Spatial aggregation and association in different resource-patch distributions: experimental analysis with Drosophila

1. Laboratory experiments using homogeneous resources were conducted to examine intra- and interspecific spatial egg distribution of D. simulans, D. auraria and D. immigrans in three different resource-patch distribution patterns: patchy, even and clustered. 2. Individuals of each species were introduced separately or simultaneously into the cage, into which artificial substrates were placed and allowed to oviposit for 24 h. Spatial analyses were performed with indices of intraspecific aggregation (J), interspecific association (C) and L-function based on Ripley's K-function. 3. Eggs were always spatially aggregated irrespective of species and the resource-patch distribution patterns. Spatial egg aggregation was influenced significantly by the resource-patch distribution pattern and tended to be weaker in the clustered resource-patch distribution than in the patchy or even resource-patch distribution. 4. Spatial extent of egg aggregation was always beyond the single resource patch scale, indicating aggregation of ovipositing females. 5. Interspecific association of egg distribution was absent or very weak. Thus, these results present experimental evidence of independent egg aggregation among drosophilids.     

Genetics of egg production in Drosophila sechellia

Drosophila sechellia, an island endemic that specializes on a single host plant, has a lower rate of egg production than its generalist sister species D. melanogaster, D. simulans, and D. mauritiana. Earlier work showed that part of this difference in egg production was due to a reduction in the number of ovarioles in D. sechellia relative to its sister species. Here, I extend this earlier work by genetically analyzing the difference in egg production between D. sechellia and D. simulans. In all, 10 genetic markers were used in several interspecific backcrosses to identify chromosome regions that affected the rate of egg production. While previously mapped factors affecting ovariole number appear to impact the rate of egg production, new, non-ovariole factors were also identified. Overall, the difference in egg production between D. sechellia and D. simulans appears to be a polygenic trait. The relationship between these factors and genes involved the adaptation of D. sechellia to its host plant is not yet clear. The data are consistent with the hypothesis that decline in egg production is, in part, a negative pleiotropic effect of genetic changes required for host specialization in D. sechellia, although finer-scale genetic analysis of both traits is needed to truly test this hypothesis.     

Olfactory response and resource utilization in Drosophila: interspecific comparisons

Olfactory response and resource utilization in Drosophila were compared among three domestic (D. melanogaster, D. simulans, D. immigrans) and one Australian endemic (D. lativittata) species. Olfactory response was measured in a choice type olfactometer (Fuyama, 1976). The following chemicals common in Drosophila resources were used as odourants: acetaldehyde, acetic acid, propionic acid, methanol, ethanol, n-propanol, isopropanol, n-butanol. Resource status of these chemicals was determined either from the literature or by adult longevity tests.
All species were attracted by acetaldehyde, while methanol, isopropanol and n-butanol were unattractive. Ethanol attracted all species except D. immigrans , while only D. lativittata and D. melanogaster were attracted to n-propanol, propionic acid and acetic acid
Methanol and isopropanol were not utilized as resources by any of the species, while D. melanogaster and D. lativittata showed greater utilization/tolerance of the other chemicals. Some correlation between resource utilization and olfactory response was found at the interspecific level, although not all chemicals utilized as resources are attractants. The adaptive significance of the interspecific variation in olfactory response is discussed, especially in relation to habitats selected. The results provide suggestions for habitat selection studies at the intraspecific level.     

Interspecific competition betweenDrosophila melanogaster andDrosophila simulans

Summary The number of eggs laid in aDrosophila culture and the survival of these eggs may depend on the number of larvae that are still inhabiting or that have already used the culture medium.A known average number ofD. simulans st larvae (designated original inhabitants) were introduced into culture vials by allowing adults to lay for 24 h (low density) or 48 h (high density). On each day for 14 days, adults of three competitor strains (D. melanogaster Or-R-C, D. melanogaster yw andD. simulans st) were added to different samples of these vials and allowed to lay for 24 h. The numbers of effective eggs (eggs expected to be laid less those withheld, cannibalised or buried) produced by competitor strains were estimated from adult emergences. Survival of original inhabitant larvae to the adult stage also was measured.At the lower density of original inhabitants, the mean proportions of effective eggs (number of effective eggs/number of eggs expected in an uninhabited culture) were not significantly different for the three competitor strains. The mean proportions were lower at the higher density (significantly so for the twoD. melanogaster competitors), and at this density, the mean proportion forD. melanogaster yw was significantly less than that forD. simulans st. These results are consistent with a simple egg destruction hypothesis, but suggest thatyw females were retaining more eggs at the higher density.Original inhabitants showed higher survival when at the higher density. Each of the three competitors caused a significantly different reduction in original inhabitant survival, which was directly related to competitor larval activity. Increasing larval activity probably reduced survival of original inhabitants by increasing pupal mortality due to drowning in the medium.Day of introduction of competitors influenced survival of original inhabitants and also the proportion of effective eggs from each competitor. The proportion of effective eggs decreased to the day 5 introduction. From day 5 to day 8, the proportion increased because the original inhabitants were pupating. After day 8, effective egg proportions again decreased, possibly due to inhibition of egg-laying or reduced survival of immature stages.     

The Rosy Region of Drosophila Melanogaster and Drosophila Simulans. I. Contrasting Levels of Naturally Occurring DNA Restriction Map Variation and Divergence

A 40-kb region around the rosy and snake loci was analyzed for restriction map variation among 60 lines of Drosophila melanogaster and 30 lines of Drosophila simulans collected together at a single locality in Raleigh, North Carolina. DNA sequence variation in D. simulans was estimated to be 6.3 times greater than in D. melanogaster (heterozygosities per nucleotide of 1.9% vs. 0.3%). This result stands in marked contrast to results of studies of phenotypic variation including proteins (allozymes), morphology and chromosome arrangements which are generally less variable and less geographically differentiated in D. simulans. Intraspecific polymorphism is not distributed uniformly over the 40-kb region. The level of heterozygosity per nucleotide varies more than 12-fold across the region in D. simulans, being highest over the hsc2 gene. Similar, though less extreme, variation in heterozygosity is also observed in D. melanogaster. Average interspecific divergence (corrected for intraspecific polymorphism) averaged 3.8%. The pattern of interspecific divergence over the 40-kb region shows some disparities with the spatial distribution of intraspecific variation, but is generally consistent with selective neutrality predictions: the most polymorphic regions within species are generally the most divergent between species. Sequence-length polymorphism is observed for D. melanogaster to be at levels comparable to other gene regions in this species. In contrast, no sequence length variation was observed among D. simulans chromosomes (limit of resolution approximately 100 bp). These data indicate that transposable elements play at best a minor role in the generation of naturally occurring genetic variation in D. simulans compared to D. melanogaster. We hypothesize that differences in species effective population size are the major determinant of the contrasting levels and patterns of DNA sequence and insertion/deletion variation that we report here and the patterns of allozyme and morphological variation and differentiation reported by other workers for these two species.     

Influence of Diaphania hyalinata (L.) (Lepidoptera: Crambidae) egg density on the parasitization capacity of Trichogramma exiguum Pinto & Platner and Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae)

The effects of the egg density of Diaphania hyalinata (L.) on several biological parameters of Trichogramma exiguum Pinto & Platner and T. pretiosum Riley were investigated. For that, 24h-old egg masses were isolated in glass tubes (15 replicates; 1 egg mass = 1 replicate), and offered to parasitization by a newly-emerged female of T. pretiosum or T. exiguum 24h at the proportion of one female to 5, 10, 15, 20, 25, 30, 35 and 40 eggs of D. hyalinata. The following parameters were evaluated: number of parasitized eggs, number of individuals per egg, viability and sex ratio. Trichogramma exiguum parasitized more eggs than T. pretiosum when more than 25 eggs were available per female. The percentage of emergence was satisfactory to T. pretiosum in densities up to 15 eggs/female and up to 20 eggs/female for T. exiguum. The number of individuals per egg was not statistically different in both species except in the density of 25 eggs/female. It can be concluded that T. exiguum performed better than T. pretiosum at larger clutch sizes, as T. exiguum parasitization capacity increased as a result of the size of the host clutch size.     

Experimental evidence for density-dependent reproductive output in a coral reef fish

We tested for density-dependent reproduction in a small coral reef fish using field manipulations of density and observational data. Males of the study species, the bridled goby (Coryphopterus glaucofraenum Gill), defend benthic nest sites, within which they spawn with females, and females can spawn repeatedly over an extended breeding season. In small areas, usually only a single male nested at any one time regardless of how many males were present, so the probability of nesting was inversely proportional to density. Nesting males were almost always the largest in the vicinity, suggesting that, for males whose home ranges overlap, social interactions dictate opportunities to nest. Both the per capita rate at which clutches were laid and the number of eggs produced per clutch declined with increasing density, so the per capita rate of egg production was also density dependent. All three measures of fecundity were better predicted by numerical density (numbers per unit area) than biomass (mass of fish per unit area), and were well described as an inverse function of the number of gobies in the vicinity. A simple hypothesis consistent with these results is that a constant number of females spawn, regardless of density. Alternately, the effect of crowding may depend primarily on the number of interacting individuals and affect all females relatively equally. This density dependence could thus contribute to population regulation at the spatial scale over which populations become reproductively closed.     

Floral resources impact longevity and oviposition rate of a parasitoid in the field

1. The use of floral resource subsidies to improve herbivore suppression by parasitoids requires certain trophic interactions and physiological changes to occur. While the longevity and fecundity of parasitoids are positively affected by nectar subsidies in laboratory studies, the impacts of floral subsidies on the fecundity and longevity of freely foraging parasitoids have not been studied. 2. We studied the longevity and per capita fecundity of naturally occurring Diadegma insulare foraging in cabbage plots with and without borders of flowering buckwheat, Fagopyrum esculentum, as well as relationships between longevity, fecundity, sugar feeding and parasitism rates on larvae of the diamondback moth, Plutella xylostella. 3. Relative longevity was estimated by counting broken setae on the fringe of the forewing. Floral borders increased the longevity of males and females in adjacent cabbage plots. 4. The egg maturation rate of D. insulare was estimated by comparing egg loads of females collected early in the day with egg loads of females held without hosts in field cages throughout the day. Females in buckwheat cages matured 2.7 eggs per hour while females in control cages resorbed 0.27 eggs over the same time period. 5. The fecundity of females collected in the afternoon was estimated by comparing their actual egg load to the estimated egg load in the absence of oviposition for females in a given plot. Females foraging in buckwheat plots had marginally fewer eggs remaining in their ovaries, and laid marginally more eggs than females in control plots. Females from both treatments carried 30-60 eggs by the afternoon and therefore were time-limited rather than egg-limited. 6. Plots where a greater proportion of females had fed on sugar had longer-lived females. This suggests that feeding enhanced longevity of D. insulare. However, plots with longer-lived and more fecund females did not exhibit higher parasitism rates, although the power of these tests were low.     

Developmental stage affects eggshell-breaking strength in two ground-nesting birds: the partridge (Alectoris rufa) and the quail (Coturnix japonica)

We examined the relationship between embryo development and egg hardness in two ground nesting bird species, the red-legged partridge (Alectoris rufa; n = 165 eggs) and the quail (Coturnix japonica; n = 148 eggs). For both species, we observed a strong effect of developmental stage on egg hardness. Eggs near hatching were significantly weaker than unincubated eggs (partridge: 18 and 23 N, respectively, and the quail 7 and 10 N, respectively). We additionally explored the effect of incubation on egg hardness in a control sample of non-fertilised quail eggs (i.e., without embryo development). The control eggs maintained in the incubator for the full incubation time (17 days) were significantly harder (7-9 N) than eggs containing fully developed chicks (5-7 N). Thus, the incubation conditions of high temperature and humidity alone seem not to have a significant effect on egg hardness, and support the important effect of calcium uptake.     

How precise is egg discrimination in weaverbirds?

The village weaverbird, Ploceus cucullatus, lays eggs of an extremely broad range of appearance between individuals. This variation is thought to have evolved as a counteradaptation to brood parasitism by the diederik cuckooChrysococcyx caprius . The primary objective of our study was to characterize the relationship between egg appearance and egg rejection in the African village weaverbird. We test predictions of three hypotheses in a study in The Gambia, West Africa: (1) interindividual egg variability permits individuals to discriminate between own and foreign eggs by rejecting eggs in proportion to the difference in appearance from their own; (2) village weavers remember the appearance of their own eggs and do not require a discordancy within their clutch, nor even the presence of one of their own eggs, in order to distinguish a foreign egg as such; and (3) colour and speckling contain the signature information by which village weavers can distinguish their eggs from foreign ones; whereas shape and mass, being less reliable, do not. We analysed rejection behaviour according to egg appearance differences by logistic regression. Results supported all three hypotheses. We estimated the predictive efficacy of our model, the amount of explained variation and the relative contribution of various egg appearance factors to discrimination by the host. These results are consistent with the hypothesis that interindividual egg variation in this species facilitates offspring recognition and is a counteradaptation to either interspecific or intraspecific brood parasitism. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Resource-Dependent Giving-Up Time of the Predatory Mite, Phytoseiulus persimilis

We examined the effect of prey (Tetranychus urticae) egg density on leaving rate of the predatory mite, Phytoseiulus persimilis, from leaf disks using predators with different feeding experiences and levels of external volatile cues related to their prey. Predators stayed longer on disks with prey eggs than on those without prey eggs. However, at each prey egg density predators stayed longer in the absence of prey-related volatiles from an external source. Starved predators stayed longer in a prey patch than those that had not experienced starvation. At each prey density, starved P. persimilis consumed a greater proportion of prey eggs than satiated predators. The total prey consumption of starved predators appears to be related to their longer residence time on source disks compared to satiated predators and also the per capita consumption rate was greater for starved predators compared to satiated predators.     

Comparative life histories of two diplostomid trematodes, Ornithodiplostomum ptychocheilus and Posthodiplostomum minimum

A lack of comparative studies limits our understanding of interspecific variation in parasite life histories, especially for species that incorporate asexual and resting stages into their life cycles. Ornithodiplostomum ptychocheilus and Posthodiplostomum minimum are sympatric diplostomid trematodes that share the same first (pond snail, Physa gyrina) and second (fathead minnow, Pimephales promelas) intermediate hosts within lakes in Alberta, Canada. Interspecific differences in the body sizes of adults, cercariae, and metacercariae were evaluated from parasites recovered from laboratory-raised chickens, snails, and minnows, respectively. Differences in adult worm fecundity and cercariae production were estimated from chickens and snails exposed to known numbers of parasite larvae. Posthodiplostomum minimum adults, metacercariae, and eggs were 97, 86, and 5% larger than those of 0. ptychocheilus, respectively. The average numbers of eggs produced per day by P. minimum was 3 times higher than for O. ptychocheilus, and P. minimum produced approximately twice as many cercariae per day. Although the larger of the 2 species had higher egg and cercariae production, conclusions regarding covariation between adult worm size and the reproductive rates of other life cycle stages require further studies on related species.     

Effect of Prey and Predator Density on Predation of Rice Leaffolder Eggs by the Cricket Metioche vittaticollis

Cage experiments were conducted to quantify the predation rate of the cricket Metioche vittaticollis (Sta ˚l) on the eggs of rice leaffolder Marasmia patnalis Bradley. Egg predation by adult females was measured in response to changes in egg density, predator density and leaf area per cage. The number of eggs consumed per predator increased with egg density, without reaching a plateau. The predation rate decreased with increasing leaf area. The functional response could be adequately described with a linear Type I model, with the effect of leaf area included. This type of response to leaffolder egg density means that predation was not limited by prey handling time or satiation, but by the search rate. The search rate is here interpreted as the leaf area effectively searched for leaffolder eggs by a single predator in one day. Estimated search rates averaged 0.13 m 2 day -1 for M. vittaticollis females. The search rate of the predators increased with prey density, but a model describing a density dependent search rate explained only 3% of the total variation in search rate. Increasing predator density per cage led to a decrease in the per capita egg predation rate when predator density was more than two per m 2 leaf area. Interference might thus reduce the potential to enhance leaffolder egg predation by conservation or augmentation of predatory cricket populations.     

Studies of sibling Drosophila species from Laguna Verde, Veracruz, Mexico: I. Species frequencies, viability, desiccation resistance, and vagility

The sibling species Drosophila melanogaster and D. simulans were collected at Laguna Verde, Veracruz, Mexico. D. melanogaster was found in significantly greater frequency than was D. simulans. Ten isofemale lines of each species were tested for egg to adult viability, desiccation resistance, and vagility. D. melanogaster surpassed D. simulans in all three characteristics. The findings are discussed with reference to the climatic conditions at Laguna Verde and the expected effect of such an environment on the relative frequencies of these species. The dichotomous results in regard to desiccation resistance and vagility that were observed between recently collected D. melanogaster and the Oregon-R laboratory stock of that species are also discussed.     

The effects of age and weather on egg laying inDanaus plexippus L. (Lepidoptera: Danaidae)

The effects of age and weather conditions on egg laying in D. plexippus were determined for caged females. Age (measured in physiological time), temperature and solar radiation influence egg laying in this species of butterfly. An algorithm taking these factors into account in presented and accounts for 88% of the daily variation in egg laying. Caged D. plexippus begin to lay eggs six—seven days after emergence, peak egg production (about 60 eggs/♀) occurs about 15 days later. Females continue to lay eggs throughout their adult life, which in a flight cage was about 40 days. This egg laying pattern is compared with other published fecundity schedules. The effect and importance of a female being prevented from laying her eggs, on her life-time egg production, is also discussed.     

Historical factors mediate the effects of interspecific competition

Biotic interactions, such as interspecific competition, are potentially important in determining whether introduced species succeed or fail to establish wild populations. Such effects may be difficult to detect, however, because the outcome of interspecific competition may depend on historical and largely unpredictable circumstances such as the timing of introductions and the number of individuals of each species introduced. I used a stochastic birth-death model to explore the effects of interspecific competition, the timing of introductions and the numbers of individuals of each species introduced, on invasion success in a two-species competitive system. I then compared the model predictions with actual data on establishment outcomes for passerine birds introduced to New Zealand, for which we have data on the timing of introductions, the size of release populations, and a measure of the strength of per capita competition (the degree of morphological similarity among species). The model and data agree well, suggesting that interspecific competition was an important determinant of invasion success in this assemblage, but that the outcome of competition depended critically on circumstances such as the timing of introductions and number of individuals released. Hence, while there is a deterministic component to invasion success in this assemblage (morphologically similar species are less likely to establish), historical circumstances played a critical role in mediating the outcomes.     

Loss of notum macrochaetae as an interspecific hybrid anomaly between Drosophila melanogaster and D. simulans.

With the aim of revealing genetic variation accumulated among closely related species during the course of evolution, this study focuses on loss of macrochaetae on the notum as one of the developmental anomalies seen in interspecific hybrids between Drosophila melanogaster and its closely related species. Interspecific hybrids between a line of D. melanogaster and D. simulans isofemale lines exhibited a wide range in the number of missing bristles. By contrast, D. mauritiana and D. sechellia lines showed almost no reduction in bristle number in hybrids with D. melanogaster. Genetic analysis showed that the D. simulans X chromosome confers a large effect on hybrid bristle loss, although X-autosome interaction may be involved. This suggests that at least one genetic factor contributing to hybrid anomalies arose recently on a D. simulans X chromosome. Moreover, the results indicate sex dependency: the male hybrids were more susceptible to bristle loss than the female hybrids were. Use of cell type markers suggests that the defect does not lie in cell fate decisions during bristle development, but in the maintenance of neural fate and/or differentiation of the descendants of sensory mother cells.     

Overcrowding,food and phosphorus limitation effects on ephipphia production and population dynamics in the invasive species <Emphasis Type="Italic">Daphnia lumholtzi</Emphasis>

Daphnia lumholtzi has been very successful in colonizing North America since its appearance in Texas in 1990. Although previous studies have sought to link its success as an invasive species with various aspects of its population biology, there is little experimental data linking the invasion success of D. lumholtzi with its autecology, specifically its reproduction strategy. In this study we sought to link food quality and quantity to diapause in D. lumholtzi through a variation in phosphorus (P) content of algae, food quantity, and light level. We also assessed the effect of Daphnia peak population densities on reproductive rates and production of resting eggs. We found that when food is abundant, per capita ephippia production may be limited by P, but under food limitation conditions, there is no significant effect of food quality on ephippia production. Our results suggest that a combination of food quality/quantity and population density may work together to induce the production of resting eggs in this invasive species. Handling editor: S. Declerk     

Repeatability of Foreign Egg Rejection: Testing the Assumptions of Co‐Evolutionary Theory

Most theoretical models of coevolution between brood parasites, whether interspecific or conspecific, and their hosts explicitly assume consistent individual behaviour in host egg‐rejection responses. Accordingly, hosts cast as acceptors always accept, whereas ejectors always reject parasitic eggs when exposed to stable ecological conditions. To date, only few studies have attempted to test this critical assumption of individual repeatability in egg‐rejection responses of hosts. Here, we studied the repeatability of egg rejection in blackbirds (Turdus merula) and song thrush (T. philomelos), species in which females are reported to reject simulated, non‐mimetic foreign eggs at intermediate frequencies at the population level. However, intermediate rates of acceptance and rejection can be consistent with either or both intra‐ and interindividual variability in rejection behaviours. Our experiments revealed generally high individual consistency in these hosts’ responses to experimentally introduced non‐mimetic and mimetic model foreign eggs. Individuals also responded faster on average to second than to first trials within the same breeding attempts, but the difference was statistically significant only in blackbirds. These results are consistent with the critical assumption of co‐evolutionary models, that statistically egg rejection is mostly individually repeatable, but also reveal that some individuals in both species change their responses even within the short time‐window of one breeding attempt. The data suggest that individuals reject foreign eggs faster when perceived parasitism risk is greater because of repeated introductions of experimental parasitic eggs. We provide methodological recommendations to facilitate experimental and meta‐analytical studies of individual egg rejection repeatability and discuss how to reduce technical constraints arising from disparate treatments and variable sample sizes for future studies.