The dynein-tubulin motor powers active oscillations and amplification in the hearing organ of the mosquito

Abstract

The design principles and specific proteins of the dynein-tubulin motor, which powers the flagella and cilia of eukaryotes, have been conserved throughout the evolution of life from algae to humans. Cilia and flagella can support both motile and sensory functions independently, or sometimes in parallel to each other. In this paper we show that this dual sensory-motile role of eukaryotic cilia is preserved in the most sensitive of all invertebrate hearing organs, the Johnston's organ of the mosquito. The Johnston's organ displays spontaneous oscillations, which have been identified as being a characteristic of amplification in the ears of mosquitoes and Drosophila. In the auditory organs of Drosophila and vertebrates, the molecular basis of amplification has been attributed to the gating and adaptation of the mechanoelectrical transducer channels themselves. On the basis of their temperature-dependence and sensitivity to colchicine, we attribute the molecular basis of spontaneous oscillations by the Johnston's organ of the mosquito Culex quinquefasciatus, to the dynein-tubulin motor of the ciliated sensillae. If, as has been claimed for insect and vertebrate hearing organs, spontaneous oscillations epitomize amplification, then in the mosquito ear, this process is independent of mechanotransduction.

Figure 1.

Schematic diagram of the mosquito hearing organ. (a) Cross section through the base of the antenna (a, flagellum; p, prong; s, scolopidium; n, Johnston's organ nerve; f, fibrillae). (b) Schematized cross section through a scolopidium of the Johnston's organ (c, cilium; b, sensory cell body; g, supporting cell; h, cuticular cap).

Figure 2.

Mechanical SOs of the flagellum. (a) Amplitude spectrum of the displacement of the flagellum without stimulation showing the frequency-specific SOs (black). The grey trace shows a spectrum recorded from the same mosquito after cooling to 5°C. The sharp peak at 800 Hz corresponds to a calibrating vibration of 2 nm in amplitude. Inset shows the waveform of the SOs in the time domain. (b) Amplitude spectrum of the electrical activity of the JO in response to a 400 Hz pure tone at increasing intensity.

Figure 3.

Temperature-dependence of the mechanical spontaneous oscillations of the Johnston's organ. (a) Arrhenius plot of the SO frequency (the natural logarithm of the SO frequency against the inverse absolute temperature). Two linear regression plots are fitted to data pooled from 12 mosquitoes with a break point at about 17°C. The activation energy values are indicated for both linear regression plots. Inset shows the Arrhenius plots for two individual mosquitoes. (b) Temperature-dependence of the amplitude of mechanical SOs. Pooled data for nine mosquitoes are shown. A simple harmonic oscillator model was fitted to the dependence of the normalized amplitude of the SOs with respect to temperature. Inset illustrates the same model fitted to the temperature dependences from two individual mosquitoes.

Figure 4.

Temperature-dependence of the electrical spontaneous oscillations of the Johnston's organ. (a) Amplitude spectrum of the electrical potential of the JO in the absence of stimulation. Inset shows the waveform of the SOs in the time domain. Spectral peaks corresponding to the frequency of SO (fSO) and its second harmonic (2fSO) are indicated by arrows. (b) Temperature-dependence of the amplitude of electrical SOs. Pooled data for 11 mosquitoes are shown. A simple harmonic oscillator model was fitted to the dependence of normalized amplitude of the electrical SO recorded from the JO. Inset illustrates the same model fitted to the temperature dependences from two individual mosquitoes.

Figure 5.

Effect of colchicine on SOs and mechanoelectrical transduction. (a–d) SOs before and after injection of colchicine. Amplitude spectrum of the displacement of the flagellum before (black) and 10 min after (grey) injection of colchicine solution (a,b) and colchicine-free solution (c,d). Sharp peaks at 700 (d) or 800 Hz (a–c) corresponds to a calibrating vibration of 1 (b,c) or 2 nm (a,d) in amplitude. Other sharp peaks in these recordings are electrical noise and pickup. (e) Electrical response of the JO to a pure tone presented between 0.2 and 0.6 s at an intensity of 1 × 10⁻⁴ ms⁻¹ at 300 Hz before and after injection of colchicine.