Pollen precedence and stigma closure: a mechanism of competition for pollination between Delphinium nelsonii and Ipomopsis aggregata

Summary Previous experiments showed that the sympatric herbs Delphinium nelsonii and Ipomopsis aggregata compete for hummingbird pollination and that deleterious effects of the former species on seed set of the latter involve interspecific pollen transfer. However, seed set was not reduced when pollen of both species was applied simultaneously to I. aggregata stigmas. Hence a competitive effect may require arrival of foreign pollen before conspecific pollen. To explore this possibility we subjected I. aggregata flowers to a competition treatment in which they received D. nelsonii pollen 6 h before I. aggregata pollen, or to a control in which they received only the conspecific pollen. Foreign pollen precedence decreased mean seed set by almost 50%, which is consistent with effects observed in previous experiments. Reduced seed set can be explained by the fact that foreign pollen often caused stigma lobes to close together within 1.5–6 h, reducing subsequent receptivity. Stigma closure was also elicited by conspecific pollen, but not by mechanical stimulation, and was influenced by size of the pollen load and identity of the plant being pollinated.     

Comparative studies of pollen and fluorescent dye transport by bumble bees visiting Erythronium grandiflorum

Summary In the Colorado Rocky Mountains the glacier lily Erythronium grandiflorum exhibits a striking dimorphism in pollen color and is commonly pollinated by the bumble bee Bombus occidentalis. We induced bees to visit sequences of flowers in a flight cage, and compared dispersal of distinctively-colored pollen and fluorescent pigment (dye) that the bee had picked up at a single donor flower. Nonparametric and parametric analyses showed that dispersal properties of pollen and dye differed; consistently less pollen was deposited and it was carried consistently shorter distances than dye. Dye thus does not provide an accurate means of assessing exacty where or how far pollen travels in this plant-pollinator system. On the other hand, both pollen and dye responded similarly to several experimental manipulations of donor and recipient flowers. Hence dye may well be of value for a qualitative investigation of how floral traits influence pollen dispersal.     

The effect of nectar guides on pollinator preference: experimental studies with a montane herb

Summary Rare albino morphs of the montane larkspur Delphinium nelsonii differ from common blue-flowered morphs in overall flower color, and in the strength of a contrasting color pattern at the center of the flower that presumably guides pollinators to concealed nectar. Previous studies showed that bumblebees and hummingbirds discriminate against albinos when presented with mixtures of the 2 morphs, and that it takes these pollinators longer to fly between successive flowers on albino than on blue-flowered inflorescences. To explore the link between these observations, we measured pollinator preferences and flower-to-flower flight times (handling times) before and after painting flowers in 2 alternative ways that enhanced albino nectar guides. In all of 16 experimental replicates discrimination against albinos was reduced or eliminated after painting, and albino handling times declined toward values for blue-flowered inflorescences. This consistent result indicates that an inferior nectar guide increases the energetic cost of foraging at albinos. Increased cost in turn explains discrimination, under the reasonable assumption that hummingbirds and bumblebees are sensitive to foraging economics.     

Experimental studies of pollen carryover: effects of floral variability in Ipomopsis aggregata

Summary In the montane herb Ipomopsis aggregata, size and placement of stamens and pistils vary substantially among flowers within plants, among nearby plants, and among groups of plants separated by 50–100 m. We trained captive hummingbirds to feed from flowers of this species in a flight cage, and explored the effects of different degrees of floral variability on carryover of fluorescent dyes that act as pollen mimics. We found that the slopes of linear dye carryover functions generally became more shallow as floral variability increased; this led to substantially longer carryover in the treatment with greatest variability. On the other hand, total amounts of dye transferred did not appear to be sensitive to the degree of variability. Floral variability may have a subtle but important effect on plant fitness by influencing the distance of pollen transfer.     

Fragments of human γG immunoglobulins produced by a porcine pancreatic esterase

The effect of a porcine pancreatic esteroproteolytic enzyme on human IgG has been described. A sequential breakdown of the molecule occurs. The first cleavage results in the formation of an F′c fragment together with an F(ab)₂ fragment. A subsequent proteolysis of the F(ab)₂ fragment liberates two Fab fragments. Each fragment has been characterized by its antigenic properties, molecular weight, and sulfhydryl content.     

COMPONENTS OF PHENOTYPIC SELECTION: POLLEN EXPORT AND FLOWER COROLLA WIDTH IN IPOMOPSIS AGGREGATA

In the hummingbird-pollinated herb Ipomopsis aggregata, selection through male function during pollination favors wide corolla tubes. We explored the mechanisms behind this selection, using phenotypic selection analysis to compare effects of corolla width on two components of male pollination success, pollinator visit rate and pollen exported per visit. During single visits by captive hummingbirds, flowers with wider corollas exported more pollen, and more dye used as a pollen analogue, to stigmas of recipient flowers. Corolla width was less strongly related to visit rate in the field, and had no direct effect on visit rate after nectar production and corolla length were controlled for. Moreover, the phenotypic selection differential was 80% higher for the effect on pollen exported per visit, suggesting that this is the more important mechanism of selection.     

Plant size,geitonogamy and seed set in Ipomopsis aggregata

Summary We used powdered fluorescent dyes to estimate receipt of self vs. outcross pollen in the self-incompatible species Ipomopsis aggregata (Polemoniaceae). Flowers on small and large plants received equal amounts of outcross pollen, whereas flowers on large plants received more self pollen, so the proportion of self pollen delivered through geitonogamy increased with plant size. In natural populations emasculation of all flowers on a plant raised average seed set per flower from 5.19 to 6.99 and also raised fruit set, though not significantly. From these results one expects a negative correlation between plant size and seeds per flower. The opposite trend was observed in a sample of plants in the field, suggesting that deleterious effects of geitonogamy on female fecundity in large plants can be overruled by other factors such as size-related fruit or seed abortion. Results are discussed in relation to the evolution of gynodioecy.     

Effects of acid irrigation and liming on two clones of Norway spruce

Summary The effect of acid irrigation on the growth of rooted cuttings ofPicea abies (L.) Karst, was investigated in a pot experiment lasting 3 years. It involved two clones of Norway spruce, H 253 Bogstad I and H 254 Bogstad II. Irrigation water of pH 5.4, 4.0, 3.0 and 2.5 was used. Liming was included in the experiment.After the experimental period, the plants of all treatments were growing reasonably well. However, those plants irrigated at pH 2.5 were slightly discoloured. The plant mortality was only 3% throughout the experiment, and was not connected to acid irrigation. The limiting growth factor was N. All other nutrient elements measured in the plants were close to optimal concentration. Plants irrigated at pH 2.5, and to some extent at pH 3, contained excessively high concentrations of Al, t-S and SO₄. The total amount of Ca, Fe and Mn taken up by the plants decreased with increasing soil acidity. The increased growth of clone H 254 relative to H 253, produces a corresponding impression on soil characteristics. Soil acidity is governed by acid irrigation and CaCO₃ application, but the clonal effects are also of importance. Norway spruce appears to be tolerant to Al concentrations as high as 50 mmol/kg in the needles.     

Convergent neuromodulation onto a network neuron can have divergent effects at the network level

Different neuromodulators often target the same ion channel. When such modulators act on different neuron types, this convergent action can enable a rhythmic network to produce distinct outputs. Less clear are the functional consequences when two neuromodulators influence the same ion channel in the same neuron. We examine the consequences of this seeming redundancy using a mathematical model of the crab gastric mill (chewing) network. This network is activated in vitro by the projection neuron MCN1, which elicits a half-center bursting oscillation between the reciprocally-inhibitory neurons LG and Int1. We focus on two neuropeptides which modulate this network, including a MCN1 neurotransmitter and the hormone crustacean cardioactive peptide (CCAP). Both activate the same voltage-gated current (I _MI ) in the LG neuron. However, I _MI-MCN1 , resulting from MCN1 released neuropeptide, has phasic dynamics in its maximal conductance due to LG presynaptic inhibition of MCN1, while I _MI-CCAP retains the same maximal conductance in both phases of the gastric mill rhythm. Separation of time scales allows us to produce a 2D model from which phase plane analysis shows that, as in the biological system, I _MI-MCN1 and I _MI-CCAP primarily influence the durations of opposing phases of this rhythm. Furthermore, I _MI-MCN1 influences the rhythmic output in a manner similar to the Int1-to-LG synapse, whereas I _MI-CCAP has an influence similar to the LG-to-Int1 synapse. These results show that distinct neuromodulators which target the same voltage-gated ion channel in the same network neuron can nevertheless produce distinct effects at the network level, providing divergent neuromodulator actions on network activity.