Dynamics of in vivo power output and efficiency of Nasonia asynchronous flight muscle.

By simultaneously measuring aerodynamic performance, wing kinematics, and metabolic activity, we have estimated the in vivo limits of mechanical power production and efficiency of the asynchronous flight muscle (IFM) in three species of ectoparasitoid wasps genus Nasonia (N. giraulti, N. longicornis, and N. vitripennis). The 0.6 mg animals were flown under tethered flight conditions in a flight simulator that allowed modulation of power production by employing an open-loop visual stimulation technique. At maximum locomotor capacity, flight muscles of Nasonia are capable to sustain 72.2 +/- 18.3 W kg(-1) muscle mechanical power at a chemo-mechanical conversion efficiency of approximately 9.8 +/- 0.9%. Within the working range of the locomotor system, profile power requirement for flight dominates induced power requirement suggesting that the cost to overcome wing drag places the primary limit on overall flight performance. Since inertial power is only approximately 25% of the sum of induced and profile power requirements, Nasonia spp. may not benefit from elastic energy storage during wing deceleration phases. A comparison between wing size-polymorphic males revealed that wing size reduction is accompanied by a decrease in total flight muscle volume, muscle mass-specific mechanical power production, and total flight efficiency. In animals with small wings maximum total flight efficiency is below 0.5%. The aerodynamic and power estimates reported here for Nasonia are comparable to values reported previously for the fruit fly Drosophila flying under similar experimental conditions, while muscle efficiency of the tiny wasp is more at the lower end of values published for various other insects.

Characterization of Danio rerio Nanog and functional comparison to Xenopus Vents.

Nanog is a homeodomain transcription factor associated with the acquisition of pluripotency. Genome analyses of lower and higher vertebrates revealed that the existence of Nanog is restricted to gnathostomata but absent from agnatha and invertebrates. To elucidate the function of Nanog in nonmammalia, we identified the Danio rerio ortholog of Nanog and characterized its role in gain and loss of function experiments. We found Nanog to be crucial for survival of early zebrafish embryos, because depletion of Nanog led to gastrulation defects with subsequent lethality. Mouse Nanog overexpression could rescue these defects. Vice versa, zebrafish Nanog was found to promote proliferation and to inhibit differentiation of mouse embryonic stem cells in the absence of leukemia inhibitory factor. These findings indicate functional conservation of Nanog from teleost fishes to mammals. However, Nanog was lost in the genome of the anurans Xenopus laevis and Xenopus tropicalis. Phylogenetic analysis revealed that deletion probably occurred in a common anuran ancestor along with chromosomal translocations. The closest homologs of Nanog in Xenopus are the Vent proteins. We, therefore, investigated whether the Xvent genes might substitute for Nanog function in Xenopus. Although we found some similarities in phenotypes after overexpression and in the regulation of several marker genes, Xvent1/2 and Nanog cannot substitute each other. Depletion of Nanog in zebrafish cannot be rescued by ectopic expression of Xvent, and Xvent depletion in Xenopus cannot be overcome by ectopic expression of zebrafish Nanog.

The significance of spiracle conductance and spatial arrangement for flight muscle function and aerodynamic performance in flying Drosophila.

During elevated locomotor activity such as flight, Drosophila satisfies its increased respiratory demands by increasing the total spiracle opening area of the tracheal gas exchange system. It has been assumed that in a diffusion-based system, each spiracle contributes to oxygen flux into and carbon dioxide flux out of the tracheal system according to the size of its opening. We evaluated this hypothesis by determining how a reduction in size and interference with the spatial distribution of gas exchange areas impair flight muscle function and aerodynamic force production in the small fruit fly Drosophila melanogaster. This was done by selectively blocking thoracic spiracles of tethered flies flying inside a flight simulator. Flow-through respirometry and simultaneous measurements of flight force production and wing kinematics revealed a negligible functional safety margin for respiration. Maximum locomotor performance was only achieved by unmanipulated flies, supporting the general assumption that at the animal's maximum locomotor capacity, maximum spiracle opening area matches respiratory need. The maximum total buffer capacity for carbon dioxide in Drosophila amounts to approximately 33.5 mul g(-1) body mass, estimated from the temporal integral of carbon dioxide release rate during the resting period after flight. By comparing flight variables in unmanipulated and 'spiracle-blocked' flies at comparable flight forces, we found that (i) stroke amplitude, stroke frequency and the chemo-mechanical conversion efficiency of the indirect flight musculature were broadly independent of the arrangement of spiracle conductance, while (ii) muscle mechanical power significantly increased, and (iii) mean lift coefficient and aerodynamic efficiency significantly decreased up to approximately 50% with an increasing number of blocked spiracles. The data suggest that Drosophila apparently maximizes the total efficiency of its locomotor system for flight by allowing oxygen delivery to the flight musculature through multiple spiracles of the thorax.

Unconventional mechanisms control cyclic respiratory gas release in flying Drosophila.

The high power output of flight muscles places special demands on the respiratory gas exchange system in insects. In small insects, respiration relies on diffusion, and for elevated locomotor performance such as flight, instantaneous gas exchange rates typically co-vary with the animal's metabolic activity. By contrast, under certain conditions, instantaneous release rate of carbon dioxide from the fruit fly Drosophila flying in a virtual-reality flight arena may oscillate distinctly at low frequency (0.37+/-0.055 Hz), even though flight muscle mechanical power output requires constant metabolic activity. Cross-correlation analysis suggests that this uncoupling between respiratory and metabolic rate is not driven by conventional types of convective flow reinforcement such as abdominal pumping, but might result from two unusual mechanisms for tracheal breathing. Simplified analytical modeling of diffusive tracheal gas exchange suggests that cyclic release patterns in the insect occur as a consequence of the stochastically synchronized control of spiracle opening area by the four large thoracic spiracles. Alternatively, in-flight motion analysis of the abdomen and proboscis using infra-red video imaging suggests utilization of the proboscis extension reflex (PER) for tracheal convection. Although the respiratory benefit of synchronized spiracle opening activity in the fruit fly is unclear, proboscis-induced tracheal convection might potentially help to balance the local oxygen supply between different body compartments of the flying animal.

Flight muscle properties and aerodynamic performance of Drosophila expressing a flightin transgene.

Flightin is a multiply phosphorylated, myosin-binding protein found specifically in indirect flight muscles (IFM) of Drosophila. A null mutation in the flightin gene (fln(0)) compromises thick filament assembly and muscle integrity resulting in muscle degeneration and lost of flight ability. Using P-element-mediated transformation with the full-length flightin gene driven by the Actin88F promoter, we have achieved rescue of all fln(0)-related ultrastructural and functional defects of the IFM. Transgenic P{fln(+)}fln(0) 'rescued' flies have fewer thick filaments per myofbril than wild-type flies (782+/-13 vs 945+/-9) but have otherwise normal IFM. Transgenic P{fln(+)}fln(+) 'tetraploid' flies have a normal number of thick filaments. The flightin protein levels in both transgenic strains are similar to wild type. By contrast, flightin levels are reduced in a myosin heavy chain tetraploid strain that produces excess myosin and excess thick filaments. These results suggest that regulation of flightin protein level is independent of gene copy number and that the number of thick filaments assembled per myofibril is influenced independently by myosin and flightin expression. We measured mechanical properties of IFM skinned fibers by sinusoidal analysis and found no significant differences in active viscoelastic properties of flightin-rescued and tetraploid transgenic flies vs wild type. The ability of the fln(+) transgene to overcome deficits in dynamic stiffness and power output in fln(0) suggest that the flightin protein contributes directly to fiber stiffness and stretch activation. However, flight parameters at maximum locomotor capacity, measured in a virtual reality flight simulator, are slightly compromised for both transgenic strains. P{fln(+)}fln(0) and P{fln(+)}fln(+) flies generated enough flight force to sustain hovering flight but showed reduced capability to produce forces in excess of hovering flight force. Both strains showed reductions in stroke frequency but only P{fln(+)}fln(+) showed reductions in stroke amplitude. Muscle and aerodynamic efficiency are similar among the two transgenic strains and wild type. These results illustrate the importance of flightin in flight muscle development and function.