The Demise of a Phylum of Protists: Phylogeny of Myxozoa and Other Parasitic Cnidaria

Abstract

The notion that members of the phylum Myxozoa Grassé, 1970 do not properly belong in classifications of protists has frequently been suggested because the infective spores of these parasites are not unicellular. Systematists have failed to be decisive about myxozoan phylogenetic affinities, either finding the suggestion of a cnidarian connection to be preposterous or considering the recent suggestion of a relationship with nematodes to be an obvious failure of molecular phylogenetics. Thus, the group has remained in classifications as a protistan phylum in its own right. The ultrastructure of the development of myxozoans was critically re-examined in order to more fully explore the possibility of morphological synapomorphies with metazoan taxa. These morphological characters, in combination with small ribosomal subunit gene sequences, were used in a phylogenetic analysis in order to assess myxozoan origins. The results unequivocally support the inclusion of myxozoans as a clade of highly derived parasitic cnidarians, and as sister taxon to the narcomedusan Polypodium hydriforme. Reassessment of myxozoans as metazoans reveals terminal differentiation, typical metazoan cellular junctions, and collagen production. Their "polar capsules" are redescribed as typical nematocysts bearing atrichous isorhiza. Insofar as taxa cannot be contained within other taxa of equal rank, the phylum Myxozoa is abandoned and it is recommended that the group as a whole be removed from all protistan classifications and placed in a more comprehensive cnidarian system.

Comprehensive phylogenomic analyses resolve cnidarian relationships and the origins of key organismal traits

“…Here we present a well-resolved cnidarian phylogeny using genomic and transcriptomic data from representatives of all classes. Our analyses support the monophyly of Anthozoa, with the enigmatic Ceriantharia placed as sister to Hexacorallia, and corroborate previous findings of a sister relationship between Medusozoa and Endocnidozoa (Myxozoa and Polypodium hydriforme) (Chang, et al 2015;Jimenez-Guri, et al 2007;Siddall, et al 1995). Importantly, our findings strongly support the existence of a clade consisting of Staurozoa, Cubozoa and Scyphozoa, which drew only weak support in previous analyses (Zapata, et al 2015).…”

Supplementary studies on Henneguya doneci Schulman, 1962 (Myxozoa: Myxosporea) infecting the gill filaments of Carassius auratus gibelio (Bloch) in China: histologic, ultrastructural, and molecular data

“…(Vita et al 2003;Adriano et al 2005;Abdel-Ghaffar et al 2005;Azevedo et al 2008;Quraishy et al 2008). The desmosome-like junctions between two valvogenic cells in the present myxosporeans prove the generally accepted viewpoint that Myxozoa are multicellular animals (Siddall et al 1995). Capsulogenesis was evidently observed in the present study.…”

“…The uncertain information are labeled as ND (Not Determined). M Myxobolus; H Henneguya; T Tetracapsuloides formation, Myxozoa are recognized as highly degenerate cnidarians, based on the diploblastic origin of the parasites (Lom 1990;Shostak 1993;Siddall et al 1995;Siddall and Whiting 1999). Polar capsules are a typical characteristic of Myxozoa.…”

Genomic insights into the evolutionary origin of Myxozoa within Cnidaria

“…position of this clade was unstable. It was either placed as the sister clade to Bilateria or nested within Cnidaria, depending on taxon sampling, alignment, optimization method, and the characters considered (13,(19)(20)(21)(22)(23). Recent phylogenomic studies support a position of Myxozoa within Cnidaria, as the sister clade to Medusozoa (24)(25)(26).…”

“…Unlike Myxozoa, the placement of the monotypic species P. hydriforme as a cnidarian has long been proposed based on morphology (17,18). With the advent of molecular phylogenetics, it was discovered that myxozoans are not protists, but instead are metazoans (19,20). The first studies, based mainly on analyses of 18S rDNA, usually recovered Myxozoa as the sister taxon to P. hydriforme.…”