Advisor: Andreas Wanninger

Master's Defensio - Wednesday, May 27th 2020

Abstract

The mesoderm is argued to be the youngest of the three germ layers that probably evolved after the cnidarian-bilaterian split. Although its morphogenesis has been studied in numerous metazoans, the molecular components underlying this process remain unknown in the majority of phyla including the highly diverse Mollusca. Here, expression of hairy and enhancer of split (HES) and Mox were investigated in the polyplacophoran mollusk Acanthochitona, together with a common regulator in animal myogenesis, Myosin heavy chain (MHC). AcaMHC is expressed during myogenesis in the early larva as well as in muscles of the late larva and the juvenile. AcaMox is expressed in the mesodermal bands and in the ventrolateral muscle, an apomorphic feature of the polyplacophoran trochophore. Two of the seven annotated AcaHES genes yielded results by in situ hybridization. AcaHESC2 is expressed in the ectoderm of the gastrula, the mesodermal bands and in putative developing neuronal tissue of the late trochophore larva. AcaHESC7 is expressed in the trochoblasts of the gastrula and during foregut formation. AcaMox expression in the mesodermal bands is similar to other lophotrochozoans such as the gastropod Haliotis asinina, the brachiopod Terebratalia transversa, and the annelid Alitta virens. A metazoan-wide comparison reveals Mox expression in the mesoderm in numerous bilaterians (except echinoderms) and in the cnidarian endoderm. In addition, Mox is involved in myogenesis in molluscs, annelids, urochordates, and craniates but not in Drosophila while nematodes have lost a Mox ortholog altogether. HES genes are expressed during a number of developmental processes such as segmentation, neurogenesis, and gut formation, providing evidence that so-called “mesodermal genes” are not confined to mesoderm formation alone. Further studies are needed to fully understand the molecular pathways underlying bilaterian mesoderm development and evolution.