Flightin maintains myofilament lattice organization required for optimal flight power and courtship song quality inDrosophila

Abstract

The indirect flight muscles (IFMs) of Drosophila and other insects with asynchronous flight muscles are characterized by a crystalline myofilament lattice structure. The high-order lattice regularity is considered an adaptation for enhanced power output, but supporting evidence for this claim is lacking. We show that IFMs from transgenic flies expressing flightin with a deletion of its poorly conserved N-terminal domain ( fln DN62 ) have reduced interthick filament spacing and a less regular lattice. This resulted in a decrease in flight ability by 33% and in skinned fibre oscillatory power output by 57%, but had no effect on wingbeat frequency or frequency of maximum power output, suggesting that the underlying actomyosin kinetics is not affected and that the flight impairment arises from deficits in force transmission. Moreover, we show that fln DN62 males produced an abnormal courtship song characterized by a higher sine song frequency and a pulse song with longer pulses and longer inter-pulse intervals (IPIs), the latter implicated in male reproductive success. When presented with a choice, wild-type females chose control males over mutant males in 92% of the competition events. These results demonstrate that flightin N-terminal domain is required for optimal myofilament lattice regularity and IFM activity, enabling powered flight and courtship song production. As the courtship song is subject to female choice, we propose that the low amino acid sequence conservation of the N-terminal domain reflects its role in fine-tuning species-specific courtship songs.

Contiguity and Structural Impacts of a Non-Myosin Protein within the Thick Filament Myosin Layers

“…A shared myosin coiled-coil binding region raises The estimated mass of the red density is less than the mass of flightin [3]. Mutant Drosophila flightin lacking the N-terminal 62 amino acid region [21,22] or the C-terminal 44 amino acid region [23] are incorporated into the fiber, indicating that the region between amino acids 63 and 138, encompassing the conserved WYR domain [24], harbors an essential myosin binding site. We hypothesize that the red density is mostly or exclusively WYR, further supported by the predicted unstructured nature of the larger N-terminal region [25].…”

Secondary Structure of the Novel Myosin Binding Domain WYR and Implications within Myosin Structure

“…The analyses of flightin mutants in D. melanogaster have revealed its important role in flight and courtship, two behaviors that underscore the evolutionary success and prolific speciation of insects [7,12,13]. Specifically, mutants that express a truncated flightin missing the C-terminal region (fln ∆C44 ) are incapable of generating a courtship song or a wing beat to propel flight, similar to the flightin null mutant, (fln 0 ).…”

“…Specifically, mutants that express a truncated flightin missing the C-terminal region (fln ∆C44 ) are incapable of generating a courtship song or a wing beat to propel flight, similar to the flightin null mutant, (fln 0 ). In contrast, mutants that express a truncated flightin missing the N-terminal region (fln ∆N62 ) have impaired flight mechanics and produce an abnormal courtship song that lessens the male's mating success [12]. Despite the truncations, the mutant flightin variants remain integral components of the fiber indicating that neither the N-terminal nor C-terminal region is necessary for flightin incorporation into the thick filament.…”

“…The N-terminal mutant (fln ∆N62 ) also exhibits slightly shorter sarcomeres that lack an evident H-zone and have a narrower M-line. Myofibrils contain more thick filaments and a more compact and less "crystalline" (regular) myofilament lattice [12,17].…”

“…IFM fibers that either lack flightin (fln 0 ) or express a mutant form (fln ∆N62 ) and (fln ∆C44 ) manifest a variety of mechanical defects that include alterations in cross-bridge cycling kinetics, viscous and elastic moduli, and power output [12,14,16]. Native IFM thick filaments isolated from these mutant strains more clearly delineate the complex role that flightin plays in dictating thick filament structure, integrity and mechanical properties [17,18].…”