Phylogenetic Placement of the Haplosporidia
Combined SSU rDNA and actin nucleotides
Other findings stemming from this work include the phylogenetic placement of another oyster parasite, Perkinsus as well as its freeliving doppleganger
Since their discovery in the late 1800s, the Haplosporidia have been a troublesome group for taxonomists and phylogeneticists. Historically, the taxon has been treated as a last resort for a diversity of spore-forming parasites that have multinucleated naked cells (plasmodia) in their life cycles and were not readily classifiable elsewhere (Sprague, 1979).
Caullery and Mesnil (1899) established the genus Haplosporidium for two parasites of marine annelids and placed the genus in the new order Haplosporida in the class Sporozoa of the phylum Protozoa. A major change in the classification of the Haplosporidia was the separation of the Haplosporidia and the Paramyxea Chatton 1911 from other "sporozoa" by establishment (Sprague, 1979) of the new phylum Ascetospora. This scheme proposed two classes: Stellatosporea, with orders Occlusosporida and Balanosporida for the families Marteiliidae and Haplosporidiidae respectively, and Paramyxea with the order Paramyxida for the family Paramyxidae. The family Haplosporidiidae contained only three genera-Haplosporidium, Minchinia and Urosporidium. Recently, the phylum Ascetospora has been abandoned and the Haplosporidia and Paramyxea have each been elevated to phylum rank (Deportes and Perkins 1990, Perkins 1990, 1991, 1996, Cavalier-Smith 1993). The phylum Haplosporidia has been generally accepted by most researchers; however, Corliss (1994) retained the phylum Ascetospora for both the Haplosporidia and Paramyxea.
Morphological characters have been of limited value in determining phylogenetic affinites of the Haplosporidia. Hypotheses that the haplosporidians and microsporidians are related because of the presence of microtubular organizing centers in the mitotic apparatus and similar nuclear cycles (Desportes and Nashed, 1983) have been discounted by Perkins (1996), and it is now known that the Microspora are among the earliest diverging eukaryotes (Sogin et al., 1989).
An attempt to determine phylogenetic affinities through the use of small subunit rRNA gene sequence of a single haplosporidian species, Haplosporidium nelsoni, was unsuccessful (Fong et al., 1993), but analyses of SSU rRNA gene sequences from two haplosporidian species (Siddall et al., 1995a) suggested an alveolate ancestry for the Haplosporidia. Another phylogenetic analyses (Flores et al., 1996) using SSU rRNA gene sequences from five haplosporidian taxa, including at least one taxon in each of the three genera, supported a phylogenetic affinity with the ciliates.
With a now more comprehensive suite of eukaryotic taxa (see above), those previous analyses must be put aside. The Haplosporidia do not obviously place with other major clades and their status as very unusual, unique protistan parasites seems to be corroborated here.