Biometrical analysis of larval digging behavior in Drosophila melanogaster

Species-specific modulation of food-search behavior by respiration and chemosensation in Drosophila larvae

Animals explore their environment to encounter suitable food resources. Despite its vital importance, this behavior puts individuals at risk by consuming limited internal energy during locomotion. We have developed a novel assay to investigate how food-search behavior is organized in Drosophila melanogaster larvae dwelling in hydrogels mimicking their natural habitat. We define three main behavioral modes: resting at the gel's surface, digging while feeding near the surface, and apneic dives. In unstimulated conditions, larvae spend most of their time digging. By contrast, deep and long exploratory dives are promoted by olfactory stimulations. Hypoxia and chemical repellents impair diving. We report remarkable differences in the dig-and-dive behavior of D. melanogaster and the fruit-pest D. suzukii. The present paradigm offers an opportunity to study how sensory and physiological cues are integrated to balance the limitations of dwelling in imperfect environmental conditions and the risks associated with searching for potentially more favorable conditions.

A circuit mechanism for the propagation of waves of muscle contraction in Drosophila

Animals move by adaptively coordinating the sequential activation of muscles. The circuit mechanisms underlying coordinated locomotion are poorly understood. Here, we report on a novel circuit for the propagation of waves of muscle contraction, using the peristaltic locomotion of Drosophila larvae as a model system. We found an intersegmental chain of synaptically connected neurons, alternating excitatory and inhibitory, necessary for wave propagation and active in phase with the wave. The excitatory neurons (A27h) are premotor and necessary only for forward locomotion, and are modulated by stretch receptors and descending inputs. The inhibitory neurons (GDL) are necessary for both forward and backward locomotion, suggestive of different yet coupled central pattern generators, and its inhibition is necessary for wave propagation. The circuit structure and functional imaging indicated that the commands to contract one segment promote the relaxation of the next segment, revealing a mechanism for wave propagation in peristaltic locomotion.

DOI:http://dx.doi.org/10.7554/eLife.13253.001

Rhythmic movements such as walking and swimming require the coordinated contraction of many different muscles. Throughout the animal kingdom, from insects to mammals, animals possess specialized circuits of neurons that are responsible for producing these patterns of muscle contraction. These circuits are known as ‘central pattern generators’.

Central pattern generators are made up of multiple types of neurons that exchange information. However, it is unclear how neurons controlling the movement of one part of the body relay information to neurons controlling the movement of other parts. To answer this question, Fushiki et al. used larvae from the fruit fly Drosophila melanogaster as a model, and combined techniques such as electrophysiology and electron microscopy with measures of the insect’s behavior.

Fruit fly larvae have bodies that are made of segments, and they can contract and relax these segments in a sequence to propel themselves forwards or backwards. The contraction of one segment is accompanied by relaxation of the segment immediately in front. Fushiki et al. found that each body segment contains a copy of the same basic neuronal circuit. This circuit is made up of excitatory and inhibitory neurons. Both types of neurons regulate movement, but the inhibitory neurons must be suppressed for movement to occur.

The experiments also showed that each circuit receives both long-range input from the brain and local sensory feedback. This combination of inputs ensures that the segments contract and relax in the correct order. Future challenges are to determine how the brain controls larval movement via its long-range projections to the body. A key step will be to map these circuits at the level of the individual neurons and the connections between them.

DOI:http://dx.doi.org/10.7554/eLife.13253.002

The neuro-ecology of Drosophila pupation behavior

Many species of Drosophila form conspecific pupa aggregations across the breeding sites. These aggregations could result from species-specific larval odor recognition. To test this hypothesis we used larval odors of D. melanogaster and D. pavani, two species that coexist in the nature. When stimulated by those odors, wild type and vestigial (vg) third-instar larvae of D. melanogaster pupated on conspecific larval odors, but individuals deficient in the expression of the odor co-receptor Orco randomly pupated across the substrate, indicating that in this species, olfaction plays a role in pupation site selection. Larvae are unable to learn but can smell, the Syn^97CS and rut strains of D. melanogaster, did not respond to conspecific odors or D. pavani larval cues, and they randomly pupated across the substrate, suggesting that larval odor-based learning could influence the pupation site selection. Thus, Orco, Syn^97CS and rut loci participated in the pupation site selection. When stimulated by conspecific and D. melanogaster larval cues, D. pavani larvae also pupated on conspecific odors. The larvae of D. gaucha, a sibling species of D. pavani, did not respond to D. melanogaster larval cues, pupating randomly across the substrate. In nature, D. gaucha is isolated from D. melanogaster. Interspecific hybrids, which result from crossing pavani female with gaucha males clumped their pupae similarly to D. pavani, but the behavior of gaucha female x pavani male hybrids was similar to D. gaucha parent. The two sibling species show substantial evolutionary divergence in organization and functioning of larval nervous system. D. melanogaster and D. pavani larvae extracted information about odor identities and the spatial location of congener and alien larvae to select pupation sites. We hypothesize that larval recognition contributes to the cohabitation of species with similar ecologies, thus aiding the organization and persistence of Drosophila species guilds in the wild.

A method for mapping intralocus interactions influencing excessive alcohol drinking

Excessive alcohol (ethanol) consumption is the hallmark of alcohol use disorders. The F1 hybrid cross between the C57BL/6J (B6) and FVB/NJ (FVB) inbred mouse strains consumes more ethanol than either progenitor strain. The purpose of this study was to utilize ethanol-drinking data and genetic information to map genes that result in overdominant (or heterotic) ethanol drinking. About 600 B6 x FVB F2 mice, half of each sex, were tested for ethanol intake and preference in a 24-h, two-bottle water versus ethanol choice procedure, with ascending ethanol concentrations. They were then tested for ethanol intake in a Drinking in the Dark (DID) procedure, first when there was no water choice and then when ethanol was offered versus water. DNA samples were obtained and genome-wide QTL analyses were performed to search for single QTLs (both additive and dominance effects) and interactions between pairs of QTLs, or epistasis. On average, F2 mice consumed excessive amounts of ethanol in the 24-h choice procedure, consistent with high levels of consumption seen in the F1 cross. Consumption in the DID procedure was similar or higher than amounts reported previously for the B6 progenitor. QTLs resulting in heightened consumption in heterozygous compared to homozygous animals were found on Chrs 11, 15, and 16 for 24-h choice 30% ethanol consumption, and on Chr 11 for DID. No evidence was found for epistasis between any pair of significant or suggestive QTLs. This indicates that the hybrid overdominance is due to intralocus interactions at the level of individual QTL.

Correlated responses to selection for faster development and early reproduction in Drosophila: the evolution of larval traits

Studies on selection for faster development in Drosophila have typically focused on the trade-offs among development time, adult weight, and adult life span. Relatively less attention has been paid to the evolution of preadult life stages and behaviors in response to such selection. We have earlier reported that four laboratory populations of D. melanogaster selected for faster development and early reproduction, relative to control populations, showed considerably reduced preadult development time and survivorship, dry weight at eclosion, and larval growth rates. Here we study the larval phase of these populations in greater detail. We show here that the reduction in development time after about 50 generations of selection is due to reduced duration of the first and third larval instars and the pupal stage, whereas the duration of the second larval instar has not changed. About 90% of the preadult mortality in the selected populations is due to larval mortality. The third instar larvae, pupae, and freshly eclosed adults of the selected populations weigh significantly less than controls, and this difference appears during the third larval instar. Thereafter, percentage weight loss during the pupal stage does not differ between selected and control populations. The minimum amount of time a larva must feed to subsequently complete development is lower in the selected populations, which also exhibit a syndrome of reduced energy expenditure through reduction in larval feeding rate, larval digging and foraging activity, and pupation height. Comparison of these results with those observed earlier in populations selected for adaptation to larval crowding and faster development under a different protocol from ours reveal differences in the evolved traits that suggest that the responses to selection for faster development are greatly affected by the larval density at which selection acts and on details of the selection pressures acting on the timing of reproduction.

The behavior of sympatric Chilean populations of Drosophila larvae during pupation

The behavioral mechanisms by which the larvae of sympatric Chilean populations of Drosophila melanogaster, D. simulans, D. hydei and D. busckii select pupation sites are described in terms of larval substrate preferences. These species share the same breeding sites in Central Valley of Chile. It is important to investigate larval substrate preferences to pupate in sympatric natural populations of species of Drosophila because such behavior could contribute to the coexistence of the species in the wild. D. busckii larvae preferred humid substrates with a smooth surface to pupate, whereas D. simulans larvae selected humid substrates with a rough surface. Larvae of D. melanogaster chose dry and humid substrates with a rough surface, whereas D. hydei larvae occupied dry substrates with a smooth surface to form puparia. D. melanogaster larvae dug deeper into dry than into humid sand, whereas D. simulans larvae dug more into humid sand. D. busckii larvae pupated in the upper layers of humid and dry sand, and D. hydei larvae dug more into humid than into dry sand. Pupae of the four Drosophila species showed aggregated distributions on the substrates. Larval prepupation behaviors of D. melanogaster, D. simulans, D. hydei and D. busckii could be important to their coexistence in the wild.

Genetics of larval urea tolerance in Drosophila melanogaster

The genetic control of larval tolerance to urea, a nitrogenous waste-product occurring naturally in crowded Drosophila cultures, was investigated in a set of five laboratory populations of D. melanogaster that had been successfully subjected to selection for increased larval urea tolerance. Larva to adult survivorship and development time at three different levels of urea were assayed on the five selected populations, their five matched controls and a set of 10 F1 hybrid populations derived from reciprocal crosses between pairs of selected and control populations. As expected from the results of previous studies, the selected populations exhibited greater larval tolerance to the toxic effects of urea, relative to their controls. Comparison of the hybrid and parental populations with respect to both survivorship and development time indicated that the genetic control of urea tolerance in the selected populations is largely dominant, and has a significant X-linked component. The data also suggested that females from the selected populations exercise a nongenetic maternal effect on the development time of their progeny, regardless of urea level.