Ichthyosporea

Note: this tree is still under construction. It does not yet contain all known Ichthyosporea subgroups.

Introduction

Ichthyosporeans, equally called mesomycetozoeans, are unicellular osmotrophic feeders. The lineage was first described in 1996 (Ragan et al.) based on four sequenced taxa, and now encompasses over 25 genera (Glocking et al. 2013) (Table 1). All described species have been isolated from or described within an animal host however, sequences are frequently detected in environmental sampling surveys, suggesting either free-living or long lived dispersal stages amongst some clades (del Campo and Ruiz-Trillo 2013). Six genera are in axenic culture (Pirum, Abeoforma, Amoebidium, Ichthyophonus, Sphaeroforma, and Creolimax (Marshall and Berbee 2011, 2013; Marshall et al. 2008; Jostensen et al. 2002; Okamoto et al. 1985; Whisler 1962). Ichthyosporeans are presumed to be the basal most lineage on the branch of the tree of life leading to the multi-cellular animals (Holozoa), and as such, are a valuable group for studying the transition from unicellular to multicellular existence. Recently, following the development of a successful transformation protocol, Creolimax fragrantissima has been proposed as a new model organism (Suga et al. 2013).

Characteristics

The ichthyosporea grow as multinucleate coenocytes and almost always have a cell wall (Figure 1). Depending on the species, environmental condition, stage of life-cycle, or other unknown cues posteriorly flagellate zoospores, spores, or motile amoebae are released (Mendoza et al. 2002) (Table 1). Binary fission and budding have been observed in a few species (Table 1). Motile plasmodia are also found in some cultured species (Okamoto et al. 1985; Marshall et al. 2008; Marshall and Berbee 2011, 2013). Asexual reproduction is predominant but scaliform conjugation, presumably leading to meiosis, was described for Enteropogon sexuale (Hibbits 1978) and genetic evidence of recombination was shown in Sphaeroforma tapetis (formerly Pseudoperkinsus) (Marshall and Berbee 2010). Sphaeroforma was shown to be haploid (Marshall and Berbee 2010), but ploidy has not been described for other species. The composition of the cell walls has not been determined. Carbohydrates are present in Amoebidium parasiticum and Ichthyophonus but neither chitin or cellulose have been identified from wall extracts (Trotter and Whisler, 1965; Franco-Sierra and Alvarez-Pellitero 1999). Other physiology, such as nutritional requirements, are largely unknown.

Ecology and Host Relationships

All described ichthyosporeans were found in association with animals. These relationships range from pathogenic to commensal, and are often not well described. They can either be found embedded within host tissues (e.g. Lohr et al. 2010; Franco-Sierra and Alvarez-Pellitero 1999; Raffel et al. 2008; Pascolini et al. 2003; Savino and Margo 1983), within the digestive lumen (Beebee 1991) or attached to the cuticle of arthropod hindguts (e.g. Mayfield and Lichtwardt 1980; Hibbits 1978). Species described from fish, amphibians, and mammals typically have negative impacts on the host whereas relationships with insect hosts are more likely commensal (Glocking et al. 2013). Population wide effects following epizootics have been described in fish (Kocan et al. 1999). Evidence of emerging pathogenicity has occurred through hitch-hiking on an invasive species (Gozlan et al. 2005) and by pathogen amplification and spillback following the introduction of a naïve host (Hershberger et al. 2010). Broad host ranges have been demonstrated for Ichthyophonus hoferi (Jones and Dawe 2002), Sphaerothecum destruens (Andreou et al. 2012), C. fragrantissima (Marshall et al. 2008), and Sphaeroforma species (Marshall and Berbee 2013). The human parasite, Rhinosporidium seeberi, has also been described in swans, dogs and horses (Kennedy et al. 1995; Leeming et al. 2007; Silva et al. 2005) but ITS variation suggests there may be host correlated strain differences (Silva et al. 2005).

Morphology

In form, ichthyosporeans range from long and thread-like to spherical. Individual cells may reach sizes of 500 micrometres in diameter for some spherical species. Members of some genera (Amphibiothecum, Dermocystidium, Amphibiocystidium, and Rhinosporidium) can form cysts composed of numerous tightly packed spherical cells within the tissues of their hosts (Ashworth 1923; Broz and Privora 1951; Olson et al. 1991; Lotman et al. 2000; Pascolini et al. 2003; Pereira et al. 2005; Raffel et al. 2008). Eccrinales and Amoebidiales secrete holdfasts through pores in their cell walls to attach to their arthropod hosts (Whisler and Fuller 1968; Mayfield and Lichtwardt 1980). The morphology of cultured genera Abeoforma, Pirum, Creolimax and Sphaeroforma is not known under in vivo conditions but Sphaeroforma cells have an external calyx which may function in adhesion (Marshall and Berbee 2013). Amoebae are variously formed with either single prominent pseudopods (C. fragrantissima) or can be more delicately branched as in Abeoforma whisleri. Large motile plasmodia and/or hyphae-like stages have been reported in Dermocystidium koi, Ichthyophonus hoferi, Sphaeroforma tapetis, Creolimax fragrantissima, and Abeoforma whisleri (Dykova and Lom 1992; Franco-Sierra and Alvarez-Pellitero 1999; Marshall et al. 2008; Marshall and Berbee 2013).

Common ultrastructure features include glycogen deposits, membrane bound extensions of the plasma membrane and microtubule organizing centres (Marshall et al 2008; Marshall and Berbee 2011, 2013). Centrioles were observed in Dermocystidium cyprini (Lotman et al. 2000), a species known to produce posteriorly flagellate zoospores. Mitochondria typically have plate-like cristae but tubular cristae are described from Ichthyophonus hoferi (Spanggaard et al. 1996; Franco-Sierra and Alvarez-Pellitero 1999). Cell walls range from thin with single layers, for example in Anurofeca richardsi (Beebee 1991), to multi-layered as in I. hoferi, Psorospermium haeckeli, and A. whisleri (Spanggaard et al 1996; Vogt and Rug, 1999; Marshall and Berbee 2011).

Discussion of Phylogenetic Relationships

The branching order of the unicellular relatives of animals is still in question but most multi-gene phylogenies position the ichthyosporea as the basal-most branch of the metazoan side of the animal-fungus divide (Paps et al. 2013; Toruella et al. 2012; Ruiz-Trillo et al 2006; Steenkamp et al 2006; Ruiz-Trillo et al 2008; Shalchian-Tabrizi et al. 2008). Within the ichthyosporea, there are two clearly distinguished lineages, most commonly referred to as the Ichthyophonida and the Dermocystida. This division is further supported by ecological and morphological differences (Mendoza et al 2002).

Nomenclature

Members of the ichthyosporea were first described in the mid-nineteenth century from arthropod hindguts (e.g. Leidy 1849). New discoveries were misplaced within the fungi, achlorophyllous green algae, or other protist groups until molecular phylogenetic analysis (Baker et al 1999; Benny and O’Donnell 2000; Ustinova 2000; Cafaro 2005). The first recognition of the Ichthyosporea as a group began with the sequencing of four parasites of fish and shellfish (Dermocystidium, the Rosette agent, Ichthyophonus, and Psorospermium), thus inaugurating the lineage with the acronym ‘DRIPs’ (Ragan et al. 1996). In 1998 the clade was given the name ‘Ichthyosporea’ by Cavalier-Smith. Herr et al, (1999), changed this to ‘Mesomycetozoa’ to reflect the phylogenetic affinity near the animal fungus divergence. In 2001, Mendoza et al amended this to ‘Class Mesomycetozoea’. Eukaryote classification schemes still use the original ‘Ichthyosporea’ (Adl et al. 2005, 2012).

Table 1

Table 1. Genera of Ichthyosporea with their hosts and reproductive mechanisms (This information was obtained from the references cited in the text.)

Other Names for Ichthyosporea

• Mesomycetozoa Herr et al 1999
• Mesomycetozoea Mendoza et al. 2001
• Vernacular Names: DRIPs

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