Insights and perspectives on the enigmatic alary muscles of arthropods.

Three types of muscles, cardiac, smooth and skeletal muscles are classically distinguished in eubilaterian animals. The skeletal, striated muscles are innervated multinucleated syncytia, which, together with bones and tendons, carry out voluntary and reflex body movements. Alary muscles (AMs) are another type of striated syncytial muscles, which connect the exoskeleton to the heart in adult arthropods and were proposed to control hemolymph flux. Developmental studies in Drosophila showed that larval AMs are specified in embryos under control of conserved myogenic transcription factors and interact with excretory, respiratory and hematopoietic tissues in addition to the heart. They also revealed the existence of thoracic AMs (TARMs) connecting to specific gut regions. Their asymmetric attachment sites, deformation properties in crawling larvae and ablation-induced phenotypes, suggest that AMs and TARMs could play both architectural and signalling functions. During metamorphosis, and heart remodelling, some AMs trans-differentiate into another type of muscles. Remaining critical questions include the enigmatic modes and roles of AM innervation, mechanical properties of AMs and TARMS and their evolutionary origin. The purpose of this review is to consolidate facts and hypotheses surrounding AMs/TARMs and underscore the need for further detailed investigation into these atypical muscles.

Alary muscles and thoracic alary-related muscles are atypical striated muscles involved in maintaining the position of internal organs.

Alary muscles (AMs) have been described as a component of the cardiac system in various arthropods. Lineage-related thoracic muscles (TARMs), linking the exoskeleton to specific gut regions, have recently been discovered in Drosophila Asymmetrical attachments of AMs and TARMs, to the exoskeleton on one side and internal organs on the other, suggested an architectural function in moving larvae. Here, we analysed the shape and sarcomeric organisation of AMs and TARMs, and imaged their atypical deformability in crawling larvae. We then selectively eliminated AMs and TARMs by targeted apoptosis. Elimination of AMs revealed that AMs are required for suspending the heart in proper intra-haemocelic position and for opening of the heart lumen, and that AMs constrain the curvature of the respiratory tracheal system during crawling; TARMs are required for proper positioning of visceral organs and efficient food transit. AM/TARM cardiac versus visceral attachment depends on Hox control, with visceral attachment being the ground state. TARMs and AMs are the first example of multinucleate striated muscles connecting the skeleton to the cardiac and visceral systems in bilaterians, with multiple physiological functions.

The Basic Plan of the Adult Heart Is Conserved Across Different Species of Adult Mosquitoes, But the Morphology of Heart-Associated Tissues Varies.

The heart is a pivotal organ in insects because it performs a number of different tasks, such as circulating nutrients, hormones, and excreta. In this study, the morphologies of the heart and associated tissues, including pericardial cells (PCs) and alary muscles (AMs), in the hematophagous mosquitoes Anopheles aquasalis Curry (Diptera: Culicidae), Aedes aegypti L. (Diptera: Culicidae), and Culex quinquefasciatus Say (Diptera: Culicidae), and the phytophagous Toxorhynchites theobaldi Dyar & Knab (Diptera: Culicidae) were compared using different microscopy techniques. Mosquito hearts are located across the median dorsal region of the whole abdomen. Paired incurrent openings in the heart wall (ostia) are found in the intersegmental regions (segments 2-7) of the abdomen, while an excurrent opening is located in the terminal cone of Ae. aegypti. The sides of the heart contain PC that are more numerous in An. aquasalis and Th. theobaldi. In these two species, PC form a cord of as closely aggregated cells, but in Ae. aegypti and Cx. quinquefasciatus, PC occur in pairs with two or four PC pairs per intersegmental region. In Th. theobaldi, AM binds to all regions of the heart, whereas in other mosquitoes they only bind in the intersegmental regions. The basic plan of the adult heart was conserved across all the adult mosquitoes investigated in this study. This conserved organization was expected because this organ plays an important role in the maintenance of individual homeostasis. However, the species had different PC and of AM morphologies. These morphological differences seem to be related to distinct physiological requirements of mosquito circulatory system.

Hox control of Drosophila larval anatomy; The Alary and Thoracic Alary-Related Muscles.

The body plan of arthropods and vertebrates involves the formation of repetitive segments, which subsequently diversify to give rise to different body parts along the antero-posterior/rostro-caudal body axis. Anatomical variations between body segments are crucial for organ function and organismal fitness. Pioneering work in Drosophila has established that Hox transcription factors play key roles both in endowing initially identical segments with distinct identities and organogenesis. The focus of this review is on Alary Muscles (AMs) and the newly discovered Thoracic Alary-Related Muscles (TARMs). AMs and TARMs are thin muscles which together connect the circulatory system and different midgut regions to the exoskeleton, while intertwining with the respiratory tubular network. They were hypothesized to represent a new type of muscles with spring-like properties, maintaining internal organs in proper anatomical positions during larval locomotion. Both the morphology of TARMs relative to AMs, and morphogenesis of connected tissues is under Hox control, emphasizing the key role of Hox proteins in coordinating the anatomical development of the larva.

The ultrastructure of the Aedes aegypti heart.

Comparative structural analyses of the heart and associated tissues in 4th instar larvae (L4), pupae and adults of Aedes aegypti were undertaken using a combination of microscopy techniques. The Ae. aegypti heart consists of cardiomyocytes arranged in a helical fashion, and it is physically associated with intersegmental groups of pericardial cells (PCs) and the alary muscles (AMs). Ramifications commonly present in AMs are more developed in adults than in the immature stages. Pericardial cells absorb and store extracellular components as shown by the uptake of carmine dye fed in larval diet. We also observed that carmine stained inclusions corresponding to electron-dense structures resembling lysosomes that were more abundant and prominent in pupae, suggestive of increase of waste accumulation during pupation. The results presented here expand on previously known aspects of the mosquito heart and describe for the first time comparative aspects of the morphology of the heart in different developmental stages.