Phylogenetic systematic inference in the Aspidogastrea (Platyhelminthes, Trematode) based on the 18s rRNA sequence

Abstract

The subclass Aspidogastrea ( Platyhelminthes, Trematoda) is an archaic and minor group among parasitic platyhelminths, and comprises 4 families, of which the Aspidogastridae Poche 1 1907 is divided into three subfamilies, and other three families i.e. Stichocotylidae Faust & Tang, 1936, Rugogastridae Schell, 1973, and Multicalycidae Giboson & Chinabut, 1984 only contains a single genus each. However, there are still arguments on the phylogenetic relationships among these four families. In the present study, the 18S rRNA sequences of three aspidogastrid species, i.e. Aspidogaster coachicola, Aspidogaster ijimai and Multicalyx elegans were determined, and all published 18S rRNA gene data for aspidogastreans in the GenBank database were downloaded, in order to examine the phylogenetic relationships within the Aspidogastrea. According to the molecular data, nucleotide variations of 18S rRNA sequences across all eight species of aspidogastreans ranged from 1.1 % to 9.1 %, among 6 species of the Aspidogastridae from 1.1 % to 7.7 %, between Rugogaster hydrolagi of the Rugogastridae and six aspido-gastrid species from 8.2% to 9.1 %, whereas between Multicalyx elegans of the Multicalycidae and six aspidogastrid species only from 2.7% to 6.8%. Phylogenetic analyses of the 18S rRNA with the digenean species Paragonimus westermani as outgroup by the maximum parsimony (MP) and the maximum likelihood (ML) methods produced two identical topological trees. R. hydrolagi of the Rugogastridae locating at the base of phylogenetic trees, appears to be the sister group of all other aspidogastrean species, which form two main clades, the first comprising the Aspigogaster and the Lobatostoma of the Aspidogastrinae, and M. elegans, the second the Multicotyle of the Aspidogastrinae, and the Cotylaspis and the Cotylogaster of the Cotylaspidinae. Recognizing Multicalycidae, however, would make the Aspidogastidae paraphyletic. It suggested that the genus Multicalyx should revert to the family Aspidogastridae, and the Multicalycidae be invalid family name; furthermore, the adhesive disc, with marginal organ, consisting of alveoli, the adhesive disc, without marginal organ, consisting of well separated sucker, and of rugae, shoud be identified as the synapomorphy of the Aspidogastridae, the Stichocotylidae, and the Rugogastridae, respectively. In addition, both the Aspidogastrinae and the Cotylaspidinae were shown to be non-monophyletic, implying that the number of longitudinal rows of alveoli is the parallel evolution character, and being unrelevant to subfamilial relationships in the Aspidogastridae. Therefore, the subfamilial classification of the Aspidogastridae needs to be revised by examining more genera and species of aspidogastrids