A New Class and Order of Myxozoans to Accommodate Parasites of Bryozoans with Ultrastructural Observations on Tetracapsula bryosalmonae (PKX Organism)

Abstract

Tetracapsula bryosalmonae, formerly PKX organism, is a myxozoan parasite that causes proliferative kidney disease in salmonid fish. Its primary hosts, in which it undergoes a sexual phase, are phylactolaemate bryozoans. It develops in the bryozoan coelomic cavity as freely floating sacs which contain two types of cells, stellate cells and sporoplasmogenic cells, which become organised as spores. Eight stellate cells differentiate as four capsulogenic cells and four valve cells which surround a single sporoplasmogenic cell. The sporoplasmogenic cell undergoes meiosis and cytoplasmic fission to produce two sporoplasms with haploid nuclei. Sporoplasms contain secondary cells. The unusual development supports previously obtained data from 18S rDNA sequences, indicating that species of Tetracapsula form a clade. It diverged early in the evolution of the Myxozoa, before the radiation that gave rise to the better known genera belonging to the two orders in the single class Myxosporea. The genus Tetracapsula as seen in bryozoans shares some of the characters unique to the myxosporean phase and others typical of the actinosporean phase of genera belonging to the class Myxosporea. However, it exhibits other features which are not found in either phase. A new class Malacosporea and order Malacovalvulida are proposed to accommodate the family Saccosporidae and genus Tetracapsula. Special features of the new class are the sac-like proliferative body, valve cells not covering the exit point of the polar filament, lack of a stopper-like structure sealing the exit, maintenance of valve cell integrity even at spore maturity, absence of hardened spore walls and unique structure of sporoplasmosomes in the sporoplasms.

New phylogenomic and comparative analyses provide corroborating evidence that Myxozoa is Cnidaria

“…They are composed of two subgroups, the Malacosporea and the Myxosporea, which include more than 2000 species (Canning and Okamura, 2004;Kent et al, 2001;Lom and Dykova, 2006). Several malacosporean species have been discovered to date, including the muscular, vermiform-like parasite Buddenbrockia plumatellae (Canning et al, 2000;Hartikainen et al, 2014;Okamura et al, 2002). The sister subgroup, Myxosporea, consists of the majority of the discovered myxozoan species (Canning and Okamura, 2004;Kent et al, 2001;Yokoyama et al, 2010).…”

Evidence that infectious stages of Tetracapsula bryosalmonae for rainbow trout Oncorhynchus mykiss are present throughout the year

“…Development of the Tetracapsula species takes place within sacs which are free in the coelomic cavity of the bryozoan host (Canning et al , 2000. The ultrastructure of the spore is unique among myxozoans in that, at maturation, the spore wall is made of 4 flattened cells instead of a hardened shell, otherwise the spore possesses the features common to myxozoans.…”

“…PKX is now known as Tetracapsula bryosalmonae (Canning et al , 2000, a myxozoan parasite of bryozoans which forms, along with the related bryo-ABSTRACT: Proliferative kidney disease (PKD) is a hyperplastic condition of the lymphoid tissue of salmonids infected with the spores of Tetracapsula bryosalmonae, a myxozoan parasite formerly designated PKX, which has recently been described as a parasite of several species of bryozoans. The occurrence of PKD is generally associated with seasonal increase in water temperature, with research indicating that transmission of the disease does not occur below 12 to 13°C.…”

Complex interaction between proliferative kidney disease, water temperature and concurrent nematode infection in brown trout

“…The spread and consequences of the disease are suspected to be enhanced by water temperature (Wahli et al 2008, Bettge et al 2009, Okamura et al 2011). The disease is caused by Tetracapsuloides bryosalmonae (Myxozoa: Malacosporea) (Hedrick et al 1993, Canning et al 2000, with bryozoans as invertebrate hosts (Anderson et al 1999, Longshaw et al 1999) and salmonids as vertebrate hosts (Feist & Bucke 1993, Hedrick et al 1993. T. bryosalmonae infects fish through the skin and gills , Longshaw et al 2002.…”