A newly uncovered phenotype associated with the fruitless gene of Drosophila melanogaster:: Aggression-like head interactions between mutant males

Male sexual behavior is regulated by the sex-determination hierarchy (SDH) in Drosophila melanogaster. The fruitless (fru) gene, one of the regulatory factors functioning downstream of other SDH factors, plays a prominent role in male sexual behavior. Here we demonstrate that fru mutations cause a previously unappreciated behavioral anomaly: high levels of head-to-head interactions between mutant males. These apparent confrontations between males are exhibited by all of the homozygous-viable foe mutants (expressing the effects of a given allele, among the four tested). Mutant dissarisfaction (dsf) males also exhibit this behavior at higher-than-normal levels, but it was barely displayed by doublesex or intersex mutants. For fru, a social component is involved in the head-interaction phenotype, while increasing age is a modifying factor for the behavior of dsf males. We suggest that head-to-head interactions, especially those per formed by fru males, are instances of putative aggression analogous to those exhibited by wildtype males and that head interactions are, to an extent, operationally separable from courtship behavior.