Myxozoa Evo Devo · Evolution and development in Myxozoa: body-plan simplification associated with endoparasitism

Evolution and development in Myxozoa: body-plan simplification associated with endoparasitism

The main objective of this project was to investigate how morphological changes occurred in organisms that have become morphologically simplified in association with the evolution of parasitism. For this project we used as a model system the Myxozoa, which are now regarded as having evolved from free-living cnidarian ancestors and are therefore related to animals like jellyfish and sea anemones. Myxozoa have complex-life cycles involving alternation between a vertebrate (mostly fish) and an invertebrate host (mostly annelid worms and freshwater colonial animals known as moss animals or bryozoans), but exhibit very simplified body-plans many of which lack fundamental characters found in other multicellular animals like, e.g. epithelial tissues, intestinal tract, gametes, gonads, nervous system and even subcellular structures called centrioles. However, within the Myxozoa, different degrees of simplification seem to exist. The vast majority of species, belonging to the subtaxon Myxosporea, lack tissue-level development and mostly occur as plasmodial stages, i.e. large cells with multiple nuclei, in the affected host tissues. The other main evolutionary branch in the Myxozoa is represented by the Malacosporea. These are so far known to use freshwater bryozoans as invertebrate hosts and the parasitic stages in this group are much more complex and morphologically diverse than those of the myxosporeans. For our investigations we compared the two closely related malacosporean species - Tetracapsuloides bryosalmonae and Buddenbrockia plumatellae. T. bryosalmonae forms large immotile sacs lined by a single epithelial tissue layer, whereas B. plumatellae is worm-like with initially two epithelial layers and between these embedded longitudinal muscle blocks that enable the worms to actively locomote. The aim was to identify differences in developmental processes or gene expression processes that can explain these large-scale morphological differences. A first step was to complete morphological data on the development of Buddenbrockia plumatellae, for which only a few studies exist. In this already published work we could demonstrate the early establishment of axial polarity in the body organisation of the worm, the existence of an independent, “mesodermal” musculature and the presence of a tetraradial architecture of the musculature. We also found differences between the two species in the positioning of the early parasite stages with respect to host tissues. A large part of the project was devoted to generating transcriptomes (i.e. libraries of nearly all of the genes expressed in an organism) from the two species studied. This involved extensive fieldwork and culturing of the infected host organisms in the lab in order to obtain enough parasite tissue material, and thus RNA, for these studies. The transcriptomes were generated using next-generation sequencing. This cutting-edge technology allows the generation of a greatly expanded range of sequences as compared to traditional Sanger sequencing. This is especially useful when working on non-model-organisms like our myxozoan system. The information from the successfully generated transcriptomes was used to identify and compare transcripts (expressed genes) between the two species and to homologous transcripts known from free-living cnidarians. Despite the lack of clearly recognisable nervous system elements we were able to find a range of transcripts related to neurotransmission suggesting that neuronal signal transduction is taking place. However, our ultrastructural studies so far indicate that neuronal cells have reduced their characteristic structural features, like axons and dendrites, as a result of body-plan simplification, miniaturisation and reduction of sensory organs. This work is in progress and a manuscript in preparation. Homologues to many developmental genes know from cnidarians and other animals (metazoans) could be identified, although many are expressed at very low levels. In summary the work on developmental genes so far shows that most genes that are used by metazoans for patterning of body-plan elements or morphological characters are still present in myxozoans despite their highly reduced morphologies. It is possible that these genes are used in a different context during development. These results are currently being prepared for a manuscript. In order to gain insights into evolutionary processes at the subcellular/protein level we have investigated the structure and formation of polar capsules, cell organelles putatively homologous to cnidarian stinging cells (nematocysts). Immunocytochemical visualisation of minicollagens and further proteins so far exclusively found in nematocysts has been conducted in various stages of polar capsule development in T. bryosalmonae. These results show highly significant similarities in the formation and protein architecture of myxozoan polar capsules and cnidarian nematocysts that support the homology of these cell organelles. A publication on these results is in an advanced stage of preparation. All the results so far have been presented at major international conferences and will be published as scientific papers shortly. The investigations undertaken in this project are a valuable contribution to the field of evolutionary developmental biology because this is the first detailed analysis of developmental processes in Myxozoa and thus advances our understanding of evolutionary processes leading to parasitism in the lower animals. The work contributed to a recent invitation by Springer Publishers to develop the first book on the Myxozoa which will be edited by the Scientist in Charge, myself and a colleague in the USA (Prof. Jerri Bartholomew, Oregon State University, Corvallis), a contract for which has just been completed. Furthermore the generated sequence data provide an invaluable data base that, once our analyses have been completed, will be made publicly available and can be mined for many further questions, e.g., genes related to host-parasite interactions, virulence, etc.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз