Analysis of 5.8S rDNA and internal transcribed spacer 1 (ITS1) sequences of ascaridoid nematodes: Phylogenetic signal and hypothesis testing

Adult forms of the nematodes belonging to the superfamily Ascaridoidea infect the vertebrate digestive system, and some of them are agents of human and stock disease. With more than 50 genera, this group of animals shows a marked variation in morphology and life histories. In this work we present a phylogenetic analysis of the ribosomal DNA region (5.8S rRNA gene and the surrounding ITSs) of the representatives of Ascaridoidea belonging to six genera and three families; eleven new sequences have been obtained in this study. We use several methods to investigate the support for the possible scenarios in the evolution of the Ascaridoidea, using the data from the 5.8S rDNA region and also previously published sequences of small and large subunit rDNA and the mitochondrial gene cox2. The present results support the hypothesis that monoxeny evolved independently among the various ascoridoid taxa. For the genera Contracaecum and Parrocaecum, the data were consistent with the monophyly of bird parasites (in the case of Contracaecum, vs. Contracaecum species that infect mammals, in the case of Porrocaecum vs. mammalian parasites in a related genus, Toxocara). This suggests that the physiology of the definitive host forms a strong selection barrier. However, our results suggest that the phylogenetic signal in the ascaridoid molecular sequence data is low. We investigate if this can be detected comparing three methods to measure the phylogenetic signal (likelihood mapping, δ plots, and split decomposition). In the case of the data analysed or re-analysed here, only a careful analysis using proper statistical methods allows to detect the situation of low signal. Using just one method for measuring the phylogenetic signal and presenting an optimal tree with high support for the interior branches without investigating the alternatives in the hypothesis-testing setting might lead to overestimating the support for some phylogenetic relationships. We conclude that an analysis of a dataset containing a large number of genes (and not just sequences from one region) may be necessary to establish the evolutionary relationships between the ascaridoid taxa.