Seasonal variations in the condition of two Daphnia species and their hybrid in a eutrophic lake: evidence for food limitation

As seston and chlorophyll concentrations in eutrophic lakesare usually high all year round, it was hypothesized in thepast that food limitation is of major importance for the seasonaldynamics of herbivorous zooplankton in such lakes. Since directmeasurements of food are hampered by the lack of knowledge onthe exact nature of the food in eutrophic conditions, indirectmeasurements are necessary to estimate the degree of food limitationin these circumstances. Hence, we used laboratory- and field-derivedrelationships between the body length and body carbon contentof different species of Daphnia, which were collected from thehighly eutrophic Tjeukemeer, the Netherlands. From a seasonalsurvey of the carbon content of the daphnids. we concluded thatin Tjeukemeer D.galeata, D.galeata x cucullata and D.cucullataare food limited during the largest part of the year. Sincethe condition of the hybrids was relatively high as comparedwith the parental species when the food concentration was high,D.galeata x cucutlaia is expected to be the more successfultaxon during periods of high food availability.     

Rare extra-pair fertilizations in the semi-colonially breeding linnet Carduelis cannabina

Microsatellite analysis was used to analyse the level of extra-pair paternity (EPP) in the semi-colonially breeding linnet Carduelis cannabina in Denmark. During the breeding seasons of 1998 and 1999 EPP accounted for only 3.8% of young (4/106, 95% CL=1–13%). These extra-pair young were found in 9.1% of the broods (2/22, 95% CL=1–29%). We hypothesize that this low level of EPP is due to an efficient use of both mate guarding and frequent copulations as paternity guards. However, experimental tests with temporary detainment of males would be needed to establish whether there is a causal relationship between the low EPP level and the double paternity guard.     

Neurotoxicity studies with the monoamine oxidase B substrate 1-methyl-3-phenyl-3-pyrroline

The neurotoxic properties of the parkinsonian inducing agent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) are dependent on its metabolic activation in a reaction catalyzed by centrally located monoamine oxidase B (MAO-B). This reaction ultimately leads to the permanently charged 1-methyl-4-phenylpyridinium species MPP(+), a 4-electron oxidation product of MPTP and a potent mitochondrial toxin. The corresponding 5-membered analogue, 1-methyl-3-phenyl-3-pyrroline, is also a selective MAO-B substrate. Unlike MPTP, the MAO-B-catalyzed oxidation of 1-methyl-3-phenyl-3-pyrroline is a 2-electron process that leads to the neutral 1-methyl-3-phenylpyrrole. MPP(+) is thought to exert its toxic effects only after accumulating in the mitochondria, a process driven by the transmembrane electrochemical gradient. Since this energy-dependent accumulation of MPP(+) relies upon its permanent charge, 1-methyl-3-phenyl-3-pyrrolines and their pyrrolyl oxidation products should not be neurotoxic. We have tested this hypothesis by examining the neurotoxic potential of 1-methyl-3-phenyl-3-pyrroline and 1-methyl-3-(4-chlorophenyl)-3-pyrroline in the C57BL/6 mouse model. These pyrrolines did not deplete striatal dopamine while analogous treatment with MPTP resulted in 65-73% depletion. Kinetic studies revealed that both 1-methyl-3-phenyl-3-pyrroline and its pyrrolyl oxidation product were present in the brain in relatively high concentrations. Unlike MPP(+), however, 1-methyl-3-phenylpyrrole was cleared from the brain quickly. These results suggest that the brain MAO-B-catalyzed oxidation of xenobiotic amines is not, in itself, sufficient to account for the neurodegenerative properties of a compound like MPTP. The rapid clearance of 1-methyl-3-phenylpyrroles from the brain may contribute to their lack of neurotoxicity.     

Genetic polymorphism in leaf-cutting ants is phenotypically plastic

Advanced societies owe their success to an efficient division of labour that, in some social insects, is based on specialized worker phenotypes. The system of caste determination in such species is therefore critical. Here, we examine in a leaf-cutting ant (Acromyrmex echinatior) how a recently discovered genetic influence on caste determination interacts with the social environment. By removing most of one phenotype (large workers; LW) from test colonies, we increased the stimulus for larvae to develop into this caste, while for control colonies we removed a representative sample of all workers so that the stimulus was unchanged. We established the relative tendencies of genotypes to develop into LW by genotyping workers before and after the manipulation. In the control colonies, genotypes were similarly represented in the large worker caste before and after worker removal. In the test colonies, however, this relationship was significantly weaker, demonstrating that the change in environmental stimuli had altered the caste propensity of at least some genotypes. The results indicate that the genetic influence on worker caste determination acts via genotypes differing in their response thresholds to environmental cues and can be conceptualized as a set of overlapping reaction norms. A plastic genetic influence on division of labour has thus evolved convergently in two distantly related polyandrous taxa, the leaf-cutting ants and the honeybees, suggesting that it may be a common, potentially adaptive, property of complex, genetically diverse societies.     

Acromyrmex Leaf-Cutting Ants Have Simple Gut Microbiota with Nitrogen-Fixing Potential

Ants and termites have independently evolved obligate fungus-farming mutualisms, but their gardening procedures are fundamentally different, as the termites predigest their plant substrate whereas the ants deposit it directly on the fungus garden. Fungus-growing termites retained diverse gut microbiota, but bacterial gut communities in fungus-growing leaf-cutting ants have not been investigated, so it is unknown whether and how they are specialized on an exclusively fungal diet. Here we characterized the gut bacterial community of Panamanian Acromyrmex species, which are dominated by only four bacterial taxa: Wolbachia, Rhizobiales, and two Entomoplasmatales taxa. We show that the Entomoplasmatales can be both intracellular and extracellular across different gut tissues, Wolbachia is mainly but not exclusively intracellular, and the Rhizobiales species is strictly extracellular and confined to the gut lumen, where it forms biofilms along the hindgut cuticle supported by an adhesive matrix of polysaccharides. Tetracycline diets eliminated the Entomoplasmatales symbionts but hardly affected Wolbachia and only moderately reduced the Rhizobiales, suggesting that the latter are protected by the biofilm matrix. We show that the Rhizobiales symbiont produces bacterial NifH proteins that have been associated with the fixation of nitrogen, suggesting that these compartmentalized hindgut symbionts alleviate nutritional constraints emanating from an exclusive fungus garden diet reared on a substrate of leaves.     

Assessing bee species richness in two Mediterranean communities: importance of habitat type and sampling techniques

The decline of bees has raised concerns regarding their conservation and the maintenance of ecosystem services they provide to bee-pollinated wild flowers and crops. Although the Mediterranean region is a hotspot for bee species richness, their status remains poorly studied. There is an urgent need for cost-effective, reliable, and unbiased sampling methods that give good bee species richness estimates. This study aims: (a) to assess bee species richness in two common Mediterranean habitat types: semi-natural scrub (phrygana) and managed olive groves; (b) to compare species richness in those systems to that of other biogeographic regions, and (c) to assess whether six different sampling methods (pan traps, variable and standardized transect walks, observation plots and trap nests), previously tested in other European biogeographic regions, are suitable in Mediterranean communities. Eight study sites, four per habitat type, were selected on the island of Lesvos, Greece. The species richness observed was high compared to other habitat types worldwide for which comparable data exist. Pan traps collected the highest proportion of the total bee species richness across all methods at the scale of a study site. Variable and standardized transect walks detected the highest total richness over all eight study sites. Trap nests and observation plots detected only a limited fraction of the bee species richness. To assess the total bee species richness in bee diversity hotspots, such as the studied habitats, we suggest a combination of transect walks conducted by trained bee collectors and pan trap sampling.     

Aphids indirectly increase virulence and transmission potential of a monarch butterfly parasite by reducing defensive chemistry of a shared food plant

Parasites and hosts live in communities consisting of many interacting species, but few studies have examined how communities affect parasite virulence and transmission. We studied a food web consisting of two species of milkweed, two milkweed herbivores (monarch butterfly and oleander aphid) and a monarch butterfly-specific parasite. We found that the presence of aphids increased the virulence and transmission potential of the monarch butterfly's parasite on one milkweed species. These increases were associated with aphid-induced decreases in the defensive chemicals of milkweed plants. Our experiment suggests that aphids can indirectly increase the virulence and transmission potential of monarch butterfly parasites, probably by altering the chemical composition of a shared food plant. These results indicate that species that are far removed from host-parasite interactions can alter such interactions through cascading indirect effects in the food web. As such, indirect effects within ecological communities may drive the dynamics and evolution of parasites.     

The use of field–based social information in eusocial foragers: local enhancement among nestmates and heterospecifics in stingless bees

Abstract. 1. Foragers of social insects can be guided to profitable food sources by social information transfer within the nest. This study showed that in addition to such an information-centre strategy, social information in the field also plays an important role in individual foraging decisions. The effect of the presence of a nestmate on individual decision-making on where to forage was investigated in six species of stingless bee that differ in their recruitment system. Some species preferred to feed close to a nestmate (local enhancement) whereas other species actively avoided landing close to a nestmate. The term local inhibition is introduced for this avoidance behaviour.
2. Local enhancement and local inhibition were species specific but were not related to the species' recruitment system.
3. Local enhancement and local inhibition were affected by the individual's experience with the food source. Newly recruited foragers of Trigona amalthea showed local enhancement whereas experienced foragers showed local inhibition.
4. These individual decision-making rules explained accurately the spatial distribution of recruited nestmates: foraging groups of T. amalthea , which shows local inhibition, were more dispersed than foraging groups of Oxytrigona mellicolor , which shows local enhancement.
5. The effect of heterospecifics on stingless bee flower choice was investigated for 18 species combinations. Landing decisions were influenced significantly by the aggressiveness and the body size of the resident bee. Larger and more aggressive heterospecifics were avoided, whereas in some cases less aggressive bees acted as an attraction cue.