The effect of insecticides on the distribution of foracing parasitoids,Diaeretiella rapae [Hym.: Braconidae] on plants

Foraging aphid parasitoids,Diaeretiella rapae M'Intosh, were exposed to sublethal doses of the insecticides pirimicarb, permethrin and malathion on brusslls sprouts plants. Observations on wasp distribution over time revealed that wasps spent less time on sprayed plants, relative to controls and, while on these plants, tended to concentrate activity on unsprayed surfaces. For permethrin and malathion, pesticide residues reversed the stereotypic upward foraging pattern of the wasp. Negative consequences of sublethal pesticide doses for parasitoid foraging efficiency are discussed.      

Ionic Requirements for Catecholamine Aggregating Actions on the Teleost Poecilia reticulata Melanophores

Norepinephrine (NE) and phenylephrine (Phe) were employed to study the ionic requirements for alpha adrenoceptor activation in the teleost Poecilia reticulata melanophores. As expected the beta adrenoceptor blocker, propranolol, increased the sensitivity of the preparation to NE (5.8 times), and was therefore employed in all the experimental procedures. Neither cocaine (a neuronal uptake blocker) nor dexamethasone (an extraneuronal uptake blocker) enhanced the sensitivity of the preparation to NE, suggesting that these inactivating mechanisms would not play a role in P. reticulata pigmentary system. However, in the absence of calcium, the dose-response curve (DRC) to NE was displaced to the left about 3.5 times, whereas the DRC to Phe was not affected. These results indicate that a neuronal uptake is active, but was not demonstrated by the classical pharmacological tools, probably due to an assymmetric display of the nervous endings. The DRC to NE was rightward displaced (14.1 times) in the presence of the calcium channel blocker Verapamil, whereas the DRC to Phe was not affected. These data suggest that P. reticulata melanophores possess a mixed population of alpha₁ and alpha₂ adrenoceptors, the activation of the latter eliciting an extracellular calcium influx. In sodium-free saline, the DRC to NE was rightward shifted (6.6 times) and the response to Phe was impaired in such a way that the maximal response was not achieved. The DRC to both NE and Phe were rightward displaced (7.9 and 2.7 times respectively) in the presence of the sodium channel blocker tetrodotoxin (TTX) 10^?7M. In potassium-free saline, the melanophore sensitivity to Phe was increased, whereas the responses to NE were not affected, suggesting a differential sensitivity of the two alpha adrenoceptor subtypes to the resulting membrane hyperpolarization. Based on the literature and on our data we propose that P. reticulata melanophores possess a mixed population of alpha₁ and alpha₂ receptors. The activation of both subtypes of alpha adrenoceptors elicits a Na⁺ ion influx through TTX-sensitive sodium channels. The stimulation of alpha₂ adrenoceptors also requires an extracellular calcium influx, through the opening of slow calcium channels.     

QUANTITATIVE GENETICS OF SPRINT RUNNING SPEED AND SWIMMING ENDURANCE IN LABORATORY HOUSE MICE (MUS DOMESTICUS)

We tested the hypothesis that locomotor speed and endurance show a negative genetic correlation using a genetically variable laboratory strain of house mice (Hsd:ICR: Mus domesticus). A negative genetic correlation would qualify as an evolutionary “constraint,” because both aspects of locomotor performance are generally expected to be under positive directional selection in wild populations. We also tested whether speed or endurance showed any genetic correlation with body mass. For all traits, residuals from multiple regression equations were computed to remove effects of possible confounding variables such as age at testing, measurement block, observer, and sex. Estimates of quantitative genetic parameters were then obtained using Shaw's (1987) restricted maximum-likelihood programs, modified to account for our breeding design, which incorporated cross-fostering. Both speed and endurance were measured on two consecutive trial days, and both were repeatable. We initially analyzed performances on each trial day and the maximal value. For endurance, the three estimates of narrow-sense heritabilities ranged from 0.17 to 0.33 (full ADCE model), and some were statistically significantly different from zero using likelihood ratio tests. The heritability estimate for sprint speed measured on trial day 1 was 0.17, but negative for all other measures. Moreover, the additive genetic covariance between speeds measured on the two days was near zero, indicating that the two measures are to some extent different traits. The additive genetic covariance between speed on trial day 1 and any of the four measures of endurance was negative, large, and always statistically significant. None of the measures of speed or endurance was significantly genetically correlated with body mass. Thus, we predict that artificial selection for increased locomotor speed in these mice would result in a decrease in endurance, but no change in body mass. Such experiments could lead to a better understanding of the physiological mechanisms leading to trade-offs in aspects of locomotor abilities.     

TWO HERBIVORES AND CONSTRAINTS ON SELECTION FOR RESISTANCE IN BRASSICA RAPA

Although most plants experience herbivory by several insect species, there has been little empirical work directed toward understanding plant responses to these simultaneous selection pressures. In an experiment in which herbivory by flea beetles (Phyllotreta cruciferae) and diamondback moths (Plutella xylostella) was manipulated in a factorial design, I found that selection for resistance to these herbivores is not independent in Brassica rapa. Specifically, the effect of flea beetle damage on B. rapa fitness depends on the amount of diamondback moth damage a plant experiences: damage by these herbivores has a nonadditive effect on plant fitness. When diamondbacks are abundant, plants that sustain high levels of damage by flea beetles are favored by natural selection, but when diamondbacks are rare, a low level of damage by flea beetles is favored. However, resistance to the later-feeding diamondback moth is not affected by the presence or absence of damage by early-feeding flea beetles. Thus, there are no plant-mediated ecological interactions between these herbivores that affect the outcome of selection for resistance. Because these herbivores do not independently affect plant fitness, neither is likely to develop a pairwise coevolutionary relationship with its host. Instead, coevolution is diffuse.     

Controlled experiments of habitat fragmentation: a simple computer simulation and a test using small mammals

Habitat fragmentation involves a reduction in the effective area available to a population and the imposition of hard patch edges. Studies seeking to measure effects of habitat fragmentation have compared populations in fragments of different size to estimate and area effect but few have examined the effect of converting open populations to closed ones (an effect of edges). To do so requires a shift in spatial scope-from comparison of individual fragments to that of fragmented versus unfragmented landscapes. Here we note that large-scale, controlled studies of habitat fragmentation have rarely been performed and are needed. In making our case we develop a simple computer simulation model based on how individual animals with home ranges are affected by the imposition of habitat edges, and use it to predict population-level responses to habitat fragmentation. We then compare predictions of the model with results from a field experiment on Peromyscus and Microtus. Our model treats the case where home ranges/territories fall entirely within or partially overlap with that of sample areas in continuous landscapes, but are restricted to areas within habitat fragments in impacted landscapes. Results of the simulations demonstrate that the imposition of hard edges can produce different population abundances for similar-sized areas in continuous and fragmented landscapes. This edge effect is disproportionately greater in small than large fragments and for species with larger than smaller home ranges. These predictions were generally supported by our field experiment. We argue that large-scale studies of habitat fragmentation are sorely needed, and that control-experiment contrasts of fragmented and unfragmented microlandscapes provide a logical starting point.     

Growth and vitality of epiphytic lichens

 We tested the hypothesis that changed microclimate at induced forest edges causes reduced growth of epiphytic lichens. Two foliose, green algal lichens were transplanted to the lower canopy of a mature Picea abies forest at six distances (2, 6.25, 12.5, 25, 50 and 100 m) from a clearcut. The biomass growth in Platismatia glauca (6.2% in 16 months) was 41% higher than in Lobaria pulmonaria (4.4%). We found no growth reduction near the forest edge. In contrast, the highest growth in both species occurred within 12 m from the edge. Further, fluorescence and chlorophyll measurements showed that lichen vitality was unaffected by distance from edge. The light intensity was 4.3 times higher at the edge than in the interior during the growing season, but there were only minor differences in air temperature and relative humidity. Monitoring of thallus water content revealed clear differences in both number and length of wetting and drying cycles. However, the total time with water content sufficient for photosynthetic activity was only slightly higher at the edge. The data thus indicate that our gradient in microclimate was too small to significantly affect lichen growth, and that lichens are largely metabolically inactive when large edge-interior contrasts in microclimate occur. Lichen response to forest edge microclimate results from intricate interactions among several biotic and abiotic factors. Linking data on lichen growth, microclimate and thallus water content with physiological measurements provides a framework for future studies of the mechanisms behind abiotic edge effects. Received: 15 April 1996 / Accepted: 21 June 1996     

Genetically-based plant resistance traits affect arthropods,fungi, and birds

We examine how the distribution of a leafgalling aphid (Pemphigus betae) affects other species associated with natural stands of hybrid cottonwoods (Populus angustifolia x P. fremontii). Aphid transfers on common-garden clones and RFLP analysis show that resistance to aphids in cottonwoods is affected by plant genotype. Because susceptible trees typically support thousands of galls, while adjacent resistant trees have few or none, plant resistance traits that affect the distribution of this abundant herbivore may directly and/or indirectly affect other species. We found that the arthropod community of aphid-susceptible trees had 31% greater species richness and 26% greater relative abundance than aphid-resistant trees. To examine direct and indirect effects of plant resistance traits on other organisms, we experimentally excluded aphids and found that abundances and/or foraging behavior of arthropods, fungi, and birds were altered. First, exclusion of gall aphids on susceptible trees resulted in a 24% decrease in species richness and a 28% decrease in relative abundance of the arthropod community. Second, exclusion of aphids also caused a 2- to 3-fold decrease in foraging and/or presence of three taxa of aphid enemies: birds, fungi, and insects. Lastly, aphidexclussion resulted in a 2-fold increase in inquilines (animals who live in abodes properly belonging to another). We also found that fungi and birds responded to variation in gall density at the branch level. We conclude plant resistance traits affect diverse species from three trophic levels supporting a bottom-up influence of plants on community structure.     

甘蓝类无蜡粉亮叶性状遗传规律及其利用的研究

我们于1987年从普通结球甘蓝“迎春”品种自交二代群体中,发现了无蜡粉亮叶甘蓝突变株, 经过多年对其遗传规律进行的研究,认为这一无蜡粉亮叶性状是由一对隐性纯合基因控制。利用这一性状可培育结球甘蓝及其它甘蓝类具有这同一性状的新类型、新品种,提高其品质,更可作一代杂种利用的标记性状,充分发挥一代杂种的优势。     

Interactions between fish,grazing invertebrates and algae in a New Zealand stream: a trophic cascade mediated by fish-induced changes to grazer behaviour?

Experiments in laboratory stream channels compared the behaviour of Deleatidium mayfly nymphs in the absence of fish with that in the presence of either native common river galaxias (Galaxias vulgaris Stokell) or introduced brown trout (Salmo trutta L.). Galaxias present similar predation risks to prey during day and night but are more active at night. Whereas, trout present a higher predation risk during the day. Deleatidium maintained a fixed nocturnal drift periodicity that is characteristic of streams containing visually feeding fish regardless of the nature of the predation regime presented in the laboratory. However, the number on the substratum surface, and therefore able to graze algae, was lower when fish were present than when they were absent. The number was lower during the day in the presence of trout, when they present the highest predation risk, and lower during the night compared to the day in trials with galaxias when galaxias activity disturbs Deleatidium from the substratum. Increases in the probability of Deleatidium leaving a patch, reductions in the proportion of mayflies on high quality patches and reductions in the distance travelled from refuge also reflected variations in the predation regime. Similar differences in positioning were observed under the same predation regimes in in situ channels in the Shag River and these were associated with differences in algal biomass. Algal ash-free dry mass (AFDM) and chlorophyll a (chl a) were higher on the tops of cobbles when fish were present. Fish also affected the biomass and the distribution of algae on cobbles as AFDM and chl a were higher on the sides of cobbles from channels with trout compared to those with galaxias. Changes in grazing behaviour, caused by predator avoidance, are likely to have been responsible for differences in algal biomass because no significant differences were detected between treatments in the biomass of Deleatidium or of total invertebrates.     

Diallel analysis for sex-linked and maternal effects

Genetic models including sex-linked and maternal effects as well as autosomal gene effects are described. Monte Carlo simulations were conducted to compare efficiencies of estimation by minimum norm quadratic unbiased estimation (MINQUE) and restricted maximum likelihood (REML) methods. MINQUE(1), which has 1 for all prior values, has a similar efficiency to MINQUE(), which requires prior estimates of parameter values. MINQUE(1) has the advantage over REML of unbiased estimation and convenient computation. An adjusted unbiased prediction (AUP) method is developed for predicting random genetic effects. AUP is desirable for its easy computation and unbiasedness of both mean and variance of predictors. The jackknife procedure is appropriate for estimating the sampling variances of estimated variances (or covariances) and of predicted genetic effects. A t-test based on jackknife variances is applicable for detecting significance of variation. Worked examples from mice and silkworm data are given in order to demonstrate variance and covariance estimation and genetic effect prediction.