Study of cellular anatomy, development and functional morphology of muscles in water bears to elucidate the evolution of muscular systems in Tardigrada and Panarthropoda

• Applicant: Professor Dr. Georg Mayer
• Subject Area: Systematics and Morphology (Zoology)
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 567662713

Tardigrades (water bears) are microscopic invertebrates whose morphology has changed little since over 500 million years. Their remarkable resistance to extreme environmental conditions, together with their phylogenetic position as the closest relatives of onychophorans (velvet worms) and arthropods, has garnered them much interest in the field of evolutionary biology. Although several studies have been conducted on tardigrade musculature, little is known about segmental identity and serial homology of muscles, especially in the head. The number of myocytes is also unknown, except for the locomotory system of the widely used model tardigrade Hypsibius exemplaris. The proposed project aims at characterizing all muscles and myocytes in this species using a variety of markers combined with super-resolution confocal microscopy and three-dimensional (3D) reconstruction. We will establish a widely applicable nomenclature for the individual muscles and myocytes. Cellular analysis will help to clarify whether there is evidence for eutely (cell constancy) in tardigrade musculature. The morphologies, position and spatial relationship of myocytes to specific nerves and motor endplates will be used to elucidate segmental identities and serial homology of muscles in tardigrade head and trunk segments. A focus on cellular characterization and proper reconstruction of stylet muscles will help to assess the hypothesis of the origin of stylets from a pair of limbs and their subsequent transformation into feeding structures. The proposed hypotheses will be tested by studying muscle development (myogenesis). Of particular interest will be the origin and growth patterns of individual myocytes during development. A cross-species comparison will reveal homologies and changes in the organization of muscles and their innervation in different tardigrade lineages. A polarization optics setup will be used to analyze the kinematics of muscle function in adult specimens. This will help to clarify, which muscles are responsible for specific body and leg movements, including elongation, shortening and bending. Elucidating these aspects will not only contribute to the understanding of the role of specific muscles but will also provide a basis for studies on the influence of drugs on muscle function. Overall, the expected outcome will make the tardigrade musculature one of the best studied organ systems in these animals. The analysis of anatomical, ontogenetic and functional aspects of the tardigrade neuromuscular system will provide insights into the ancestral body plan and its various modifications in panarthropods, including the evolution of segmentation, tagmosis, and cephalization.

• DFG Programme: Research Grants