Adriana Villella

Control of Male Sexual Behavior and Sexual Orientation in Drosophila by the fruitless Gene

Sexual orientation and courtship behavior in Drosophila are regulated by fruitless (fru), the first gene in a branch of the sex-determination hierarchy functioning specifically in the central nervous system (CNS). The phenotypes of new fru mutants encompass nearly all aspects of male sexual behavior. Alternative splicing of fru transcripts produces sex-specific proteins belonging to the BTB-ZF family of transcriptional regulators. The sex-specific fru products are produced in only about 500 of the 10(5) neurons that comprise the CNS. The properties of neurons expressing these fru products suggest that fru specifies the fates or activities of neurons that carry out higher order control functions to elicit and coordinate the activities comprising male courtship behavior.

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 ∼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.

Isoform-Specific Control of Male Neuronal Differentiation and Behavior in Drosophila by the fruitless Gene

BACKGROUND How the central nervous system (CNS) develops to implement innate behaviors remains largely unknown. Drosophila male sexual behavior has long been used as a model to address this question. The male-specific products of fruitless (fru) are pivotal to the emergence of this behavior. These putative transcription factors, containing one of three alternative DNA binding domains, determine the neuronal substrates for sexual behavior in male CNS. RESULTS We isolated the first fru coding mutation, resulting in complete loss of one isoform. At the neuronal level, this isoform alone controls differentiation of a male-specific muscle and its associated motorneuron. Conversely, a combination of isoforms is required for development of serotonergic neurons implicated in male copulatory behavior. Full development of these neurons requires the male-specific product of doublesex, a gene previously thought to act independently of fru. At the behavioral level, missing one isoform leads to diminished courtship behavior and infertility. We achieved the first rescue of a distinct fru behavioral phenotype, expressing a wild-type isoform in a defined subset of its normal expression pattern. CONCLUSION This study exemplifies how complex behaviors can be controlled by a single locus through multiple isoforms regulating both developmental and physiological pathways in different neuronal substrates.

Reassessment of the effect of biological rhythm mutations on learning in Drosophila melanogaster

SummaryA link between learning deficits and circadian period-lengthening mutations in Drosophila melanogaster previously has been reported. Mutant long-period males performed poorly in two learning assays involving experience-dependent courtship inhibition. In one, normal males that have courted fertilized females subsequently show courtship inhibition with virgin females. In the other, normal males that have courted sexually immature males subsequently fail to court other immature males. Those results have been reassessed in an extended study of genetic variants involving the period gene.1.Long-period perL1 males demonstrated poor conditioned courtship inhibition when exposed to fertilized females; they showed normal courtship conditioning when exposed to immature males. This could be due to a perL1 -associated olfactory deficit with fertilized females, since perL1 males were unable to discriminate behaviorally between fertilized and virgin females.2.Other long-period males, including perL2 males and transgenic perL1 males bearing a truncated form of the per+ gene, were conditioned normally by fertilized females. Thus, the courtship inhibition defect is specific to the perL1 mutant strain.3.perL1 (and other per mutant) flies showed normal acquisition and retention of a classically conditioned olfactory avoidance response.4.Results from a new conditioned courtship inhibition experiment are presented; males exposed to fertilized females during training showed further courtship inhibition during subsequent exposure to fertilized females. From the perspective of learning theory, this can be viewed as a savings experiment.

Impaired clock output by altered connectivity in the circadian network

Substantial progress has been made in elucidating the molecular processes that impart a temporal control to physiology and behavior in most eukaryotes. In Drosophila, dorsal and ventral neuronal networks act in concert to convey rhythmicity. Recently, the hierarchical organization among the different circadian clusters has been addressed, but how molecular oscillations translate into rhythmic behavior remains unclear. The small ventral lateral neurons can synchronize certain dorsal oscillators likely through the release of pigment dispersing factor (PDF), a neuropeptide central to the control of rhythmic rest-activity cycles. In the present study, we have taken advantage of flies exhibiting a distinctive arrhythmic phenotype due to mutation of the potassium channel slowpoke (slo) to examine the relevance of specific neuronal populations involved in the circadian control of behavior. We show that altered neuronal function associated with the null mutation specifically impaired PDF accumulation in the dorsal protocerebrum and, in turn, desynchronized molecular oscillations in the dorsal clusters. However, molecular oscillations in the small ventral lateral neurons are properly running in the null mutant, indicating that slo is acting downstream of these core pacemaker cells, most likely in the output pathway. Surprisingly, disrupted PDF signaling by slo dysfunction directly affects the structure of the underlying circuit. Our observations demonstrate that subtle structural changes within the circadian network are responsible for behavioral arrhythmicity.

Neurotoxic protein expression reveals connections between the circadian clock and mating behavior in Drosophila

To investigate the functions of circadian neurons, we added two strategies to the standard Drosophila behavioral genetics repertoire. The first was to express a polyglutamine-expanded neurotoxic protein (MJDtr78Q; MJD, Machado–Joseph disease) in the major timeless (tim)-expressing cells of the adult brain. These Tim-MJD flies were viable, in contrast to the use of cell-death gene expression for tim neuron inactivation. Moreover, they were more arrhythmic than flies expressing other neurotoxins and had low but detectable tim mRNA levels. The second extended standard microarray technology from fly heads to dissected fly brains. By combining the two approaches, we identified a population of Tim-MJD-affected mRNAs. Some had been previously identified as sex-specific and relevant to courtship, including mRNAs localized to brain-proximal fat-body tissue and brain courtship centers. Finally, we found a decrease in the number of neurons that expressed male-specific forms of the fruitless protein in the laterodorsal region of the brain. The decrease was not a consequence of toxic protein expression within these specialized cells but a likely effect of communication with neighboring TIM-expressing neurons. The data suggest a functional interaction between adjacent circadian and mating circuits within the fly brain, as well as an interaction between circadian circuits and brain-proximal fat body.

fruitless Gene products truncated of their male-like qualities promote neural and behavioral maleness in Drosophila if these proteins are produced in the right places at the right times.

To bring GAL4 production under the control of the sex promoter (P1) contained within Drosophilas fruitless gene, a gal4 cassette was previously inserted downstream of P1. This insert should eliminate male-specific FRUM proteins, which normally contain 101 amino acids (aas) at their N termini. Thus males homozygous for the P1-gal4 insert should be courtless, as was briefly stated to be so in the initial report of this transgenic type. But XY flies whose only fru form is P1-gal4 have now been found to court vigorously. P1-gal4 females displayed no appreciable male-like actions except courtship rejection behaviors; yet, they developed a male-specific abdominal muscle. No immunoreactivity against the male-specific aas was detectable in P1-gal4 flies. But male-like neural signals were observed in XY or XX P1-gal4 pupae and adults after applying an antibody that detects all FRU isoforms; transgenic females displayed reduced expression of such proteins. RT-PCRs rationalized these findings: P1 transcripts include anomalous splice forms from which gal4 was removed, allowing FRUs lacking M aas to be produced in male-like patterns in both sexes. Within males, such defective proteins promote neural differentiation and function that is sufficient to support spirited P1-gal4 courtship. But dispensability of the male-specific FRU N-terminus is tempered by the finding that intra-fru sequences encoding these 101 aas are highly conserved among interspecific relatives of D. melanogaster.

Defective transfer of seminal-fluid materials during matings of semi-fertile fruitless mutants in Drosophila

In context of the semi-sterility exhibited by Drosophila males expressing certain mating-enabling fruitless (fru) mutant genotypes, we examined the transfer of seminal fluid using a transgene that encodes the Sex Peptide (SP) oligopeptide fused to Green Fluorescent Protein (GFP). We found that this fusion construct expresses SP-GFP in a valid manner within accessory glands of the male reproductive system in normal and fru-mutant males. Transfer of SP-GFP to live females was readily detectable during and after copulation. With respect to the pertinent combinations of fru mutations, we demonstrated that these abnormal genotypes cause males to transmit mating-related materials in two aberrant ways: one involving whether any seminal-fluid entities are transferred at all during a given mating; the other revealing an intriguing aspect of these fruitless effects, such that the mutations in question cause males to transfer female-affecting materials in a manner that varies among copulations. In this regard, certain mutant males that do not transfer SP nevertheless are able to transfer sperm: a fru-mated female possessing no GFP who was not fecund initially could produce progeny when seminal-fluid proteins were subsequently supplied by mating with a male that was spermless owing to the effects of a tudor mutation.

Wing-Beat Frequency Mutants and Courtship Behavior in Drosophila melanogaster Males

Flightless mutations that affect wing-beat frequency (WBF) of Drosophila melanogaster were examined for their effect on male courtship. WBFs were measured using a fixed-wire tether that completely supports the fly in an attitude similar to hovering flight. The two spontaneous mutations, one of which reduces WBF to one-half normal and the other to zero, were placed on an isogenic background and were compared to an isogenic wildtype strain and to a genetically heterogeneous wildtype strain. Time to mating under noncompetitive conditions (single pair matings) was not significantly different among the four male types in one experiment. In a second experiment, although the time to mating varied significantly among the four male types, there was no association between the WBF that was characteristic of a male type and the length of time to mating. Time to mating was not significantly correlated with WBF, wing size, or body mass in either experiment. Genetically heterogeneous wild-type females were significantly more receptive (had shorter times to mating) than inbred wild-type females toward all four male types. During the time-to-mating tests, all four male types appeared to show typical courtship behaviors. Therefore, the male types were compared for possible differences in four components of the male courtship song: sine song frequency, interpulse interval, intrapulse frequency (=carrier frequency), and wing cycles per pulse. One or another of these components showed significant differences among the four male types (e.g., genetically heterogeneous, wild-type males showed a significantly higher sine song frequency and intrapulse frequency than males of the three isogenic types). However, the average values for all four male types were within reported wild-type ranges for each courtship song component. Although the two mutations drastically reduce WBF during tethered flight, they do not have any similar major effect on courtship. Apparently they affect muscles or neuronal control mechanisms that are not common elements shared by the wing movement of flight and male courtship song.