A phylogenetic analysis of the Metazoa with special consideration of the Acanthocephala

grantor: University of TorontoThe purpose of this study was to examine phylogenetic relationships among the Metazoa, identify appropriate outgroups to the Acanthocephala, and make a phylogenetic analysis of the Acanthocephala. A database of 144 characters for the metazoan phyla was obtained by examination of primary and secondary literature. The Choanoflagellata was used as outgroup for 33 multicellular phyla. Phylogenetic analysis yielded 17 equally parsimonious trees. Using successive weighting, these were reduced to two trees (Length = 361; CI = 0.46; HI = 0.58) differing only in placement of the Platyhelminthes. Monophyly of the Mesozoa was not supported but mesozoans were placed basal to the Porifera + (Cnidaria + (Ctenophora + remaining Metazoans)). The remaining Metazoans formed two major clades. The Acanthocephala was the basal member of a clade, the Ecdysozoa, containing Rotifera + (Gastrotricha + ((Nematoda + Nematomorpha) + (Pentastomida + (Kinorhynchida + (Tardigrada + (Onychophora + (Chelicerata + (Uniramia + Crustacea)))))))). The second clade consisted of two sub-clades; Chaetognatha basal to Chordata + (Echinodermata + (Phornida + Brachiopoda)) and Priapulida basal to Gnathostomulida + (Platyhelminthes + Nemertea + Mollusca + (Sipuncula + (Echiura + (Pogonophora + (Clitellata + Polychaeta))))). A novel approach to identification of early cleavage patterns involved coding of individual cleavage events as independent characters. Reinterpretation of the classic assignment of 'spiral' cleavage to arthropods supported placement within the Ecdysozoa. Deuterostomes were only monophyletic by including protostomes, thus 'spiral' cleavage is derived from 'radial' cleavage. The position of the Ecdysozoa as sister-group to the deuterostomes + protostomes supports the conclusion that 'modified-spiral' cleavage is not derived from 'spiral' cleavage. Priapulida and Rotifera were supported as outgroups for the analysis of relationships among the Acanthocephala. Direct examination of specimens identified 138 characters for 21 well-defined genera. Phylogenetic analysis yielded a single tree (Length = 404; CI = 0.545; HI = 0.455) which provided support for the monophyly of the Palaeacanthocephala and Eoacanthocephala but not the Archiacanthocephala, which were placed basal to the two other classes. To rectify taxonomic problems encountered while characters were being evaluated, 'Koronacantha', was described with a single species, ' K. mexicana'. 'Tegorhynchus brevis' was redescribed and 'T. pectinarius' transferred to 'Koronacantha'. Results supported placement of 'T. pectinarius' in ' Koronacantha' and retention of both 'Tegorhynchus' and 'Illiosentis' as distinct genera.Ph.D