Identifying the lethal fish egg parasiteIchthyodinium chabelardias a member of Marine Alveolate Group I

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SummaryCells of the parasitic, unicellular eukaryoteIchthyodinium chabelardiwere isolated from eggs of sardine (Sardina pilchardus) and from a previously unrecognized host, bogue (Boops boops), off the Atlantic coast of Portugal. Immediately after release from the infected fish egg or newly hatched larva,I. chabelardicells were spherical and non-motile. After few minutes, spherical cells became flagellated and motile. Following 2–3 days of incubation and several divisions, spherical flagellated cells developed a twisted elongate shape and moved vigorously. Sequences of the small-subunit ribosomal RNA gene (SSU rDNA) were identical forI. chabelardiof both hosts and so were sequences of ITS1, ITS2 and the 5.8S rRNA gene. This genetic similarity suggests that eggs of sardine and bogue were infected by one single population ofI. chabelardi. The SSU rRNA gene sequence ofI. chabelardiwas, in turn, 97% similar to those of two identical Asian isolates ofIchthyodiniumsp. Phylogenetic analyses showed high support for the inclusion ofIchthyodiniumin the so-called Marine Alveolate Group I (MAGI). Two morphologically well-described genera, namelyIchthyodiniumandDubosquella, have now been shown to belong to this group of seemingly exclusively parasitic alveolates.

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