Review: Thermal preference in Drosophila

Robustness of a rhythmic circuit to short- and long-term temperature changes

Recent computational and experimental work has shown that similar network performance can result from variable sets of synaptic and intrinsic properties. Because temperature is a global perturbation that differentially influences every biological process within the nervous system, one might therefore expect that individual animals would respond differently to temperature. Nonetheless, the phase relationships of the pyloric rhythm of the stomatogastric ganglion (STG) of the crab, Cancer borealis, are remarkably invariant between 7 and 23°C (Tang et al., 2010). Here, we report that, when isolated STG preparations were exposed to more extreme temperature ranges, their networks became nonrhythmic, or "crashed", in a reversible fashion. Animals were acclimated for at least 3 weeks at 7, 11, or 19°C. When networks from the acclimated animals were perturbed by acute physiologically relevant temperature ramps (11-23°C), the network frequency and phase relationships were independent of the acclimation group. At high acute temperatures (>23°C), circuits from the cold-acclimated animals produced less-regular pyloric rhythms than those from warm-acclimated animals. At high acute temperatures, phase relationships between pyloric neurons were more variable from animal to animal than at moderate acute temperatures, suggesting that individual differences across animals in intrinsic circuit parameters are revealed at high temperatures. This shows that individual and variable neuronal circuits can behave similarly in normal conditions, but their behavior may diverge when confronted with extreme external perturbations.

Innate preference in Drosophila melanogaster

Innate preference behaviors are fundamental for animal survival. They actually form the basis for many animal complex behaviors. Recent years have seen significant progresses in disclosing the molecular and neural mechanism underlying animal innate preferences, especially in Drosophila. In this review, I will review these studies according to the sensory modalities adopted for preference assaying, such as vision, olfaction, thermal sensation. The behavioral strategies and the theoretic models for the formation of innate preferences are also reviewed and discussed.

Dopamine modulates metabolic rate and temperature sensitivity in Drosophila melanogaster

Homeothermal animals, such as mammals, maintain their body temperature by heat generation and heat dissipation, while poikilothermal animals, such as insects, accomplish it by relocating to an environment of their favored temperature. Catecholamines are known to regulate thermogenesis and metabolic rate in mammals, but their roles in other animals are poorly understood. The fruit fly, Drosophila melanogaster, has been used as a model system for the genetic studies of temperature preference behavior. Here, we demonstrate that metabolic rate and temperature sensitivity of some temperature sensitive behaviors are regulated by dopamine in Drosophila. Temperature-sensitive molecules like dTrpA1 and shi^ts induce temperature-dependent behavioral changes, and the temperature at which the changes are induced were lowered in the dopamine transporter-defective mutant, fumin. The mutant also displays a preference for lower temperatures. This thermophobic phenotype was rescued by the genetic recovery of the dopamine transporter in dopamine neurons. Flies fed with a dopamine biosynthesis inhibitor (3-iodo-L-tyrosine), which diminishes dopamine signaling, exhibited preference for a higher temperature. Furthermore, we found that the metabolic rate is up-regulated in the fumin mutant. Taken together, dopamine has functions in the temperature sensitivity of behavioral changes and metabolic rate regulation in Drosophila, as well as its previously reported functions in arousal/sleep regulation.

Biogeography of species richness gradients: linking adaptive traits, demography and diversification

Here we review how adaptive traits contribute to the emergence and maintenance of species richness gradients through their influence on demographic and diversification processes. We start by reviewing how demographic dynamics change along species richness gradients. Empirical studies show that geographical clines in population parameters and measures of demographic variability are frequent along latitudinal and altitudinal gradients. Demographic variability often increases at the extremes of regional species richness gradients and contributes to shape these gradients. Available studies suggest that adaptive traits significantly influence demographic dynamics, and set the limits of species distributions. Traits related to thermal tolerance, resource use, phenology and dispersal seem to play a significant role. For many traits affecting demography and/or diversification processes, complex mechanistic approaches linking genotype, phenotype and fitness are becoming progressively available. In several taxa, species can be distributed along adaptive trait continuums, i.e. a main axis accounting for the bulk of inter-specific variation in some correlated adaptive traits. It is shown that adaptive trait continuums can provide useful mechanistic frameworks to explain demographic dynamics and diversification in species richness gradients. Finally, we review the existence of sequences of adaptive traits in phylogenies, the interactions of adaptive traits and community context, the clinal variation of traits across geographical gradients, and the role of adaptive traits in determining the history of dispersal and diversification of clades. Overall, we show that the study of demographic and evolutionary mechanisms that shape species richness gradients clearly requires the explicit consideration of adaptive traits. To conclude, future research lines and trends in the field are briefly outlined.

Taxonomic chauvinism revisited: insight from parental care research

Parental care (any non-genetic contribution by a parent that appears likely to increase the fitness of its offspring) is a widespread trait exhibited by a broad range of animal taxa. In addition to influencing the fitness of parent(s) and offspring, parental care may be inextricably involved in other evolutionary processes, such as sexual selection and the evolution of endothermy. Yet, recent work has demonstrated that bias related to taxonomy is prevalent across many biological disciplines, and research in parental care may be similarly burdened. Thus, I used parental care articles published in six leading journals of fundamental behavioral sciences (Animal Behaviour, Behavioral Ecology, Behavioral Ecology and Sociobiology, Ethology, Hormones and Behavior, and Physiology & Behavior) from 2001–2010 (n = 712) to examine the year-to-year dynamics of two types of bias related to taxonomy across animals: (1) taxonomic bias, which exists when research output is not proportional to the frequency of organisms in nature, and (2) taxonomic citation bias, which is a proxy for the breadth of a given article—specifically, the proportion of articles cited that refer solely to the studied taxon. I demonstrate that research on birds likely represents a disproportionate amount of parental care research and, thus, exhibits taxonomic bias. Parental care research on birds and mammals also refers to a relatively narrow range of taxonomic groups when discussing its context and, thus, exhibits taxonomic citation bias. Further, the levels of taxonomic bias and taxonomic citation bias have not declined over the past decade despite cautionary messages about similar bias in related disciplines— in fact, taxonomic bias may have increased. As in Bonnet et al. (2002), my results should not be interpreted as evidence of an ‘ornithological Mafia’ conspiring to suppress other taxonomic groups. Rather, I generate several rational hypotheses to determine why bias persists and to guide future work.

Temperature and parasitism by Asobara tabida (Hymenoptera: Braconidae) influence larval pupation behaviour in two Drosophila species

In holometabolous insects, pupation site selection behaviour has large consequences for survival. Here, we investigated the combined effects of temperature and parasitism by the parasitoid Asobara tabida on larval pupation behaviour in two of its main Drosophila sp. hosts differing in their climate origin. We found that larvae of Drosophila melanogaster--a species with a (sub)tropical origin--placed at 25°C pupated higher in rearing jars than those placed at 15°C. The opposite pattern was observed for Drosophila subobscura larvae--a species from temperate regions--which pupated lower, i.e. on or near the substrate at 25°C, than those placed at 15°C. When placed at 25°C, parasitized larvae of both species pupated closer to the substrate than unparasitized ones. Moreover, the Drosophila larvae that had been exposed and probably stung by A. tabida, but were not parasitized, pupated lower than the control unparasitized larvae. These results provide new insights of host behaviour manipulation by A. tabida larvae.

Opposite effect of capsaicin and capsazepine on behavioral thermoregulation in insects

Transient receptor potential channels are implicated in thermosensation both in mammals and insects. The aim of our study was to assess the effect of mammalian vanilloid receptor subtype 1 (TRPV1) agonist (capsaicin) and antagonist (capsazepine) on insect behavioral thermoregulation. We tested behavioral thermoregulation of mealworms larvae intoxicated with capsaicin and capsazepine in two concentrations (10(-7) and 10(-4) M) in a thermal gradient system for 3 days. Our results revealed that in low concentration, capsaicin induces seeking lower temperatures than the ones selected by the insects that were not intoxicated. After application of capsazepine in the same concentration, the mealworms prefer higher temperatures than the control group. The observed opposite effect of TRPV1 agonist and antagonist on insect behavioral thermoregulation, which is similar to the effect of these substances on thermoregulation in mammals, indicates indirectly that capsaicin may act on receptors in insects that are functionally similar to TRPV1.

Anatomical characterization of thermosensory AC neurons in the adult Drosophila brain

Temperature preference is vital for the survival of all animals. A small set of warm-activated anterior cell (AC) neurons acting as an internal thermosensor in the Drosophila brain is critical for optimal temperature selection ( Hamada et al., 2008 , Nature, 454, 217-220). Here, the authors analyze the circuit components of the AC neurons by characterization of its spatial distribution, dendrite-axon polarity, and the putative type of neurontransmitter released. The results show that the AC neurons are serotonergic, do not have any dendrites, and send axons bilaterally to the superior dorsofrontal protocerebrum (SDFP). Searching the FlyCircuit database for neurons with serotonin receptor and dendrites in the SDFP, the authors found a dorsal-anterior-lateral (DAL) neuron as a candidate postsynaptic partner of the AC neurons. In conclusion, by morphological analysis of the AC neurons, the authors show a general strategy for predicting brain circuits orchestrating thermosensory behaviors.

Preference versus performance: body temperature of the intertidal snail Chlorostoma funebralis

Evolutionary theory predicts that, in variable environments, it is advantageous for ectothermic organisms to prefer a body temperature slightly below the physiological optimum. This theory works well for many terrestrial organisms but has not been tested for animals inhabiting the hypervariable physical environment of intertidal shores. In laboratory experiments, we allowed the intertidal snail Chlorostoma funebralis to position itself on a temperature gradient, then measured its thermal preference and determined an index of how its performance varied with temperature. Snails performed a biased random walk along the temperature gradient, which, contrary to expectations, caused them to aggregate where body temperature was 15 to 17 °C below their temperature of optimum performance and near the species' lower thermal limit. This "cold-biased" behavioral response may guide snails to refuges in shaded cracks and crevices, but potentially precludes C. funebralis from taking full advantage of its physiological capabilities.

Genetic constraints for thermal coadaptation in Drosophila subobscura

Background

Behaviour has been traditionally viewed as a driver of subsequent evolution because behavioural adjustments expose organisms to novel environments, which may result in a correlated evolution on other traits. In Drosophila subobscura, thermal preference and heat tolerance are linked to chromosomal inversion polymorphisms that show parallel latitudinal clines worldwide, such that "cold-climate" ("warm-climate") chromosome arrangements collectively favour a coherent response to colder (warmer) settings as flies carrying them prefer colder (warmer) conditions and have lower (higher) knock out temperatures. Yet, it is not clear whether a genetic correlation between thermal preference and heat tolerance can partially underlie such response.

Results

We have analyzed the genetic basis of thermal preference and heat tolerance using isochromosomal lines in D. subobscura. Chromosome arrangements on the O chromosome were known to have a biometrical effect on thermal preference in a laboratory temperature gradient, and also harbour several genes involved in the heat shock response; in particular, the genes Hsp68 and Hsp70. Our results corroborate that arrangements on chromosome O affect adult thermal preference in a laboratory temperature gradient, with cold-climate O_st carriers displaying a lower thermal preference than their warm-climate O₃₊₄ and O₃₊₄₊₈ counterparts. However, these chromosome arrangements did not have any effect on adult heat tolerance and, hence, we putatively discard a genetic covariance between both traits arising from linkage disequilibrium between genes affecting thermal preference and candidate genes for heat shock resistance. Nonetheless, a possible association of juvenile thermal preference and heat resistance warrants further analysis.

Conclusions

Thermal preference and heat tolerance in the isochromosomal lines of D. subobscura appear to be genetically independent, which might potentially prevent a coherent response of behaviour and physiology (i.e., coadaptation) to thermal selection. If this pattern is general to all chromosomes, then any correlation between thermal preference and heat resistance across latitudinal gradients would likely reflect a pattern of correlated selection rather than genetic correlation.

Running hot and cold: behavioral strategies, neural circuits, and the molecular machinery for thermotaxis in C. elegans and Drosophila

Like other ectotherms, the roundworm Caenorhabditis elegans and the fruit fly Drosophila melanogaster rely on behavioral strategies to stabilize their body temperature. Both animals use specialized sensory neurons to detect small changes in temperature, and the activity of these thermosensors governs the neural circuits that control migration and accumulation at preferred temperatures. Despite these similarities, the underlying molecular, neuronal, and computational mechanisms responsible for thermotaxis are distinct in these organisms. Here, we discuss the role of thermosensation in the development and survival of C. elegans and Drosophila, and review the behavioral strategies, neuronal circuits, and molecular networks responsible for thermotaxis behavior.

Promoter complexity and tissue-specific expression of stress response components in Mytilus galloprovincialis, a sessile marine invertebrate species

The mechanisms of stress tolerance in sessile animals, such as molluscs, can offer fundamental insights into the adaptation of organisms for a wide range of environmental challenges. One of the best studied processes at the molecular level relevant to stress tolerance is the heat shock response in the genus Mytilus. We focus on the upstream region of Mytilus galloprovincialis Hsp90 genes and their structural and functional associations, using comparative genomics and network inference. Sequence comparison of this region provides novel evidence that the transcription of Hsp90 is regulated via a dense region of transcription factor binding sites, also containing a region with similarity to the Gamera family of LINE-like repetitive sequences and a genus-specific element of unknown function. Furthermore, we infer a set of gene networks from tissue-specific expression data, and specifically extract an Hsp class-associated network, with 174 genes and 2,226 associations, exhibiting a complex pattern of expression across multiple tissue types. Our results (i) suggest that the heat shock response in the genus Mytilus is regulated by an unexpectedly complex upstream region, and (ii) provide new directions for the use of the heat shock process as a biosensor system for environmental monitoring.

Physiological climatic limits in Drosophila: patterns and implications

Physiological limits determine susceptibility to environmental changes, and can be assessed at the individual, population or species/lineage levels. Here I discuss these levels in Drosophila, and consider implications for determining species susceptibility to climate change. Limits at the individual level in Drosophila depend on experimental technique and on the context in which traits are evaluated. At the population level, evidence from selection experiments particularly involving Drosophila melanogaster indicate high levels of heritable variation and evolvability for coping with thermal stresses and aridity. An exception is resistance to high temperatures, which reaches a plateau in selection experiments and has a low heritability/evolvability when temperatures are ramped up to a stressful level. In tropical Drosophila species, populations are limited in their ability to evolve increased desiccation and cold resistance. Population limits can arise from trait and gene interactions but results from different laboratory studies are inconsistent and likely to underestimate the strength of interactions under field conditions. Species and lineage comparisons suggest phylogenetic conservatism for resistance to thermal extremes and other stresses. Plastic responses set individual limits but appear to evolve slowly in Drosophila. There is more species-level variation in lower thermal limits and desiccation resistance compared with upper limits, which might reflect different selection pressures and/or low evolvability. When extremes are considered, tropical Drosophila species do not appear more threatened than temperate species by higher temperatures associated with global warming, contrary to recent conjectures. However, species from the humid tropics may be threatened if they cannot adapt genetically to drier conditions.

Ambient Air Temperature Does Not Predict whether Small or Large Workers Forage in Bumble Bees (Bombus impatiens)

Bumble bees are important pollinators of crops and other plants. However, many aspects of their basic biology remain relatively unexplored. For example, one important and unusual natural history feature in bumble bees is the massive size variation seen between workers of the same nest. This size polymorphism may be an adaptation for division of labor, colony economics, or be nonadaptive. It was also suggested that perhaps this variation allows for niche specialization in workers foraging at different temperatures: larger bees might be better suited to forage at cooler temperatures and smaller bees might be better suited to forage at warmer temperatures. This we tested here using a large, enclosed growth chamber, where we were able to regulate the ambient temperature. We found no significant effect of ambient or nest temperature on the average size of bees flying to and foraging from a suspended feeder. Instead, bees of all sizes successfully flew and foraged between 16°C and 36°C. Thus, large bees foraged even at very hot temperatures, which we thought might cause overheating. Size variation therefore could not be explained in terms of niche specialization for foragers at different temperatures.

Clinal patterns of chromosomal inversion polymorphisms in Drosophila subobscura are partly associated with thermal preferences and heat stress resistance

Latitudinal clines in the frequency of various chromosomal inversions are well documented in Drosophila subobscura. Because these clines are roughly parallel on three continents, they have undoubtedly evolved by natural selection. Here, we address whether individuals carrying different chromosomal arrangements also vary in their thermal preferences (T(p)) and heat stress tolerance (T(ko)). Our results show that although T(p) and T(ko) were uncorrelated, flies carrying "cold-adapted" gene arrangements tended to choose lower temperatures in the laboratory or had a lower heat stress tolerance, in line with what could be expected from the natural patterns. Different chromosomes were mainly responsible for the underlying genetic variation in both traits, which explains why they are linearly independent. Assuming T(p) corresponds closely with temperatures that maximize fitness our results are consistent with previous laboratory natural selection experiments showing that thermal optimum diverged among thermal lines, and that chromosomes correlated with T(p) differences responded to selection as predicted here. Also consistent with data from the regular tracking of the inversion polymorphism since the colonization of the Americas by D. subobscura, we tentatively conclude that selection on tolerance to thermal extremes is more important in the evolution and dynamics of clinal patterns than the relatively "minor" adjustments from behavioral thermoregulation.