Behavior and cytogenetics of fruitless in Drosophila melanogaster: different courtship defects caused by separate, closely linked lesions.

The fruitless (fru) courtship mutant was dissected into three defects of male reproductive behavior, which were separable as to their genetic etiologies by application of existing and newly induced chromosomal aberrations. fru itself is a small inversion [In(3R) 90C; 91B] on genetic and cytological criteria. Uncovering the fru distal breakpoint with deletions usually led to males with two of the fru courtship abnormalities: no copulation attempts with females (hence, behavioral sterility) and vigorous courtship among males, including the formation of "courtship chains." However, certain genetic changes involving region 91B resulted in males who formed courtship chains but who mated with females. Uncovering the fru proximal breakpoint led to males that passively elicit inappropriately high levels of courtship. This elicitation property was separable genetically from the sterility and chain formation phenotypes and provisionally mapped to the interval 89F-90F, which includes the fru proximal breakpoint. Behavioral sterility and chaining were also observed in males expressing certain abnormal genotypes, independent of the fru inversion. These included combinations of deficiencies, each with a breakpoint in 91B, and a transposon inserted in 91B.

Reduced reproductive success for a conditioning mutant in experimental populations of Drosophila melanogaster.

Male Drosophila melanogaster that have courted newly-emerged males can modify their subsequent courtship behavior to avoid further courtship with immature males for up to 6 hr (previously reported). Here, it was hypothesized that such an experience-dependent modification would afford a mating advantage to normal males over males that carried a mutation that affects learning and memory. Coisogenic lines were constructed which varied at the dunce gene (dnc+ and dncM14 alleles) in order to test this hypothesis. Whether previously experienced with immature males or not, dnc+ and dncM14 males were indistinguishable in their response and mating efficiency when individually paired with virgin females. However, courtship performance of dnc+ and dncM14 males was different if they were first experienced with immature males and were then individually tested in an artificial population of nine immature males and one virgin female. In this situation, dnc+ males spent much less time in courtship with immature males and achieved copulation in one-third the time required for dncM14 males. As a control, the behavior and mating efficiency of courtship-naive dnc+ and dncM14 males in the artificial population was indistinguishable. In competition for a single virgin female, experienced dncM14 males showed a slight mating advantage over experienced dnc+ males. But when competition by experienced males for a single virgin female took place in the presence of nine immature males, dnc+ males were the successful maters in three-fourths of the trials.

Male courtship in Drosophila: the conditioned response to immature males and its genetic control.

Experimentally naive male Drosophila melanogaster respond to sexually immature males with intense courtship. However, this response decreases markedly in a short period of time, and "experienced" males then avoid further courtship with immature males for 4 hr. This subsequent inhibition of the courtship response is specific to immature males; the response to virgin females remains intact. This experience-dependent modification in courtship behavior is designated as "conditioned courtship." Seven mutant strains isolated for their inability to express avoidance conditioning (on criteria independent of courtship) were all found to be mutant with respect to expression of conditioned courtship. The potential application of this phenomenon to mosaic analysis of these mutations is posed. Other results indicate that immature males constitutively release a chemical signal that is sufficient for the expression of conditioned courtship. The interpretation of conditioned courtship as a component of fitness is discussed.

Conditioning Mutations in DROSOPHILA MELANOGASTER Affect an Experience-Dependent Behavioral Modification in Courting Males.

One aspect of courtship in male Drosophila melanogaster has been reported to be experience dependent. Males that have courted fertilized females are virtually unresponsive to virgin females for 2-3 hr. Here, this response was utilized as an assay for the effects of conditioning mutations on experience-dependent courtship. Seven strains expressing conditioning mutations (previously isolated and characterized for learning or memory defects in an electrical shock-odor association paradigm, independent of courtship) were all found to be mutant in expression of this experience-dependent change in courtship behavior. By comparison, three control strains that were unselected for conditioning defects all expressed normal experience-dependent courtship. Other results indicate that males of the conditioning-defective strains are able to elicit necessary cues from fertilized females, yet do not then modify their courtship with virgin females. Thus, it is suggested that experience-dependent modification of courtship and the previously reported associative olfactory conditioning with electric shock share common elements of processing. The possibility that experience-dependent courtship represents adaptive behavior is discussed.

Construction of an unstable Ring-X chromosome bearing the autosomal dopa decarboxylase gene in Drosophila melanogaster and analysis of Ddc mosaics.

An unstable Ring-X chromosome, Ddc+ -Ring-X carrying a cloned Dopa decarboxylase (Ddc) encoding segment was constructed. The construction involved a double recombination event between the unstable Ring-X, R(1)wvC and a Rod-X chromosome which contained a P-element mediated Ddc+ insert. The resulting Ddc+ -Ring-X chromosome behaves similarly to the parent chromosome with respect to somatic instability. The Ddc+ -Ring-X chromosome was used to generate Ddc mosaics. Analyses of Ddc mosaics revealed that while there was no absolute requirement for the Ddc+ expression in either the epidermis or the nervous system, very large mutant clones did affect the viability of the mosaic.

Extended reproductive roles of the fruitless gene in Drosophila melanogaster revealed by behavioral analysis of new fru mutants.

The fruitless mutants fru3 and fru4 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. fru3 and fru4 males are less stimulated to court females than fru1 and fru2. No courtship pulse song is generated by either fru3 or fru4 males, even though they perform brief wing extensions. fru3 and fru4 males display significantly less chaining behavior than do fru1 males. The hierarchy of courtship responses by fru males directed toward females vs. males, when presented with both sexes simultaneously, is that fru1 males perform vigorous and indiscriminant courtship directed at either sex; fru4 males are similarly indiscriminant, but courtship levels were lower than fru1; fru2 males prefer females; fru3 males show a courtship bias toward males. fru3 and fru4 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.

Molecular genetic dissection of the sex-specific and vital functions of the Drosophila melanogaster sex determination gene fruitless.

A multibranched hierarchy of regulatory genes controls all aspects of somatic sexual development in Drosophila melanogaster. One branch of this hierarchy is headed by the fruitless (fru) gene and functions in the central nervous system, where it is necessary for male courtship behavior as well as the differentiation of a male-specific abdominal structure, the muscle of Lawrence (MOL). A preliminary investigation of several of the mutations described here showed that the fru gene also has a sex-nonspecific vital function. The fru gene produces a complex set of transcripts through the use of four promoters and alternative splicing. Only the primary transcripts produced from the most distal (P1) promoter are sex-specifically spliced under direction of the sex-determination hierarchy. We have analyzed eight new fru mutations, created by X-ray mutagenesis and P-element excision, to try to gain insight into the relationship of specific transcript classes to specific fru functions. Males that lack the P1-derived fru transcripts show a complete absence of sexual behavior, but no other defects besides the loss of the MOL. Both males and females that have reduced levels of transcripts from the P3 promoter develop into adults but frequently die after failing to eclose. Analysis of the morphology and behavior of adult escapers showed that P3-encoded functions are required for the proper differentiation and eversion of imaginal discs. Furthermore, the reduction in the size of the neuromuscular junctions on abdominal muscles in these animals suggests that one of fru's sex-nonspecific functions involves general aspects of neuronal differentiation. In mutants that lack all fru transcripts as well as a small number of adjacent genes, animals die at an early pupal stage, indicating that fru's function is required only during late development. Thus, fru functions both in the sex-determination regulatory hierarchy to control male sexual behavior through sex-specific transcripts and sex-nonspecifically to control the development of imaginal discs and motorneuronal synapses during adult development through sex-nonspecific transcript classes.

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.

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

A 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.

Reassessment of 79B actin gene expression in the abdomen of adult Drosophila melanogaster.

The expression pattern of the 79B actin gene in Drosophila melanogaster has been inferred previously by means of a reporter gene in which 79B actin promoter sequences drive the lacZ coding sequences. Although the 79B actin gene is expressed primarily in muscles of the thorax and first abdominal segment of the adults of both sexes, expression in the remaining abdominal segments appears limited to the male genital muscles and the male-specific Muscle of Lawrence (MOL). This reported abdominal expression pattern has been reassessed. By varying parameters of tissue preparation and lacZ reporter gene detection, expression of the 79B actin gene has been revealed in most dorsal abdominal longitudinal fibres and genital muscles of both females and males. These new results suggest that there are differences in the level of 79B actin gene expression among the various abdominal muscles of both sexes, and that abdominal expression is not limited primarily to male sex structures.

Spectral analysis of courtship songs in behavioral mutants of Drosophila melanogaster.

Spectral analyses were applied to the courtship songs of the mutants cacophony (cac), dissonance (diss), fruitless (fru), and period (per), as well as to the double mutant cac diss. Aberrant intervals between song pulses were observed in diss, cac, cac diss, and fru songs. diss males displayed a defect in song hums manifested by an irregular sine wave, although the fundamental frequencies were normal. Sine song frequencies and intrapulse frequencies were aberrant in cac diss males. Two per mutant alleles (pero1 and pers) were associated with normal song [corrected] pulses and hums. These findings are discussed with regard to the mechanisms of song production and the role of these sounds in Drosophila reproduction.

The muscle of lawrence in Drosophila: a case of repeated evolutionary loss.

The muscle of Lawrence (MOL) is a bilaterally symmetrical muscle spanning the tergite of the fifth abdominal segment of adult male Drosophila melanogaster. It is not, however, a general feature of male-specific development within the subfamily Drosophilinae. Of 95 species surveyed within this subfamily, 67 exist with no MOL at all. By drawing comparisons with published cladograms of species relatedness, three conclusions regarding the evolutionary history of the MOL are made: (i) The MOL predates the major radiations of the genus Drosophila, given its presence in earlier-branching Chymomyza and Scaptodrosophila; the MOL has been subsequently excluded in at least one present species of each of these two primitive genera. (ii) Within the genus Drosophila the MOL is present sporadically in the radiation of the subgenus Sophophora, showing repetitive loss even in very close evolutionary lineages. (iii) The MOL may have been entirely excluded from the prolific radiation of the subgenus Drosophila. Thus the MOL shows a uniquely incongruous pattern of presence or absence relative to accepted drosophilid phylogeny.