Shortening deactivation: quantifying a critical component of cyclical muscle contraction

A muscle undergoing cyclical contractions requires fast and efficient muscle activation and relaxation to generate high power with relatively low energetic cost. To enhance activation and increase force levels during shortening, some muscle types have evolved stretch activation (SA), a delayed increased in force following rapid muscle lengthening. SA's complementary phenomenon is shortening deactivation (SD), a delayed decrease in force following muscle shortening. SD increases muscle relaxation, which decreases resistance to subsequent muscle lengthening. While it might be just as important to cyclical power output, SD has received less investigation than SA. To enable mechanistic investigations into SD and quantitatively compare it to SA, we developed a protocol to elicit SA and SD from Drosophila and Lethocerus indirect flight muscles (IFM) and Drosophila jump muscle. When normalized to isometric tension, Drosophila IFM exhibited a 118% SD tension decrease, Lethocerus IFM dropped by 97%, and Drosophila jump muscle decreased by 37%. The same order was found for normalized SA tension: Drosophila IFM increased by 233%, Lethocerus IFM by 76%, and Drosophila jump muscle by only 11%. SD occurred slightly earlier than SA, relative to the respective length change, for both IFMs; but SD was exceedingly earlier than SA for jump muscle. Our results suggest SA and SD evolved to enable highly efficient IFM cyclical power generation and may be caused by the same mechanism. However, jump muscle SA and SD mechanisms are likely different, and may have evolved for a role other than to increase the power output of cyclical contractions.

Age-Related Changes of Gene Expression Profiles in Drosophila

An individual’s gene expression profile changes throughout their life. This change in gene expression is shaped by differences in physiological needs and functions between the younger and older organism. Despite intensive studies, the aging process is not fully understood, and several genes involved in this process may remain to be identified. Here we report a transcriptomic analysis of Drosophila melanogaster using microarrays. We compared the expression profiles of two-day-old female adult flies with those of 45-day-old flies. We identified 1184 genes with pronounced differences in expression level between young and old age groups. Most genes involved in muscle development/maintenance that display different levels of expression with age were downregulated in older flies. Many of these genes contributed to sarcomere formation and function. Several of these genes were functionally related to direct and indirect flight muscles; some of them were exclusively expressed in these muscles. Conversely, several genes involved in apoptosis processes were upregulated in aging flies. In addition, several genes involved in resistance to toxic chemicals were upregulated in aging flies, which is consistent with a global upregulation of the defense response system in aging flies. Finally, we randomly selected 12 genes among 232 genes with unknown function and generated transgenic flies expressing recombinant proteins fused with GFP protein to determine their subcellular expression. We also found that the knockdown of some of those 12 genes can affect the lifespan of flies.

Wings and halteres act as coupled dual-oscillators in flies

The mechanics of Dipteran thorax is dictated by a network of exoskeletal linkages which, when deformed by the flight muscles, generate coordinated wing movements. In Diptera, the forewings power flight, whereas the hindwings have evolved into specialized structures called halteres which provide rapid mechanosensory feedback for flight stabilization. Although actuated by independent muscles, wing and haltere motion is precisely phase-coordinated at high frequencies. Because wingbeat frequency is a product of wing-thorax resonance, any wear-and-tear of wings or thorax should impair flight ability. How robust is the Dipteran flight system against such perturbations? Here, we show that wings and halteres are independently-driven, coupled oscillators. We systematically reduced the wing length in flies and observed how wing-haltere synchronization was affected. The wing-wing system is a strongly-coupled oscillator, whereas the wing-haltere system is weakly-coupled through mechanical linkages which synchronize phase and frequency. Wing-haltere link acts in a unidirectional manner; altering wingbeat frequency affects haltere frequency, but not vice-versa. Exoskeletal linkages are thus key morphological features of the Dipteran thorax which ensure wing-haltere synchrony, despite severe wing damage.

Octopamine drives honeybee thermogenesis

In times of environmental change species have two options to survive: they either relocate to a new habitat or they adapt to the altered environment. Adaptation requires physiological plasticity and provides a selection benefit. In this regard, the Western honeybee (Apis mellifera) protrudes with its thermoregulatory capabilities, which enables a nearly worldwide distribution. Especially in the cold, shivering thermogenesis enables foraging as well as proper brood development and thus survival. In this study, we present octopamine signaling as a neurochemical prerequisite for honeybee thermogenesis: we were able to induce hypothermia by depleting octopamine in the flight muscles. Additionally, we could restore the ability to increase body temperature by administering octopamine. Thus we conclude, that octopamine is necessary and sufficient for thermogenesis. Moreover, we show that these effects are mediated by β octopamine receptors. The significance of our results is highlighted by the fact the respective receptor genes underlie enormous selective pressure due to adaptation to cold climates. Finally, octopamine signaling in the service of thermogenesis might be a key strategy to survive in a changing environment.

Kinematics of wings from Caudipteryx to modern birds

This study seeks to better quantify the parameters that drove the evolution of flight from non-volant winged dinosaurs to modern birds. In order to explore this issue, we used fossil data to model the feathered forelimbs of Caudipteryx, the most basal non-volant maniraptoran dinosaur with elongated pennaceous feathers that could be described as forming proto-wings. In order to quantify the limiting flight factors, we created three hypothetical wing profiles for Caudipteryx with incrementally larger wingspans. We compared them with what revealed through fossils in wing morphology. These four models were analyzed under varying air speed, wing beat amplitude, and wing beat frequency to determine lift, thrust potential, and metabolic requirements. We tested these models using theoretical equations in order to mathematically describe the evolutionary changes observed during the evolution of modern birds from a winged terrestrial theropod like Caudipteryx. Caudipteryx could not fly, but this research indicates that with a large enough wing span, Caudipteryx-like animal could have flown. The results of these analyses mathematically confirm that during the evolution of energetically efficient powered flight in derived maniraptorans, body weight had to decrease and wing area/wing profile needed to increase together with the flapping angle and surface area for the attachment of the flight muscles. This study quantifies the morphological changes that we observe in the pennaraptoran fossil record in the overall decrease in body size in paravians, the increased wing surface area in Archaeopteryx relative to Caudipteryx, and changes observed in the morphology of the thoracic girdle, namely, the orientation of the glenoid and the enlargement of the sternum.

PLoS Computational Biology Issue Image | Vol. 17(9) October 2021

The tomato flowers are characterized by possessing poricidal anthers, which restrict the exit of the pollen to a tiny opening on the apex of the anther. To extract pollen efficiently, some visiting bees grasp the anthers and quickly contracting their flight muscles, producing vibrations and an audible sound. The vibrations are transferred to the anthers, shaking and stimulating the pollen inside them to leave by the pores, a phenomenon known as floral sonication or buzz-pollination. DOI: pcbi.1009426 Image Credit: Priscila de CE1;ssia Souza AraFA;jo (co-author of the manuscript) photographed this bee visiting flowers of tomato plants grown at the experimental fields of the Federal University of ViE7;osa (Minas Gerais State, Brazil). We confirm that the image can publish under the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/). The authors own the copyright for the image and confirm that agree with open Access License of PLOS Computational Biology.

PLoS Computational Biology Issue Image | Vol. 1(9) October 2021

The tomato flowers are characterized by possessing poricidal anthers, which restrict the exit of the pollen to a tiny opening on the apex of the anther. To extract pollen efficiently, some visiting bees grasp the anthers and quickly contracting their flight muscles, producing vibrations and an audible sound. The vibrations are transferred to the anthers, shaking and stimulating the pollen inside them to leave by the pores, a phenomenon known as floral sonication or buzz-pollination. DOI: pcbi.1009426 Image Credit: Priscila Souza AraFA;jo (co-author of the manuscript) photographed this bee visiting flowers of tomato plants grown at the experimental fields of the Federal University of ViE7;osa (Minas Gerais State, Brazil). We confirm that the image can publish under the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/). The authors own the copyright for the image and confirm that agree with open Access License of PLOS Computational Biology.

Experimental Studies of Carpenter Bee (Xylocopa: Apidae) Thorax Mechanics During Defensive Buzzing

Bees and other Hymenoptera utilize thorax vibration to realize an extensive range of behaviors ranging from flight to pollination. Strong indirect flight muscles contract to deform the thoracic walls and the resulting oscillation is sustained through a mechanism called stretch activation. While the mechanics of the insect thorax and muscles have been studied extensively during flight, relatively little is known about the thorax mechanics during non-flight behaviors. In this work, we investigate the thorax mechanics of the carpenter bee Xylocopa californica during defensive buzzing. During defensive buzzing, the insect folds its wings over its abdomen and rapidly fires it flight muscles, resulting in a loud audible buzz and large forces intended to deter predators. We devised a novel experiment to measure thorax oscillation and directional force production from a defensively buzzing carpenter bee. The largest peak forces were on average 175 mN and were oriented with the insect's dorsal-ventral muscle group. Peak forces oriented with the insect's dorsal-longitudinal muscle group averaged 117 mN. Thorax velocities were about 90 mm s^-1 p-p and velocity amplitude was positively correlated to peak force. Thorax oscillation frequency averaged 132 Hz but was highly variable both within individuals and across the tested population. From our measurements, we estimated the peak mechanical power required by defensive buzzing at 8.7 mW, which we hypothesize is greater than the power required during flight. Overall, this study provides insight into the function and capabilities of the Hymenopteran indirect flight muscle during non-flight behaviors.

How and why do bees buzz? Implications for buzz pollination

Buzz pollination encompasses the evolutionary convergence of specialised floral morphologies and pollinator behaviour in which bees use vibrations (floral buzzes) to remove pollen. Floral buzzes are one of several types of vibrations produced by bees using their thoracic muscles. Here I review how bees can produce these different types of vibrations and discuss the implications of this mechanistic understanding for buzz pollination. I propose that bee buzzes can be categorised according to their mode of production and deployment into: (1) thermogenic, which generate heat with little mechanical vibration; (2) flight buzzes, which combined with wing deployment and thoracic vibration, power flight, and (3) non-flight buzzes in which the thorax vibrates but the wings remain folded, and include floral, defence, mating, communication, and nest-building buzzes. I hypothesise that the characteristics of non-flight buzzes, including floral buzzes, can be modulated by bees via modification of the biomechanical properties of the thorax through activity of auxiliary muscles, changing the rate of activation of the indirect flight muscles, and modifying flower handling behaviours. Thus, bees should be able to fine-tune mechanical properties of their floral vibrations, including frequency and amplitude, depending on flower characteristics and pollen availability to optimise energy use and pollen collection.