Abstract
The phylogeny of Kinorhyncha was analyzed using morphology and the molecular loci 18S rRNA and 28S rRNA. The different datasets were analyzed separately and in combination, using maximum likelihood and Bayesian Inference. Bayesian inference of molecular sequence data in combination with morphology supported the division of Kinorhyncha into two major clades: Cyclorhagida comb. nov. and Allomalorhagida nom. nov. The latter clade represents a new kinorhynch class, and accommodates Dracoderes, Franciscideres, a yet undescribed genus which is closely related with Franciscideres, and the traditional homalorhagid genera. Homalorhagid monophyly was not supported by any analyses with molecular sequence data included. Analysis of the combined molecular and morphological data furthermore supported a cyclorhagid clade which included all traditional cyclorhagid taxa, except Dracoderes that no longer should be considered a cyclorhagid genus. Accordingly, Cyclorhagida is divided into three main lineages: Echinoderidae, Campyloderidae, and a large clade, ‘Kentrorhagata’, which except for species of Campyloderes, includes all species with a midterminal spine present in adult individuals. Maximum likelihood analysis of the combined datasets produced a rather unresolved tree that was not regarded in the following discussion. Results of the analyses with only molecular sequence data included were incongruent at different points. However, common for all analyses was the support of several major clades, i.e., Campyloderidae, Kentrorhagata, Echinoderidae, Dracoderidae, Pycnophyidae, and a clade with Paracentrophyes + New Genus and Franciscideres (in those analyses where the latter was included). All molecular analyses including 18S rRNA sequence data furthermore supported monophyly of Allomalorhagida. Cyclorhagid monophyly was only supported in analyses of combined 18S rRNA and 28S rRNA (both ML and BI), and only in a restricted dataset where taxa with incomplete information from 28S rRNA had been omitted. Analysis of the morphological data produced results that were similar with those from the combined molecular and morphological analysis. E.g., the morphological data also supported exclusion of Dracoderes from Cyclorhagida. The main differences between the morphological analysis and analyses based on the combined datasets include: 1) Homalorhagida appears as monophyletic in the morphological tree only, 2) the morphological analyses position Franciscideres and the new genus within Cyclorhagida near Zelinkaderidae and Cateriidae, whereas analyses including molecular data place the two genera inside Allomalorhagida, and 3) species of Campyloderes appear in a basal trichotomy within Kentrorhagata in the morphological tree, whereas analysis of the combined datasets places species of Campyloderes as a sister clade to Echinoderidae and Kentrorhagata.
Highlights
The metazoan phylum Kinorhyncha was discovered by the French naturalist Felix Dujardin [1]
18S rRNA and 28S rRNA were the most commonly used loci for phylogenetic analyses of taxa with deep splits, and even though a different approach would have been used if the study was about to be started over today, the choice of loci should be understood in light of the long-term sampling effort
We find that this can be justified, since the two trees are based on the same morphological information, and that the molecular data in this context can be seen as supplementary information that enables a better outgroup comparison and rooting of the tree
Summary
The metazoan phylum Kinorhyncha was discovered by the French naturalist Felix Dujardin [1]. More than 170 years after Dujardin’s discovery, the phylum accommodates about 222 described species, distributed on 23 genera (see [3], [4] for the most recent reviews of kinorhynch classification and taxonomy). Kinorhynchs are considered part of the Ecdysozoa. This is supported by morphological [6], as well as molecular evidence [7], [8], [9]. Kinorhynchs are accommodated within the group Scalidophora [6], [10], together with Priapulida and Loricifera, and even though scalidophoran monophyly has been questioned in regard to the loriciferans [11], [12], we can at least consider Kinorhyncha and Priapulida as closely related and potential sister groups
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