Polacanthoderes, Sorensen, 2008

Yamasaki, Hiroshi, Fujimoto, Shinta, Tanaka, Hayato, Shimada, Daisuke, Ito, Masato, Tokuda, Yuki & Tsujimoto, Megumu, 2022, Taxonomy, genetic diversity, and phylogeny of the Antarctic mud dragon, Polacanthoderes (Kinorhyncha: Echinorhagata: Echinoderidae), Zoologischer Anzeiger (Zool. Anz.) 301, pp. 42-58 : 52

3.3. Phylogenetic position of Polacanthoderes

The BI and ML analyses produced trees of very similar topology, hence we show the ML tree with nodal support (ML bootstrap values (BS) and BI posterior probability (PP)) in Fig. 10 . The resulted topology is mostly congruent with those of the previous phylogenetic and phylogenomic studies of kinorhynchs ( Dal Zotto et al., 2013; Herranz et al., 2022; Sørensen et al., 2015; Yamasaki et al., 2013). The only remarkable difference among the new and previous results is the position of the clade Paracentrophyes + Franciscideridae ( Franciscideres and Gracilideres ): it is represented as the sister clade of Pycnophyidae in the analyses based on the 18S and 28S gene sequences ( Yamasaki et al., 2013; Sørensen et al., 2015; this study), whereas it composes the Anomoirhaga clade together with Dracoderes and Cateria in the phylogenomic analyses ( Herranz et al., 2022). Aside from the difference between this and previous analyses found in the Allomalorhagida clade, the relationships of cyclorhagids ( Echinorhagata , Kentrorhagata , and Xenosomata ) are consistent throughout all analyses.

In our results, the Echinorhagata / Echinoderidae clade including Polacanthoderes , Cephalorhyncha , Echinoderes , and Meristoderes was detected with high support values (nodal support bootstrap value/posterior probability = 100/1.00). In this clade, Polacanthoderes branches off most basally, representing the sister taxon to a clade with the remaining three genera. The clade Cephalorhyncha + Echinoderes + Meristoderes was also supported with high values (99/1.00), however, the monophyly of any of the three genera was not supported.

The phylogenetic position of Polacanthoderes detected by this study does not conflict with the previous analyses based on morphology ( Sørensen, 2008b) or the total evidence analysis ( Sørensen et al., 2015) but indicates its position more clearly. Both of the previous studies suggested the inclusion of Polacanthoderes in the family Echinoderidae , which is strongly supported by the current results. Sørensen (2008b) also suggested that Polacanthoderes is the most basal taxon of the family, being the sister taxon of the clade consisting of three echinoderid genera ( Cephalorhyncha , Echinoderes , and Fissuroderes ). In the total-evidence analysis in Sørensen et al. (2015), Polacanthoderes composed a clade together with Fissuroderes , but the root of the family appeared as an unresolved polytomy and failed to clarify the relationships between the Polacanthoderes + Fissuroderes clade and the remaining three echinoderid genera. Our new results support that Polacanthoderes is one of the most basal taxa of Echinoderidae , and Cephalorhyncha , Echinoderes , and Meristoderes are more closely related to each other than to Polacanthoderes . Because our analyses lack Fissuroderes data, the phylogenetic position of Fissuroderes remains unclear. Nevertheless, considering the phylogenetic position of Polacanthoderes , there is no doubt that the genus is one of the key taxa to detect the evolution of Echinoderidae . Further analysis with broader taxon sampling, including Fissuroderes and various Echinoderes species, will provide a more detailed understanding of the phylogenetic relationships within the family Echinoderidae and their evolutionary history in the group.

Figures

Fig. 10. Maximum-likelihood tree of Kinorhyncha based on 18S (1353 bp) + 28S (2750 bp) dataset. Numbers near nodes are the maximum-likelihood bootstrap (BS) and Bayesian posterior probability (PP) values, respectively; asterisks indicate the node with the full support values (BS = 100% and PP = 1.00); BS values lower than 50% indicate as polytomy; PP values lower than 0.95 are indicated by dashes. The scale bar indicates branch length in substitutions per site.