Advisor: Georg Brenneis
Master's Defensio
Friday, December 13th, 2024, 14:00 CET
SR 4.1, UBB
Djerassiplatz 1, 1030 Vienna
Abstract
Pycnogonids, as the sister group to all other Chelicerata, have the potential to provide unique insights into arthropod evolution. In recent years, the development of the pycnogonid species Pycnogonum litorale (Strøm, 1762) was studied in more detail, providing new insight in their anamorphic phase of development. This, for arthropods probably ancestral, developmental mode is characterised by the addition of new body segments and walking legs with each molt. However, the developmental process of myogenesis (muscle formation) remains so far unexplored. Indeed, studies on arthropod myogenesis at the cell level are generally scarce, outside of insects and some crustacean taxa. The “founder cell model” of myogenesis in the fruit fly Drosophila melanogaster describes how mononucleated founder cells fuse with myoblasts to form multinucleated muscle precursors. Similar processes where recently discovered in crustacean taxa. However, due to the lack of comparable data on myogenesis for Chelicerata and Myriapoda, it is unclear whether these patterns are representative of all arthropods. This study aimed to describe myogenetic patterns in P. litorale during its anamorphic development, focusing on muscle formation hot-spots like the walking legs, trunk musculature, and proctodeal dilator muscles. Additionally, changes in the proboscis musculature were explored, being predicted to occur with an obligatory feeding host shift from hydrozoans to sea anemones during P. litorale development. Using fluorescent histochemistry and immunohistochemistry to analyze myosin heavy chain and F-actin proteins, as well as HCR-FISH to study the expression patterns of myosin heavy chain (striated type 2) and the transcription factor myocyte-enhancer-factor 2 (MEF2), the study reveals new muscle structures and sequences of muscle formation. For the first time in a chelicerate, mononucleated muscle pioneer cells are identified, which were observed to differentiate into singular, seemingly multinucleated muscle fibers, reminiscent of the founder cell model. However, myoblast fusion, as well as multinucleated muscle precursor cells could not be conclusively documented. Muscle cells in P. litorale appear to remain mononucleated for an extended period before undergoing rapid nuclear addition to form multinucleated muscle fibers. This work contributes new insights into pycnogonid myogenesis and its potential relevance to broader arthropod developmental processes.
