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 rol e in male sexual behavior. Here we demonstrate that fru mutations cause a p reviously unappreciated behavioral anomaly: high levels of head-to-head int eractions 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 co mponent 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 instanc es of putative aggression analogous to those exhibited by wildtype males an d that head interactions are, to an extent, operationally separable from co urtship behavior.