No abstract

We have developed a genetic approach to examine the role of spontaneous activity and synaptic release in the establishment and maintenance of an olfactory sensory map. Conditional expression of tetanus toxin light chain, a molecule that inhibits synaptic release, does not perturb targeting during development, but neurons that express this molecule in a competitive environment fail to maintain appropriate synaptic connections and disappear. Overexpression of the inward rectifying potassium channel, Kir2.1, diminishes the excitability of sensory neurons and more severely disrupts the formation of an olfactory map. These studies suggest that spontaneous neural activity is required for the establishment and maintenance of the precise connectivity inherent in an olfactory sensory map.

Individual olfactory sensory neurons express only a single odorant receptor from a large family of genes, and this singularity is an essential feature in models of olfactory perception. We have devised a genetic strategy to examine the stability of receptor choice. We observe that immature olfactory sensory neurons that express a given odorant receptor can switch receptor expression, albeit at low frequency. Neurons that express a mutant receptor gene switch receptor transcription with significantly greater probability, suggesting that the expression of a functional odorant receptor elicits a feedback signal that terminates switching. This process of receptor gene switching assures that a neuron will ultimately express a functional receptor and that the choice of this receptor will remain stable for the life of the cell.

Energy theory posits three processes that link local abundance of ectotherms to geographical gradients in temperature. A survey of 49 New World habitats found a two order of magnitude span in the abundance (nests m ¡2 ) of ground nesting ants (Formicidae). Abundance increased with net primary productivity (r 2ˆ0 .55), a measure of the baseline supply of harvestable energy. Abundance further increased with mean temperature (r 2ˆ0 .056), a constraint on foraging activity for this thermophilic taxon. Finally, for a given mean temperature, ants were more abundant in seasonal sites with longer, colder winters (r 2ˆ0 .082) that help ectotherm taxa sequester harvested energy in non-productive months. All three variables are currently changing on a global scale. All should be useful in predicting biotic responses to climate change.

Species richness describes the number of species of a given taxon in a given time and space. The energy limitation hypothesis links the species richness of consumer taxa to net primary productivity (NPP) through two relationships: NPP limits a taxon's density, and taxon density limits species richness. We study both relationships with a survey of 15 ground ant assemblages, along a productivity gradient from deserts to rain forests. Ant density (colonies m-2) was a positive, decelerating function of net aboveground productivity (NAP). A stepwise regression suggests that the efficiency with which NAP is converted to ant colonies increases with maximum summer temperature and decreases with precipitation. Ant species richness was a positive decelerating function of density at three spatial scales. This supports the energy limitation hypothesis' assumption that average population densities are higher in environments that are more productive. These two nonlinear functions (NAP-density and density-species richness) combine to create, at a variety of scales, positive, decelerating, productivity-diversity curves for a common, ecologically dominant taxon across the terrestrial productivity gradient. However, variance in the density and diversity explained by NAP decreases with scale, suggesting that energy limitation of diversity predominates at small spatial scales (<1 ha).

No abstract

Wasps (Vespidae) exhibit a range of social complexity, from solitary living to eusocial colonies, and thus are exemplary for studies of the evolutionary origin and maintenance of social behavior in animals. Integral to the definition of eusociality is the presence of reproductive castes, group members that differ qualitatively in their ability to reproduce in a social setting. Behavioral and morphological evidence suggests that caste determination, the developmental process by which differences in fecundity are established, occurs to a large extent before adult emergence (pre-imaginally) in many species of Vespidae, in both basal and advanced taxa within the clade (Vespinae+Polistinae), which includes most eusocial species. Pre-imaginal determination has been documented in many taxa (e.g. independent-founding Polistinae) where it was not thought to occur. Correlative and experimental studies indicate that differences in nutrition during larval development are often the basis of pre-imaginal caste determination. Pre-imaginal caste determination has important implications for the roles of subfertility and manipulation by nest mates in the evolution of eusocial behavior.

Summary1. Models that predict organismal and population responses to climate change may be improved by considering ecological factors that affect species thermal tolerance. Species differences in microhabitat use can expose animals to diverse thermal selective environments at a given site and may cause sympatric species to evolve different thermal tolerances. 2. We tested the hypothesis that species differences in body size and microhabitat use (abovevs. below-ground activity) would correspond to differences in thermal tolerance (maximum critical temperatures: CT max ). Thermal buffering effects of soil can reduce exposure to extreme high temperatures for below-ground active species. We predicted larger-bodied individuals and species would have higher CT max and that species mean CT max would covary positively with degree of above-ground activity. We used Neotropical army ants (Formicidae: Ecitoninae) as models. Army ants vary in microhabitat use from largely subterranean to largely above-ground active species and are highly size polymorphic. 3. We collected data on above-and below-ground temperatures in habitats used by army ants to test for microhabitat temperature differences, and we conducted CT max assays for army ant species with varying degrees of surface activity and with different body sizes within and between species. We then tested whether microhabitat use was associated with species differences in CT max and whether microhabitat was a better predictor of CT max than body size for species that overlapped in size. 4. Microhabitat use was a highly significant predictor of species' upper thermal tolerance limits, both for raw data and after accounting for the effects of phylogeny. Below-ground species were more thermally sensitive, with lower maximum critical temperatures (CT max ). The smallest workers within each species were the least heat tolerant, but the magnitude of CT max change with body size was greater in below-ground species. Species-typical microhabitat was a stronger predictor of CT max than body size for species that overlapped in size. Compared to the soil surface, 10-cm subsoil was a significantly moderated thermal environment for below-ground army ants, while maximum surface raid temperatures sometimes exceeded CT max for the most thermally sensitive army ant castes. 5. We conclude sympatric species differences in thermal physiology correspond to microhabitat use. These patterns should be accounted for in models of species and community responses to thermal variation and climate change.