Modulation of Aggressive Behavior Through Acoustic Signals COMMENTARY Ronald R

COMMENTARY Sound and fury: Modulation of aggressive behavior through acoustic signals COMMENTARY Ronald R. Hoya,1 and David L. Deitchera In any sexually reproducing animals, one expects ag- Here, a deafened male was placed in an arena with an gressive encounters to occur between/among males intact male and the interactions were video-recorded so because of their adaptive value; competition over that the number and level of aggressive encounters resources (food, territory, mates, social status) plays could be assessed. The assay proved to be sufficiently a critical role in both natural and sexual selection in robust that reliable data could be obtained within 5 min most species of animals (1, 2). Male–male interaction, of pairing. First, the investigators selectively extirpated when behaviorally enacted, is labeled as aggression, one JO (unilateral aristectomy) and then both JOs (bi- and it varies in expression from subtle gestures to lateral aristectomy). Compared with intact, control, outright physically combative encounters. Moreover, males, the males that had the double aristectomy pro- aggressive behavior is usually mediated by species- cedure showed reduced numbers of aggressive en- specific signals in one or more sensory modalities. counters. Males with bilateral aristal lesions exhibited In PNAS, Versteven et al. (3) continue the study of slightly less aggressivity than the unilaterally lesioned intermale acoustic signals in Drosophila melanogaster males. A second, less drastic hearing-disruption pro- [e.g., Jonsson et al. (4)], advancing it by providing cedure was tested by gluing (both unilaterally and neurosensory mechanistic data by deploying classic bilaterally) the second and third antennae segments, lesion as well as genetic methods to probe the role thus drastically reducing their mechanical mobility, of the fly’s “ears” in expressing aggressive behavior. and presumably the sensitivity of the JO to acoustic The authors (3) show that the ability of hearing can stimulation. This procedure diminished aggressivity, affect the behavioral outcome of dyadic male–male compared with controls, but to a lesser extent than encounters in laboratory contest arenas. the aristectomy procedure. The most dramatic effect The auditory basis of hearing in D. melanogaster has was on the duration of the aggressive interaction been subject to numerous studies at multiple levels of (“fighting time”). Whether the bilateral disruption to biological organization over many size scales (5). The hearing was through aristectomy or gluing of the auditory hearing organ, known as Johnston’sorgan aristae, the male–male interaction time was dimin- (JO), is the modified second segment of the antenna. ished nearly fivefold compared with intact controls Previous work has shown that the JO is stimulated by (5 s vs. 25 s). Although neither procedure entirely sound-induced vibrations, such as airflow from the beat- eliminated aggressive interaction, auditory/JO in- ing wings of a nearby fly. The salient acoustic signals for tactness had a pronounced effect on aggressivity. communication are produced by the male extending Auditory input clearly influences level and intensity one wing and briefly fluttering it at the target receiver of aggressive acts. This finding is not entirely surpris- fly. This stereotyped gesture takes place with the sender ing per se; however, Versteven et al. (3) pushed the and receiver at very close proximity, within one or at analysis to the neurosensory level by applying genetic most two body lengths. The JO is easily visible and very techniques that permitted them to genetically lesion accessible for experimental manipulation, which made neurons within the JO itself, while presumably sparing deafening the fly relative easy using classic techniques the intactness of the rest of the fly, including behavior. that prevent acoustic vibrations from setting the JO into Versteven et al. (3) performed several comple- motion or simply ablating it (figure 1 in ref. 3). mentary genetic perturbations that strongly support the role of hearing in aggressive behavior and bolster Drosophila the mechanical treatment results. First, the authors Versteven et al. (3) investigated the consequences of used a GAL4 line specific for AB neurons of the JO and deafening socially experienced D. melanogaster male GAL80ts to drive expression of tetanus toxin light flies by placing them in an experimental arena that chain, only at the adult stage. AB neurons transduce normally promotes male–male aggressive interactions. sound-evoked vibration. At the permissive temperature aDepartment of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853 Author contributions: R.R.H. and D.L.D. wrote the paper. The authors declare no conflict of interest. See companion article 10.1073/pnas.1605946114. 1To whom correspondence should be addressed. Email: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1700520114 PNAS Early Edition | 1of2 Downloaded by guest on September 26, 2021 of 18 °C, tetanus toxin expression is blocked by GAL80ts and males acoustic duetting behavior occurs and thus is even more complex in the aggression assay display normal levels of aggression. than the analogous signals in D. melanogaster (7). This is worth noting However, at the nonpermissive temperature of 25 °C, tetanus because D. melanogaster is a model system in which modern tools of toxin is expressed, neurotransmitter release from the AB neurons the genomics revolution are being applied to reveal the genetic is blocked, and aggression is significantly decreased. Another control of the neural systems that underlie behavior, as indicated by GAL4, also from the JO, but involved in gravity sensing—not the Versteven et al. (3) report. The decades-long hopes of making hearing—does not have a significant effect on aggression. transgenic animals through gene transfer from species of “known Versteven et al. (3) initially examined genetic mutants that might genetics” to related species with homologous neural systems and alter aggression. From earlier work (6), a set of genes altered in behavior might even be on the horizon, as the CRISPR-Cas9 hyperaggression mutants was compared with genes expressed in the technology is developed. Two examples of interesting “non- auditory system and it was subsequently narrowed to those involved melanogaster” drosophilids are presented to show interesting evolu- in signal transduction. The reduced list contained nan, iav, trpl, Arr2, tionary variations on the theme of acoustic signaling in social behavior. inaD,andnompC (based on other criterion). All but the nan mutant Territorial songs and courtship duets are known in Drosophila virilis altered aggression. However, nan and iav are broadly expressed and and the drosophilid Zaprionus genus (8). The second example comes may have pleiotropic effects outside of the JO, and trpl, Arr2,and from the most spectacular species radiation in Drosophila and is found inaD are important players in the visual transduction, a modality in the Hawaiian Drosophila (9). It has been firmly established that ter- known to be important in aggression. ritorial intermale aggression displays occur in the Drosophila-fauna of To address the question of whether these mutant effects were Hawaii, especially in the members of the Planitibia complex. Two specific, an experiment similar to the tetanus toxin study was per- members of this complex, D. heteroneura and D. silvestris,fromthe formed, except this time, RNAi-mediated knockdown for each of big island of Hawaii are thought to be lekking species (9). When males the candidate genes was driven by the same JO AB-neuron GAL4 come into close proximity to each other they engage in conspicuous line in the presence of GAL80ts. In every instance, as in the RNAi acoustic displays that can be recorded as substrate vibrational signals experiment, males displayed reduced aggression if the genes iav, and such signals play a conspicuous role in their intersexual courtship, Arr2, nompC, nan, inaD,ortrpl were knocked down. Thus, the as demonstrated several decades ago (10). However, the auditory specific RNAi knockdown matched the genetic mutant analysis in all mechanism of their extensive signaling behavior, for either sound cases except nan, firmly supporting the idea that sound-evoked production or ears for hearing the sounds, is not yet known. transduction is an important determinant of aggressive behavior. Studies of acoustic behavior in insects outside of Drosophila Rarely does a single sensory cue strictly control the release of is extensive, particularly in crickets (11). Aggression songs ac- a complex behavioral syndrome, such as social behavior, and in company and punctuate the spectacular intermale cricket fights, D. melanogaster it has long been known that in its mating be- where aggression escalates to a level of viciousness that the havior, courtship by males can involve cues from multiple sensory behavior was “elevated” to human spectator status and inevi- modalities, like vision, olfaction, taste, touch, in addition to hearing tably to the domain of blood sport gambling. The custom is (7). It is reasonable to assume that aggressive behavior also en- ancient: cricket fighting tournaments are as old the Tang dynasty gages multisensory cues. Versteven et al. (3) address such possi- (approximately AD 600) (12). It has been reported that in cricket bilities in their paper. The authors performed aggression assays fights, the winner emits longer and more intense song bouts, using the

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