A CLADISTIC ANALYSIS OF PSEUDOCOELOMATE (ASCHELMINTH) MORPHOLOGY

摘要

We investigated possible relationships among pseudocoelomates (aschelminths) by using cladistic analysis to uncover all most-parsimonious trees from a data set comprising 45 morphological characters of 11 phyla (3 acoelomate, 7 pseudocoelomate, and 1 coelomate). Analysis of our matrix yielded 2 most-parsimonious trees, differing only in relative placement of 3 taxa (Kinorhyncha, Loricifera, and Priapulida). Tree topology suggests that the pseudocoel either evolved twice, or that it evolved once and was then independently lost in Gastrotricha and in Gnathostomulida. Tree topology also is consistent with derivation of the pseudocoel from a coelom. From our analysis and that of other morphologically-based studies, we conclude that there are 2 main lines of pseudocoelomate evolution: (1) a minor clade (Acanthocephala + Rotifera) defined principally by a syncytial epidermis, intracytoplasmic lamina, and sperms with anterior flagella; (2) a major clade (the 5 remaining phyla), defined primarily by a cuticle that is molted. Within the major clade are 2 subclades (Nematoda + Nematomorpha; Kinorhyncha + Loricifera + Priapulida). Using morphological data, the chief issues remaining to be resolved are placement of Gastrotricha and Gnathostomulida, and evolution of the pseudocoel. No analysis based solely on morphological data has so far produced a tree congruent with those based on molecular data. Nevertheless, there is a general consensus that (1) the pseudocoel is of little phylogenetic significance and (2) the pseudocoelomates are probably polyphyletic. [References: 66]