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Orphan Worm Finds a Home: Buddenbrockia is a Myxozoan

2002; Oxford University Press; Volume: 19; Issue: 6 Linguagem: Inglês

10.1093/oxfordjournals.molbev.a004155

1537-1719

Ana Sara Monteiro, Beth Okamura, Peter W. H. Holland,

Protist diversity and phylogeny

The strange wormlike Buddenbrockia plumatellae and the spore-forming myxozoans are among the most enigmatic animals known to science. Dumortier and van Beneden (1850) first noted worm-shaped parasites inside freshwater bryozoan colonies; these animals were later described by Schroder (1910, 1912) and named B. plumatellae (fig. 1). This species is almost unique in never having been confidently assigned to an animal phylum, nor has a monotypic phylum been erected for it. Indeed, Nielsen (1995, p. 437) lists Buddenbrockia as one of the last ‘‘five enigmatic taxa.’’ The vermiform shape and the presence of four longitudinal muscle blocks clearly suggest a placement within the Bilateria, perhaps related to nematodes, although it should be noted that Buddenbrockia has neither a gut nor a clear central nervous system. Myxozoans, in contrast, form plasmodia or hollow sacs in which infective spores are produced. The spores attach to new hosts using penetrative filaments everted from characteristic polar capsules. Because of their anatomical simplicity, myxozoans were long placed within the protists, but rDNA and Hox gene sequences now indicate they are metazoans (Smothers et al. 1994; Anderson, Canning, and Okamura 1998). The precise phylogenetic placement of Myxozoa within the Metazoa is controversial. If polar capsules are homologues of nematocysts, this character argues for common ancestry with Cnidaria. Such a placement was supported by a combined analysis of rDNA and morphological data (Siddall et al. 1995), although several other rDNA analyses have not supported this phylogenetic position (Smothers et al. 1994; Schlegel et al. 1996; Kim, Kim, and Cunningham 1999). In addition, myxozoans have been shown to possess central class Hox genes which is thought to be a uniquely bilaterian character (Anderson, Canning, and Okamura 1998; Ferrier and Holland 2001). Here, we report 18S rDNA sequences of B. plumatellae from three widely separate geographical locations and demonstrate near identity with the previously reported 18S rDNA of the myxozoan Tetracapsula bryozoides. We conclude that Buddenbrockia is a myxozoan, a contention also made recently from ultrastructural studies (Okamura et al. 2002). We suggest that some species of Myxozoa have two alternative morphological forms, occurring as motile worms and as sporeforming sacs. Furthermore, the anatomy of Buddenbrockia provides strong support for the bilaterian affinities of the Myxozoa.