Male-Specific Expression of the Fruitless Protein is not Common to all Drosophila Species

Sex-specific behavioral patterns must be a result of sexual differences in the structure and/or function of the central nervous system (CNS). Male Drosophila melanogaster mutants for the fruitless (fru) locus exhibit enhanced male-to-male courtship. The fru mutant males are accompanied by malformation of the male-specific muscle of Lawrence (MOL), which, in wild-type males, is induced by male motoneurons innervating it. These two phenotypes are the consequences of impaired sex determination of CNS neurons. In D. melanogaster, although the fru mRNAs are transcribed in the CNS of both the male and female, the Fru protein is only translated in the male CNS. This male-specific translation of Fru was also observed in D. simulans, D. yakuba, D. pseudoobscura and D. virilis; however, in D. suzukii, the Fru protein expression was detected even in the female CNS.     

Elements of the fruitless locus regulate development of the muscle of Lawrence, a male-specific structure in the abdomen of Drosophila melanogaster adults

A genetically defined element of the fruitless (fru) locus in Drosophila melanogaster regulates the development of a male-specific muscle spanning the fifth abdominal segment in adult males, the 'muscle of Lawrence' (MOL). The region is defined by two cytological deletions, each with a breakpoint that co-maps with previously described mutant courtship phenotypes at cytogenetic interval 91B on the third chromosome. Flies that carry both of these deletions are viable, and males express abnormalities of courtship similar to those caused by the fru inversion breakpoint at 91B. In addition, these double-deletion males show the complete absence of the MOL, suggesting that they have little or no gene expression of a postulated MOL determinant; the musculature in the fifth abdominal segment of these mutants to indistinguishable from that of a normal female. Other mutant combinations that produce fruitless courtship phenotypes--including deletion and inversion breakpoints, and a marked transposon inserted at 91B--produce intermediate forms of the MOL. A new genetic variant, induced by imprecise excision of the marked transposon, is homozygous lethal and disrupts fru functions related to courtship and the MOL. The MOL is shown to be dispensable for fertility and is therefore not the causative factor of fru-induced behavioral sterility. These genetic variants and their phenotypic results are discussed with regard to a model for the organization of the fru locus.     

New reproductive anomalies in fruitless-mutant Drosophila males: extreme lengthening of mating durations and infertility correlated with defective serotonergic innervation of reproductive organs

Several features of male reproductive behavior are under the neural control of fruitless (fru) in Drosophila melanogaster. This gene is known to influence courtship steps prior to mating, due to the absence of attempted copulation in the behavioral repertoire of most types of fru-mutant males. However, certain combinations of fru mutations allow for fertility. By analyzing such matings and their consequences, we uncovered two striking defects: mating times up to four times the normal average duration of copulation; and frequent infertility, regardless of the time of mating by a given transheterozygous fru-mutant male. The lengthened copulation times may be connected with fru-induced defects in the formation of a male-specific abdominal muscle. Production of sperm and certain seminal fluid proteins are normal in these fru mutants. However, analysis of postmating qualities of females that copulated with transheterozygous mutants strongly implied defects in the ability of these males to transfer sperm and seminal fluids. Such abnormalities may be associated with certain serotonergic neurons in the abdominal ganglion in which production of 5HT is regulated by fru. These cells send processes to contractile muscles of the male's internal sex organs; such projection patterns are aberrant in the semifertile fru mutants. Therefore, the reproductive functions regulated by fruitless are expanded in their scope, encompassing not only the earliest stages of courtship behavior along with almost all subsequent steps in the behavioral sequence, but also more than one component of the culminating events.     

The Orthologue of the Fruitfly Sex Behaviour Gene Fruitless in the Mosquito Aedes aegypti: Evolution of Genomic Organisation and Alternative Splicing

In Drosophila melanogaster the doublesex (dsx) and fruitless (fru) regulatory genes act at the bottom of the somatic sex determination pathway. Both are regulated via alternative splicing by an upstream female-specific TRA/TRA-2 complex, recognizing a common cis element. dsx controls somatic sexual differentiation of non-neural as well as of neural tissues. fru, on the other hand, expresses male-specific functions only in neural system where it is required to built the neural circuits underlying proper courtship behaviour. In the mosquito Aedes aegypti sex determination is different from Drosophila. The key male determiner M, which is located on one of a pair of homomorphic sex chromosomes, controls sex-specific splicing of the mosquito dsx orthologue. In this study we report the genomic organization and expression of the fru homologue in Ae. aegypti (Aeafru). We found that it is sex-specifically spliced suggesting that it is also under the control of the sex determination pathway. Comparative analyses between the Aeafru and Anopheles gambiae fru (Angfru) genomic loci revealed partial conservation of exon organization and extensive divergence of intron lengths. We find that Aeadsx and Aeafru share novel cis splicing regulatory elements conserved in the alternatively spliced regions. We propose that in Aedes aegypti sex-specific splicing of dsx and fru is most likely under the control of splicing regulatory factors which are different from TRA and TRA-2 found in other dipteran insects and discuss the potential use of fru and dsx for developing new genetic strategies in vector control.     

fruitless splicing specifies male courtship behavior in Drosophila

All animals exhibit innate behaviors that are specified during their development. Drosophila melanogaster males (but not females) perform an elaborate and innate courtship ritual directed toward females (but not males). Male courtship requires products of the fruitless (fru) gene, which is spliced differently in males and females. We have generated alleles of fru that are constitutively spliced in either the male or the female mode. We show that male splicing is essential for male courtship behavior and sexual orientation. More importantly, male splicing is also sufficient to generate male behavior in otherwise normal females. These females direct their courtship toward other females (or males engineered to produce female pheromones). The splicing of a single neuronal gene thus specifies essentially all aspects of a complex innate behavior.     

Fruitless and doublesex coordinate to generate male-specific neurons that can initiate courtship

Biologists postulate that sexual dimorphism in the brain underlies gender differences in behavior, yet direct evidence for this has been sparse. We identified a male-specific, fruitless (fru)/doublesex (dsx)-coexpressing neuronal cluster, P1, in Drosophila. The artificial induction of a P1 clone in females effectively provokes male-typical behavior in such females even when the other parts of the brain are not masculinized. P1, located in the dorsal posterior brain near the mushroom body, is composed of 20 interneurons, each of which has a primary transversal neurite with extensive ramifications in the bilateral protocerebrum. P1 is fated to die in females through the action of a feminizing protein, DsxF. A masculinizing protein Fru is required in the male brain for correct positioning of the terminals of P1 neurites. Thus, the coordinated actions of two sex determination genes, dsx and fru, confer the unique ability to initiate male-typical sexual behavior on P1 neurons.     

Extended Reproductive Roles of the Fruitless Gene in Drosophila Melanogaster Revealed by Behavioral Analysis of New Fru Mutants

The fruitless mutants fru(3) and fru(4) were assessed for sex-specific reproductive-behavioral phenotypes and compared to the previously reported fru mutants. Among the several behavioral anomalies exhibited by males expressing these relatively new mutations, some are unique. fru(3) and fru(4) males are less stimulated to court females than fru(1) and fru(2). No courtship pulse song is generated by either fru(3) or fru(4) males, even though they perform brief wing extensions. fru(3) and fru(4) males display significantly less chaining behavior than do fru(1) males. The hierarchy of courtship responses by fru males directed toward females vs. males, when presented with both sexes simultaneously, is that fru(1) males perform vigorous and indiscriminant courtship directed at either sex; fru(4) males are similarly indiscriminant, but courtship levels were lower than fru(1); fru(2) males prefer females; fru(3) males show a courtship bias toward males. fru(3) and fru(4) males essentially lack the Muscle of Lawrence (MOL). On several reproductive criteria, there was no difference between fru-variant females and fru(+). The increases in phenotypic severity measured for the new mutants are discussed in the context of the emerging molecular genetics of fru and with regard to the gene's position within the sex-determination pathway.     

Sexual receptivity in Anopheles gambiae mosquitoes: absence of control by male accessory gland substances

Although male accessory gland substances modulate female receptivity in most mosquito genera that have been studied, they are not active in the development of sexual refractoriness in the malaria vectors, Anopheles gambiae or Anopheles albimanus. Neither the implantation of male accessory glands nor the injection of gland homogenates affected the insemination rate of unmated females. Similarly, portions of the male genital tract and spermathecae from mated females were also inactive when introduced into these females. Interruption of nervous transmission from the brain reduced the tendency of females to become inseminated and removal of the terminal abdominal ganglion completely abolished insemination. Oviposition behavior did not occur unless the spermatheca containing sperm was present, suggesting that this may be the mechanism that mediates the switchover to mated behavior.