Tissue characteristics and development in myxozoa

Abstract

For most of the time that they have been recognised, myxozoans were viewed to lack any tissue-level of development. However, the discovery of malacosporean stages in freshwater bryozoans revealed recognisable tissues in the form of epithelial sheets and musculature. In this chapter we compare and contrast myxozoan tissues with those of other metazoans and review the scattered literature on myxozoan development in order to explore differences and similarities to normal development in cnidarians and bilaterians. Malacosporean trophic stages possess a bona fide epithelium (including a basal lamina). Close inspection, however, demonstrates that some epithelial features are found in all myxozoan spores (cell-junctions) and even in syncytial plasmodial stages of myxosporeans (polarity, directed transport). Vestiges of tissue-level traits of their free-living ancestors can therefore be observed in all myxozoans. Resorptive and secretory tissues in myxozoans and muscle tissues in malacosporeans are evaluated with respect to typical cnidarian or bilaterian tissues. Elements of neurotransmission pathways identified in a transcriptomic survey suggest that muscle activity in myxoworms is coordinated by nervous signal transduction. Nerve cells may therefore be highly reduced and have not been recognised in structural investigations so far. Gametogenesis and embryogenesis in myxozoans have clearly been highly modified but remain poorly understood. Outstanding issues that remain to be resolved include the identification and formation of blastula and gastrula stages and the orientation of the ectoderm and endoderm (gastrodermis) of myxozoans.