Acoustic communication and evolution in Drosophila: roles for a nuclear receptor and its regulon
Dr Joerg Albert
All behaviour is guided, or restricted, by the senses. Sense organs have evolved in multiple ways to extract and pre-process information from the external world. However, molecular mechanisms of sense organ specification and their evolutionary origins have remained unclear. We have used closely-related Drosophila species to explore how ears can contribute to evolution - and how evolution, in turn, has shaped ears.
In flies (Diptera), hearing is mediated by Johnston’s Organ (JO) neurons in the second antennal segment (1). In Drosophilids, the spectral tuning of the flies’ antennal ears correlates with the spectral composition of song pulses produced by conspecific males (2). Laser-Doppler vibrometric analysis of sound receiver mechanics and extracellular recordings of compound action potentials from the antennal nerve show that the species-specific auditory tuning is partly the result of variations in the molecular modules for mechanotransduction in JO neurons.
RNA-Seq based transcriptomics of the JOs from six closely-related Drosophila species combined with predictive bioinformatics (i-cisTarget and iRegulon) identified a particular type of transcription factor from the nuclear hormone receptor family as important contributor to inter-specific variation in Drosophilid ears.
Nuclear hormone receptor proteins are also required for normal sex organ development. The investigated mutants showed sexually dimorphic defects in auditory function (both with regard to auditory mechanics and auditory nerve responses). On the sender side of Drosophila acoustic communication, in turn, mutant males displayed severe defects in song production (both in their propensity to produce songs and with regard to song structure). The duality of its contributions presents this nuclear receptor gene as a potential substrate for genetic coupling in the Drosophila acoustic communication system.
Making an effort to listen: mechanical amplification by ion channels and myosin motors in hair cells of the inner ear
Professor Jim Hudspeth, The Rockefeller Univeristy, USA
Human hearing is enhanced by an active process that amplifies the ear's mechanical inputs several hundredfold, sharpens frequency tuning to allow the discrimination of tones differing in frequency by less than 0.2 %, and compresses six orders of magnitude in the amplitude of sounds into only two orders of magnitude in neural output. In addition, spontaneous otoacoustic emissions emerge from ears in a very quiet environment, an indication that the active process can be so exuberant as to become unstable. Cooperativity between mechanoelectrical-transduction channels confers negative stiffness on the hair bundle, which together with myosin-based adaptation motors elicits a dynamical instability that underlies the active process. Experiments on individual hair bundles indicate that the bundle's operation near this instability, a Hopf bifurcation, accounts for the four characteristics of the active process.
Novel synaptic transmission from vestibular hair cells to calyceal afferents serves fast reflexes in amniotes
Professor Ruth Anne Eatock
The vestibular type I hair cell and its distinctive calyceal synapse are found only in the inner ears of reptiles, birds and mammals. Like the cochlea, the type I – calyx synapse may represent adaptations to life on land. Over the past 20 years, evidence has accrued that these unusual-looking synapses are also functionally remarkable, featuring not just chemical (quantal) transmission of vesicle-bound glutamate from ribbon synapses but also a form of non-quantal transmission that depends on currents through ion channels in dense arrays on presynaptic (hair cell) and postsynaptic (calyceal) membranes. Quantal and non-quantal transmission filter the transmitted mechanosensory signal in distinct ways and have been recorded both together and separately, suggesting unexpectedly rich possibilities for shaping vestibular inputs to the brain.
The role of the auditory brainstem in understanding speech in challenging listening conditions
Dr Tobias Reichenbach, Impertial College London, UK
Humans excel at selectively listening to a target speaker in background noise such as competing voices. While the encoding of speech in the auditory cortex is modulated by selective attention, it remains debated whether such modulation occurs already in subcortical auditory structures. Investigating the contribution of the human brainstem to attention has, in particular, been hindered by the tiny amplitude of the brainstem response. Its measurement normally requires a large number of repetitions of the same short sound stimuli, which may lead to a loss of attention and to neural adaptation. This talk describes a mathematical method to measure the auditory brainstem response to running speech, an acoustic stimulus that does not repeat and that has a high ecological validity. This research employs this method to assess the brainstem's activity when a subject listens to one of two competing speakers, and show that the brainstem response is consistently modulated by attention.