A genetic screen for regulators of muscle morphogenesis in Drosophila

The mechanisms that determine the final topology of skeletal muscles remain largely unknown. We have been developing Drosophila body wall musculature as a model to identify and characterize the pathways that control muscle size, shape, and orientation during embryogenesis (Johnson et al., 2013; Williams et al., 2015; Yang et al., 2020a; Yang et al., 2020b). Our working model argues muscle morphogenesis is regulated by (1) extracellular guidance cues that direct muscle cells toward muscle attachment sites, and (2) contact dependent interactions between muscles and tendon cells. While we have identified several pathways that regulate muscle morphogenesis, our understanding is far from complete. Here we report the results of a recent EMS-based forward genetic screen that identified a myriad of loci not previously associated with muscle morphogenesis. We recovered new alleles of known muscle morphogenesis genes, including back seat driver, kon-tiki, thisbe, and tumbleweed, arguing our screen had the depth and precision to uncover myogenic genes. We also identified new alleles of spalt-major, barren, and patched that presumably disrupt independent muscle morphogenesis pathways. Equally as important, our screen shows that at least 11 morphogenetic loci remain to be mapped and characterized. Our screen has developed exciting new tools to study muscle morphogenesis, which may provide future insights into the mechanisms that regulate skeletal muscle topology.

A genetic screen for regulators of muscle morphogenesis in Drosophila

The mechanisms that determine the final topology of skeletal muscles remain largely unknown. We have been developing Drosophila body wall musculature as a model to identify and characterize the pathways that control muscle size, shape, and orientation during embryogenesis (Johnson et al., 2013; Williams et al., 2015; Yang et al., 2020a; Yang et al., 2020b). Our working model argues muscle morphogenesis is regulated by (1) extracellular guidance cues that direct muscle cells toward muscle attachment sites, and (2) contact dependent interactions between muscles and tendons. While we have identified several pathways that regulate muscle morphogenesis, our understanding is far from complete. Here we report the results of a recent EMS-based forward genetic screen that identified a myriad of loci not previously associated with muscle morphogenesis. We recovered new alleles of known muscle morphogenesis genes, including bsd, kon, ths, and tum, arguing our screening strategy was effective and efficient. We also identified and sequenced new alleles of salm, barr, and ptc that presumably disrupt independent pathways directing muscle morphogenesis. Equally as important, our screen shows that at least 11 morphogenetic loci remain to be identified. This screen has developed exciting new tools to study muscle morphogenesis, which may provide future insights into the mechanisms that determine skeletal muscle topology.

The skeleton of Congruus kitcheneri , a semiarboreal kangaroo from the Pleistocene of southern Australia

The macropodine kangaroo, Wallabia kitcheneri , was first described in 1989 from a Pleistocene deposit within Mammoth Cave, southwestern Australia, on the basis of a few partial dentaries and maxilla fragments. Here, we recognize W. kitcheneri within the Pleistocene assemblages of the Thylacoleo Caves, south-central Australia, where it is represented by several cranial specimens and two near-complete skeletons, a probable male and female. We reallocate this species to the hitherto monotypic genus Congruus . Congruus kitcheneri differs from all other macropodid species by having a highly unusual pocket within the wall of the nasal cavity. It is distinguished from C. congruus by having a longer, narrower rostrum, a taller occiput and a deeper jugal. Congruus is closest to Protemnodon in overall cranial morphology but is smaller and less robust. In most postcranial attributes, Congruus also resembles Protemnodon , including general limb robustness and the atypical ratio of 14 thoracic to five lumbar vertebrae. It is distinguished by the high mobility of its glenohumeral joints, the development of muscle attachment sites for strong adduction and mobility of the forelimb, and large, robust manual and pedal digits with strongly recurved distal phalanges. These adaptations resemble those of tree-kangaroos more than ground-dwelling macropodines. We interpret this to imply that C. kitcheneri was semiarboreal, with a propensity to climb and move slowly through trees. This is the first evidence for the secondary adoption of a climbing habit within crown macropodines.

Scx-positive tendon cells are required for correct muscle patterning

SummaryThe elaborate movement of the vertebrate body is supported by the precise connection of muscle, tendon and bone. Each of the >600 distinct skeletal muscles in the human body has unique attachment sites; however, the mechanism through which muscles are reproducibly attached to designated partner tendons during embryonic development is incompletely understood. We herein show that Screlaxis-positive tendon cells have an essential role in correct muscle attachment in mouse embryos. Specific ablation of Screlaxis-positive cells resulted in dislocation of muscle attachment sites and abnormal muscle bundle morphology. Step-by-step observation of myogenic cell lineage revealed that post-fusion myofibers, but not migrating myoblasts, require tendon cells for their morphology. Furthermore, muscles could change their attachment site, even after the formation of the insertion. Our study demonstrated an essential role of tendon cells in the reproducibility and plasticity of skeletal muscle patterning, in turn revealing a novel tissue-tissue interaction in musculoskeletal morphogenesis.Graphical abstract

Intrinsic control of muscle attachment sites matching

Myogenesis is an evolutionarily conserved process. Little known, however, is how the morphology of each muscle is determined, such that movements relying upon contraction of many muscles are both precise and coordinated. Each Drosophila larval muscle is a single multinucleated fibre whose morphology reflects expression of distinctive identity Transcription Factors (iTFs). By deleting transcription cis-regulatory modules of one iTF, Collier, we generated viable muscle identity mutants, allowing live imaging and locomotion assays. We show that both selection of muscle attachment sites and muscle/muscle matching is intrinsic to muscle identity and requires transcriptional reprogramming of syncytial nuclei. Live-imaging shows that the staggered muscle pattern involves attraction to tendon cells and heterotypic muscle-muscle adhesion. Unbalance leads to formation of branched muscles, and this correlates with locomotor behavior deficit. Thus, engineering Drosophila muscle identity mutants allows to investigate, in vivo, physiological and mechanical properties of abnormal muscles.

Muscle attachment site patterns for species determination in West Palaearctic Wohlfahrtia (Diptera: Sarcophagidae) of medical and veterinary importance

The flesh fly genus Wohlfahrtia Brauer & Bergenstamm contains at least six species of medical and veterinary importance. Traditional methods of species identification in specimens of Wohlfahrtia, however, are restricted mostly to adult forms. Muscle attachment site (MAS) patterns allow for species determination in larval forms. MAS patterns in third instar larvae of six common West Palearctic species of Wohlfahrtia have been analyzed for this study. As in previously investigated Calliphoridae and Sarcophagidae, MAS patterns were found to be species specific. A genus pattern was established to be used as base for comparison in further species determination. For the first time a tool is provided for species identification of such broad range in larvae of Wohlfahrtia species. Wohlfahrtia patterns are composed of a significantly higher number of MAS than patterns found in Sarcophaga. Specifics of the six species analyzed are explained in detail. The larvae of the well-known species W. magnifica, an obligate traumatic myiasis agent, had to be excluded from the analysis as a great number of spines on the outside obscure muscle attachment sites on the inside of the cuticle.