenic plants and fed for 1 or 2 weeks, respectively, on wild‐type plants, confirming that silencing had a prolonged impact. Reduced shp expression correlated with a decline in growth, reproduction and survival rates. Remarkably, morphological and physiological aberrations such as winged adults and delayed maturation were maintained over seven aphid generations feeding on wild‐type plants. Targeting shp expression therefore appears to cause strong transgenerational effects on feeding, development and survival in S. avenae, suggesting that the HIGS technology has a realistic potential for the control of aphid pests in agriculture.     

Abscisic acid deficiency increases defence responses against Myzus persicae in Arabidopsis

Comparison of Arabidopsis thaliana (Arabidopsis) gene expression induced by Myzus persicae (green peach aphid) feeding, aphid saliva infiltration and abscisic acid (ABA) treatment showed a significant positive correlation. In particular, ABA‐regulated genes are over‐represented among genes that are induced by M. persicae saliva infiltration into Arabidopsis leaves. This suggests that the induction of ABA‐related gene expression could be an important component of the Arabidopsis–aphid interaction. Consistent with this hypothesis, M. persicae populations induced ABA production in wild‐type plants. Furthermore, aphid populations were smaller on Arabidopsis aba1‐1 mutants, which cannot synthesize ABA, and showed a significant preference for wild‐type plants compared with the mutant. Total free amino acids, which play an important role in aphid nutrition, were not altered in the aba1‐1 mutant line, but the levels of isoleucine (Ile) and tryptophan (Trp) were differentially affected by aphids in wild‐type and mutant plants. Recently, indole glucosinolates have been shown to promote aphid resistance in Arabidopsis. In this study, 4‐methoxyindol‐3‐ylmethylglucosinolate was more abundant in the aba1‐1 mutant than in wild‐type Arabidopsis, suggesting that the induction of ABA signals that decrease the accumulation of defence compounds may be beneficial for aphids.     

Prey preference and egg production of the carabid beetleAgonum dorsale

In order to elucidate whether the aphidRhopalosiphum padi is low quality food for the carabid beetleAgonum dorsale, as it has previously been found to be for cereal spiders, we performed a series of experiments using fruit fliesDrosophila melanogaster as a standard alternative prey (‘control prey’): 1) Prey preference, 2) Aphid consumption for satiated and starved beetles, 3) Egg production on different diets and 4) Preference experiments with fruit flies coated with a taste of different prey types. Other alternative prey types used were earthworms and sciarid midges, all potential prey in the field.A. dorsale has a low preference for aphids compared to fruit flies. Apparently there is an upper limit to the consumption of aphids independent of hunger and much below the beetles' total food demand. Egg production on a pure diet of aphids is lower than on a pure diet of fruit flies; it is extremely low on a pure diet of earthworms; the highest fecundity is found on a mixed diet. As fruit flies coated with a taste of aphids or sciarid midges are less prefered than flies covered with a taste of fruit flies there may be a chemical factor, acting through taste, involved in determining prey preference.     

PARENTAL EFFECTS ON SEED DEVELOPMENT AND SEED YIELD IN RAPHANUS RAPHANISTRUM: IMPLICATIONS FOR NATURAL AND SEXUAL SELECTION

The possibility that sexual selection operates in angiosperms to effect evolutionary change in polygenic traits affecting male reproductive success requires that there is additive genetic variance for these traits. I applied a half-sib breeding design to individuals of the annual, hermaphroditic angiosperm, wild radish (Raphanus raphanistrum: Brassicaceae), to estimate paternal genetic effects on, or, when possible, the narrow-sense heritability of several quantitative traits influencing male reproductive success. In spite of significant differences among pollen donors with respect to in vitro pollen tube growth rates, I detected no significant additive genetic variance in male performance with respect to the proportion of ovules fertilized, early ovule growth, the number of seeds per fruit, or mean individual seed weight per fruit. In all cases, differences among maternal plants in these traits far exceeded differences among pollen donors. Abortion rates of pollinated flowers and fertilized ovules also differed more among individuals as maternal plants than as pollen donors, suggesting strong maternal control over these processes. Significant maternal phenotypic effects in the absence of paternal genetic or phenotypic effects on reproductive traits may be due to maternal environmental effects, to non-nuclear or non-additive maternal genetic effects, or to additive genetic variance in maternal control over offspring development, independent of offspring genotype. While I could not distinguish among these alternatives, it is clear that, in wild radish, the opportunity for natural or sexual selection to effect change in seed weight or seed number per fruit appears to be greater through differences in female performance than through differences in male performance.     

Are Defenses of Wild Radish Populations Well Matched with Variability and Predictablity of Herbivory?

Theory predicts that plants should employ constitutive (fixed) defenses when herbivory is consistently strong among years and induced (plastic) defenses when herbivory varies among years but is predictable within a season. We tested this theory by examining the herbivore species and damage censused over three seasons for 20 populations of wild radish in northern California. We conducted assays of constitutive resistance by challenging undamaged plants from these 20 populations with their common herbivores in the greenhouse. We assayed induced resistance by comparing the performance of herbivores on plants that had been experimentally damaged to undamaged plants from the same populations. Following damage, plants generally became more resistant to chewing herbivores (caterpillars) but more susceptible to sucking herbivores (aphids). Constitutive resistance to caterpillars was not stronger for populations that had high levels of herbivory that varied little among years, contrary to theory. Induced resistance may be stronger for plants from populations where herbivory varied more among years, consistent with expectations, although low power makes this conclusion equivocal. Induced resistance was not stronger for populations where early herbivory was a good predictor of late season herbivory. This lack of support for theory could have been caused by inadequacies with the experimental tests or with the theory and its assumptions. The theory assumes a coevolutionary equilibrium; however, high gene flow that has been reported for wild radish could disrupt matches between risk of herbivory and plant defense. The theory also assumes that resistance traits evolved as defenses against herbivory although these traits also serve other functions. Finally, the correlation we measured between early and late season herbivory may be at a temporal scale that is irrelevant since wild radish appears to adjust its defenses very rapidly.     

POSTPOLLINATION EFFECTS ON SEED PATERNITY: MECHANISMS IN ADDITION TO MICROGAMETOPHYTE COMPETITION OPERATE IN WILD RADISH

After pollen arrives on a stigma, the paternity of seeds may be influenced by microgametophyte competition, maternal choice, genetic complementation between parents, and embryo competition. While microgametophyte competition has been well accepted, the other mechanisms are more difficult to demonstrate, and their effects are often confounded. Here, wild radish plants were pollinated with single and mixed pollen loads, and some plants were stressed such that reproduction was reduced. Effects of pollen donors, maternal families, maternal × paternal interaction, pollen donor number, and stress on fruit abortion, seed number per fruit, seed weight, and total seed weight per fruit were measured. Maternal-plant × pollen-donor interaction effects were found for all variables, indicating that genetic complementation or maternal choice occurred. Values of the components of reproduction were generally higher for multiply sired fruits than for singly sired fruits, indicating that either competition among embryos changed under multiple paternity or maternal choice for multiply sired fruits occurred. Finally, when maternal plants were stressed, the components of reproduction were more strongly affected by seed and fruit paternity. This result indicates that either competitive regimes among embryos were affected by stress or maternal plants become more selective under stress. In both cases where embryo competition might have been an explanation of the results, variation in seed weight within fruits was unaffected, suggesting that competitive regimes were unchanged. Clearly, mechanisms in addition to microgametophyte competition are important in sorting the pollen that arrives on stigmas of wild radish. These data suggest that maternal choice is likely to be important. In addition, these processes are likely to occur in the field, since the effects are stronger in stressed than in control plants.     

Expression of the <Emphasis Type="Italic">ipt</Emphasis> Gene with the AGPase S1 Promoter in Tomato Results in Unbranched Roots and Delayed Leaf Senescence

Transgenic tomato plants were produced with the isopentenyl transferase gene (ipt) ligated to a promoter that is active exclusively in sink tissue. Initially, transgenic plants had smaller, round-scale leaves, swollen stems, and exhibited early development of lateral shoots compared to wild type. Expression of the ipt gene resulted in the formation of unbranched roots on cuttings and delayed senescence in excised leaves. Callus and root formation occurred on excised leaves and leaf discs during dark incubation. The retention percentage of chlorophyll, as well as cytokinin in excised leaves or discs was significantly greater than wild type. Transgenic tomato fruit had elevated levels of cytokinins in the first days after fruit set and these levels were maintained longer during fruit development.     

MEASUREMENTS OF NATURAL SELECTION ON FLORAL TRAITS IN WILD RADISH (RAPHANUS RAPHANISTRUM). I. SELECTION THROUGH LIFETIME FEMALE FITNESS

Although the role of natural selection in the evolution of floral traits has been of great interest to biologists since Darwin, studies of selection on floral traits through differences in lifetime fitness have been rare. We measured selection acting on flower number, flower size, stigma exsertion, and ovule number per flower using field data on lifetime female fitness (seed production) in wild radish, Raphanus raphanistrum. The patterns of selection were reasonably consistent across three field seasons, with strong directional selection for increased flower production in all three years, weaker selection for increased ovule number per flower in two years, and selection for increased flower size in one year. The causes of the selection were investigated using path analysis combined with multiplicative fitness components. Increased flower production increased fruit production directly, and increased numbers of ovules per flower increased the number of seeds per fruit in all three years; pollinator visitation did not influence either of these fitness components. Increased flower size was associated with increases in both the number of fruit and the number of seeds per fruit in one year, with the latter relationship being stronger. Total lifetime seed production was affected more strongly by differences in fruit production than by differences in either the number of seeds per fruit or the proportion of fertilized seeds that were viable, but all three fitness components were positively correlated with total seed production.     

DEVELOPMENTAL AND GENETIC SOURCES OF SEED WEIGHT VARIATION IN RAPHANUS RAPHANISTRUM L. (BRASSICACEAE)

Seed weight is known to have a marked impact on emergence and post-emergence productivity in wild radish (Raphanus raphanistrum). In this paper, I describe several levels of seed weight variation in plants taken from a natural population in Hamden, Connecticut. Six maternal plants from the 1981 season were analyzed in detail: the weights and positions of all seeds within a fruit were recorded, and some of these seeds were used the following summer for competition studies and progeny analysis. Within a plant, average seed weight decreased as the number of seeds within a fruit increased, suggesting that developing embryos compete for maternal resources. Seed weight also varied significantly among the six maternal plants used in the study. Comparison of the average weights of seeds produced by offspring of those six plants with the average weights of seeds borne by the maternal plant revealed a significant genetic component to seed weight variation. Seed weight varied up to six-fold within single fruits of R. raphanistrum; large seeds tend to occur near the pedicel or in the middle positions. Seed size variation seen within single fruits is of sufficient magnitude to result in differential reproductive output among closely related seeds under competitive field conditions.     

Suppression of aphids by augmentative release of larvae of flightless Harmonia axyridis

The release of the larvae of flightless ladybird beetles could extend the duration of effective control because of the longer presence of adult stages, and the per individual production cost for juvenile stages is lower than that for adult stages. A preliminary release experiment was conducted to assess the effectiveness in controlling two aphid species, Aphis gossypii and Aulacorthum solani, using second instars of a flightless strain of Harmonia axyridis. The number of A. gossypii was suppressed in greenhouses that contained the flightless strain compared with greenhouses that contained the wild‐type strain. In one of two replicates, more flightless adults were observed on the plants for longer than wild‐type adults. However, it was not clear whether the extension of the residence period contributed to the effectiveness in controlling aphids directly, because the population of ladybirds in the greenhouses consisted of both larvae and adults during parts of the experimental period. In flightless H. axyridis, the release of larvae was more effective in suppressing A. solani than the release of adults. These results suggest that it may be more effective to release larvae of flightless H. axyridis than wild‐type larvae or flightless adults.