Median bundle neurons coordinate behaviours during Drosophila male courtship

Control of male sexual behavior in Drosophila by the sex determination pathway

Understanding how genes influence behavior, including sexuality, is one of biology's greatest challenges. Much of the recent progress in understanding how single genes can influence behavior has come from the study of innate behaviors in the fruit fly Drosophila melanogaster. In particular, the elaborate courtship ritual performed by the male fly has provided remarkable insights into how the neural circuitry underlying sexual behavior--which is largely innate in flies--is built into the nervous system during development, and how this circuitry functions in the adult. In this review we will discuss how genes of the sex determination pathway in Drosophila orchestrate the developmental events necessary for sex-specific behaviors and physiology, and the broader lessons this can teach us about the mechanisms underlying the development of sex-specific neural circuitry.

Veela defines a molecular link between Cryptochrome and Timeless in the light-input pathway to Drosophila's circadian clock

Organisms use the daily cycles of light and darkness to synchronize their internal circadian clocks with the environment. Because they optimize physiological processes and behavior, properly synchronized circadian clocks are thought to be important for the overall fitness. In Drosophila melanogaster, the circadian clock is synchronized with the natural environment by light-dependent degradation of the clock protein Timeless, mediated by the blue-light photoreceptor Cryptochrome (Cry). Here we report identification of a genetic variant, Veela, which severely disrupts this process, because these genetically altered flies maintain behavioral and molecular rhythmicity under constant-light conditions that usually stop the clock. We show that the Veela strain carries a natural timeless allele (ls-tim), which encodes a less-light-sensitive form of Timeless in combination with a mutant variant of the F-box protein Jetlag. However, neither the ls-tim nor the jetlag genetic variant alone is sufficient to disrupt light input into the central pacemaker. We show a strong interaction between Veela and cryptochrome genetic variants, demonstrating that the Jetlag, Timeless, and Cry proteins function in the same pathway. Veela also reveals a function for the two natural variants of timeless, which differ in their sensitivity to light. In combination with the complex array of retinal and extraretinal photoreceptors known to signal light to the pacemaker, this previously undescribed molecular component of photic sensitivity mediated by the two Timeless proteins reveals that an unexpectedly rich complexity underlies modulation of this process.

Blueprints for behavior: genetic specification of neural circuitry for innate behaviors

Innate behaviors offer a unique opportunity to use genetic analysis to dissect and characterize the neural substrates of complex behavioral programs. Courtship in Drosophila involves a complex series of stereotyped behaviors that include numerous exchanges of multimodal sensory information over time. As we will discuss in this review, recent work has demonstrated that male-specific expression of Fruitless transcription factors (Fru(M) proteins) is necessary and sufficient to confer the potential for male courtship behaviors. Fru(M) factors program neurons of the male central and peripheral nervous systems whose function is dedicated to sexual behaviors. This circuitry seems to integrate sensory information to define behavioral states and regulate conserved neural elements for sex-specific behavioral output. The principles that govern the circuitry specified by Fru(M) expression might also operate in subcortical networks that govern innate behaviors in mammals.

A Drosophila Protein Specific to Pheromone-Sensing Gustatory Hairs Delays Males' Copulation Attempts

In insects, increasing evidence suggests that small secreted pheromone binding proteins (PBPs) and odorant binding proteins (OBPs) are important for normal olfactory detection of airborne pheromones and odorants far from their source. In contrast, it is unknown whether extracellular ligand binding proteins participate in perception of less volatile chemicals, including many pheromones, that are detected by direct contact with chemosensory organs. CheB42a, a small Drosophila melanogaster protein unrelated to known PBPs or OBPs, is expressed and likely secreted in only a small subset of gustatory sensilla on males' front legs, the site of gustatory perception of contact pheromones. Here we show that CheB42a is expressed specifically in the sheath cells surrounding the taste neurons expressing Gr68a, a putative gustatory pheromone receptor for female cuticular hydrocarbons that stimulate male courtship. Surprisingly, however, CheB42a mutant males attempt to copulate with females earlier and more frequently than control males. Furthermore, CheB42a mutant males also attempt to copulate more frequently with other males that secrete female-specific cuticular hydrocarbon pheromones, but not with females lacking cuticular hydrocarbons. Together, these data indicate that CheB42a is required for a normal gustatory response to female cuticular hydrocarbon pheromones that modulate male courtship.

GAL4 enhancer trap targeting of the Drosophila sex determination gene fruitless

The fru4 allele of the sex determination gene fruitless is induced by insertion of a P[lacZ,ry+] enhancer trap element. This insert also acts to disrupt expression of the fru P1 promoter derived male-specific proteins, consequently impairing male courtship behavior. fru4 maps less than 2 kb upstream of the fru P3 promoter, whose function is essential for viability. We replaced this insert with a GAL4 element, P[GAL4,w+], recovering two lines with insertions in opposite orientations at the locus, one of which demonstrated fru-specific mutant phenotypes. Reporter expression of these lines recapitulated that of P3- and P4-derived proteins which, when correlated with a developmental and tissue specific survey of fru promoters' activities, uncovered a previously unsuspected complexity of fru regulation. These novel fru alleles provide the tools for manipulation of fru-expressing cells, allowing the consequent effects to be related back to specific fru functions and the regulatory units controlling these activities.

Functional analysis of fruitless gene expression by transgenic manipulations of Drosophila courtship

A gal4-containing enhancer-trap called C309 was previously shown to cause subnormal courtship of Drosophila males toward females and courtship among males when driving a conditional disrupter of synaptic transmission (shi(TS)). We extended these manipulations to analyze all features of male-specific behavior, including courtship song, which was almost eliminated by driving shi(TS) at high temperature. In the context of singing defects and homosexual courtship affected by mutations in the fru gene, a tra-regulated component of the sex-determination hierarchy, we found a C309/tra(F) combination also to induce high levels of courtship between pairs of males and "chaining" behavior in groups; however, these doubly transgenic males sang normally. Because production of male-specific FRU(M) protein is regulated by TRA, we hypothesized that a fru-derived transgene encoding the male (M) form of an Inhibitory RNA (fru(MIR)) would mimic the effects of tra(F); but C309/fru(MIR) males exhibited no courtship chaining, although they courted other males in single-pair tests. Double-labeling of neurons in which GFP was driven by C309 revealed that 10 of the 20 CNS clusters containing FRU(M) in wild-type males included coexpressing neurons. Histological analysis of the developing CNS could not rationalize the absence of tra(F) or fru(MIR) effects on courtship song, because we found C309 to be coexpressed with FRU(M) within the same 10 neuronal clusters in pupae. Thus, we hypothesize that elimination of singing behavior by the C309/shi(TS) combination involves neurons acting downstream of FRU(M) cells.

Neural circuitry that governs Drosophila male courtship behavior

Male-specific fruitless (fru) products (Fru(M)) are both necessary and sufficient to "hardwire" the potential for male courtship behavior into the Drosophila nervous system. Fru(M) is expressed in approximately 2% of neurons in the male nervous system, but not in the female. We have targeted the insertion of GAL4 into the fru locus, allowing us to visualize and manipulate the Fru(M)-expressing neurons in the male as well as their counterparts in the female. We present evidence that these neurons are directly and specifically involved in male courtship behavior and that at least some of them are interconnected in a circuit. This circuit includes olfactory neurons required for the behavioral response to sex pheromones. Anatomical differences in this circuit that might account for the dramatic differences in male and female sexual behavior are not apparent.

Behavior pattern (innate action) of individuals in fish schools generating efficient collective evasion from predation

The schooling of fishes is one typical animal social behavior. One primary function of fish school is to protect members when attacked by predators. One main way that the school reduces the predator's chance of making a successful kill is to confuse the predator as it makes its strike. This may be accomplished by collective evasion behaviors organized through integration of motions of individual fish made based on their innate actions (behavior patterns). In order to investigate what kind of behavior pattern of individuals can generate the efficient collective evasion of a school, we present a model of evasion behavior pattern which consists of three component behavior patterns, schooling, cooperative escape, and selfish escape behavior patterns and the rule for choice of one among them with proper timing. Each fish determines its movement direction taking into account simultaneously three kinds of elemental motions, mimicking its neighbors, avoiding collisions with its nearest neighbors, and escaping from an approaching predator. The weights of three elemental motions are changed depending on which component behavior pattern the fish carries out. The values of the weights for three component behavior patterns can be definitively determined under the condition that the collective evasion of the school becomes the most efficient, that is, the probability that any member is eaten by the predator becomes minimum.

Male-specific fruitless specifies the neural substrates of Drosophila courtship behaviour

Robust innate behaviours are attractive systems for genetically dissecting how environmental cues are perceived and integrated to generate complex behaviours. During courtship, Drosophila males engage in a series of innate, stereotyped behaviours that are coordinated by specific sensory cues. However, little is known about the specific neural substrates mediating this complex behavioural programme. Genetic, developmental and behavioural studies have shown that the fruitless (fru) gene encodes a set of male-specific transcription factors (FruM) that act to establish the potential for courtship in Drosophila. FruM proteins are expressed in approximately 2% of central nervous system neurons, at least one subset of which coordinates the component behaviours of courtship. Here we have inserted the yeast GAL4 gene into the fru locus by homologous recombination and show that (1) FruM is expressed in subsets of all peripheral sensory systems previously implicated in courtship, (2) inhibition of FruM function in olfactory system components reduces olfactory-dependent changes in courtship behaviour, (3) transient inactivation of all FruM-expressing neurons abolishes courtship behaviour, with no other gross changes in general behaviour, and (4) 'masculinization' of FruM-expressing neurons in females is largely sufficient to confer male courtship behaviour. Together, these data demonstrate that FruM proteins specify the neural substrates of male courtship.

Drosophila retained/dead ringer is necessary for neuronal pathfinding, female receptivity and repression of fruitless independent male courtship behaviors

Mutations in the Drosophila retained/dead ringer (retn) gene lead to female behavioral defects and alter a limited set of neurons in the CNS. retn is implicated as a major repressor of male courtship behavior in the absence of the fruitless (fru) male protein. retn females show fru-independent male-like courtship of males and females, and are highly resistant to courtship by males. Males mutant for retn court with normal parameters, although feminization of retn cells in males induces bisexuality. Alternatively spliced RNAs appear in the larval and pupal CNS, but none shows sex specificity. Post-embryonically, retn RNAs are expressed in a limited set of neurons in the CNS and eyes. Neural defects of retn mutant cells include mushroom body beta-lobe fusion and pathfinding errors by photoreceptor and subesophageal neurons. We posit that some of these retn-expressing cells function to repress a male behavioral pathway activated by fruM.

Gender-selective patterns of aggressive behavior in Drosophila melanogaster

Complex behaviors, such as aggression, are comprised of distinct stereospecific behavioral patterns (modules). How such patterns get wired into nervous systems remains unknown. Recently, we reported on a quantitative analysis of fighting behavior in male flies of the common Canton-S strain of Drosophila melanogaster. Here, we report a similar analysis of fighting behavior in females of the same species. Fights were carried out between pairs of virgin and pairs of mated females in competition for a yeast resource. Each fight was videotaped and analyzed by using transition matrices and Markov chain analyses. We observe only small difference in fighting intensity between virgin and mated females. In contrast to what is seen in male fights, however, no clear hierarchical relationship is formed in the female fights. A further comparison of the behavioral patterns making up male and female fights reveals that some modules are shared by both sexes, whereas others are highly selective. Within the shared components, transitions between the modules also show gender-selective differences. By using the powerful genetic methods available for examining behavior in fruit flies, it should be possible to use the gender-selective differences in fighting behavior to address the question of how these behavioral patterns get established in the brains of fruit flies.