The initiation of life cycle complexity in the Myxozoa: evolutionary events and potential mechanisms

LISNEROVÁ, MARTINA; FIALA, IVAN; CANTATORE, DELFINA M P; IRIGOITIA MANUEL MARCIAL; TIMI JUAN TOMÁS; PECKOVÁ HANA; BARTOVÁ-SOJKOVÁ P.; HOLZER ASTRID S.

Porto

Conferencia; 19th International Conference on Diseases of Fish and Shellfish; 2019

Introduction: Chondrichthyes as some of the first fishes play a significant role in the reconstruction of the early evolutionary history of myxozoan parasites in vertebrates. It seems that life cycle complexity in myxozoans originated by inclusion of vertebrates into simple life cycles exploiting aquatic invertebrate hosts. Nevertheless, nothing is known about the mechanisms of this event, even though it supported massive parasite diversifications. The aim of this study was to do a comprehensive search and enrich myxozoan parasite SSU rDNA sequences data from evolutionary old fishes, discover new lineages, morphotypes and reconstruct the basic branches in the phylogenetic tree of myxozoans.Methodology: Twenty one species of sharks and rays were collected from 3 geographical areas: the Gulf of Mexico, the Atlantic off South Carolina and the Atlantic off Mar del Plata (Argentina). We analyzed myxozoan biodiversity by microscopy and molecular methods (SSU rDNA). We used phylogenetic, cophylogenetic (CoRe-PA) and character mapping (s-DIVA) methods to characterize the origins of myxozoans in Chondrichthyes, as well as host-parasite coevolution and biogeographic origins.Results: The acquisition of 21 new unique SSU rDNA sequences allowed for new views into the evolutionary origin ofthe Myxozoa in sharks, skates and rays. We identified seven lineages of myxozoans, 2 of them seem to be restricted to Chondrichthyes only (Bipteria and basal Chloromyxum). With respect to a large monophyletic dataset of Chloromyxum spp. we demonstrate that host and parasite phylogenies are strongly correlated and that tectonic changes explain phylogeographic patterns in more recent species of skates and softnose skates. The most basal myxozoan lineages are bile parasites so we suggest trophic transmission and feed-integration as a likely mechanism establishing a complex life cycle, with entry from the gut to bile via the bile duct, following the consumption of infected invertebrates by palaeozoic predators feeding predominantly on benthic organisms (e.g. chimaeras).Conclusion: For the first time we analyse the course, the drivers and the early evolutionary history of the oldest metazoan parasites known in their oldest vertebrate host group.