On the position of Archigetes and its bearing on the early evolution of the tapeworms.
June 27th, 2008 | by admin |On the position of Archigetes and its bearing on the early evolution of the tapeworms.
The tapeworm Archigetes sieboldi Leuckart, 1878 (Platyhelminthes: Cestoda: Caryophyllidea) has been oft cited as representative of the \’protocestode\’ condition owing to its lack of segmentation and ability to attain sexual maturity in the invertebrate host (aquatic oligochaetes). The idea has been variously amplified or rejected in the literature, albeit never before has the actual phylogenetic position of the species been investigated. New collections of Archigetes from both its vertebrate and invertebrate hosts provided the opportunity to do so and prompted new analyses aimed at assessing the early diversification of the Cestoda. Additional collections representing the Amphilinidea, Caryophyllidea and Gyrocotylidea were combined with published data to construct data sets of complete 18S (110 taxa) and partial (D1-D3) 28S (107 taxa) rDNA sequences, including 8 neodermatan outgroup taxa. Estimates resulting from Bayesian inference and maximum parsimony bootstrapping of the separate and combined data sets supported a derived position of the genus within the Caryophyllidea, and thus reject the idea that Archigetes may exemplify a \’primitive\’ condition. Topological constraint analyses differed somewhat between methods, but generally rejected alternative positions of the genus (e.g., as the earliest branching taxon of the Caryophyllidea or of the Eucestoda) whilst supporting basal positions of the non-segmented Caryophyllidea and Spathebothriidea relative to other major lineages of the Eucestoda, implying that segmentation was a derived feature of the common ancestor of the di- and tetrafossate eucestodes. However, both methods used for constraint analyses showed the data to be inconclusive with regard to the precise branching patterns of the cestodarian and mono- and difossate lineages, as has been the case with previous studies based on rDNA. Nevertheless, phylogenetic analyses favour the interpretation that sexual maturity of Archigetes in the invertebrate host, and similar examples in members of the Spathebothriidea, are the result of progenesis and have little if any bearing on understanding the \’protocestode\’ condition.
Olson P, Poddubnaya L, Littlewood TD, Scholz T.