To Sing or to Fly: Role of Muscle Proteins in <i>Drosophila</i> Song and Flight Behaviors.

To Sing or to Fly: Role of Muscle Proteins in Drosophila Song and Flight Behaviors. Samya Chakravorty, Bertrand Tanner, Matthew Rosenthal, Jim Vigoreaux. University of Vermont.

Complex behaviors using wings have facilitated the evolutionary success and diversification of insects. Drosophila uses indirect flight muscles (IFM) to power their wings for flight, a behavior subject to natural selection. Drosophila IFM also gets neurally activated to generate sound by wing vibrations for species-specific male courtship song, a behavior subject to sexual selection. To gain insight into how competing selection regimes are manifested at the molecular level, we investigated the effect on flight and mating behaviors of mutations affecting two contractile proteins essential for IFM function, flightin (FLN) and myosin regulatory light chain (MLC2). A deletion of 62 N-terminal amino acids of FLN (fln^N62), the faster evolving region of the protein (dN/dS= 0.4 vs 0.08 for rest of protein), results in myofilament lattice disorder and reduced flight ability (flight score: 2.80.1 vs 4.20.4 for fln⁺ rescued control) despite a normal wing beat frequency. fln^N62 males sing with an abnormal interpulse interval (IPI, 562.5 vs 370.7 ms for fln⁺) and a reduced pulse duty cycle (PDC, 2.60.2 vs 7.30.2 % for fln⁺), suggesting that FLN N-terminal region fine-tunes sexually selected song parameters in D. melanogaster. Unlike FLN, mutations of the highly conserved MLC2 (N-terminal 46 amino acid deletion and mutations of myosin light chain kinase phosphorylation sites) result in flight impairments through their effect on actin-myosin contractile kinetics and subtle but significant changes in myofilament lattice spacing. The MLC2 mutations do not affect sexually selected song parameters (IPI and PDC). Our data suggest that the highly conserved amino acid regions of FLN and MLC2 are under purifying selection to support the IFMs myofilament lattice structure and contractile function necessary for flight, whereas the fast evolving FLN N-terminal region is under positive selection to optimize IFMs biological performance in flight and species-specific song.