Time to re-assess pharynx evolution in flatworms. Homology of pharynx rosulatus in rhabdocoel flatworms Dalytyphloplanida and Kalyptorhynchia

Pharynx structure was traditionally considered a major systematic character in turbellarian Plathelminthes. Recent appearance of a robust phylogenetic tree of flatworms necessitates a total re-assessment of our views on pharynx evolution and on the homology of specific types of pharynges in different groups, especially that of pharynx plicatus and subtypes of pharynx bulbosus. In the present study, basing on morphology and on the recent phylogenies we test the homology of the rosette-shaped pharynx (pharynx rosulatus) in Rhabdocoela. Using immunohistochemistry, phalloidin staining and confocal laser scanning microscopy we compare pharynx musculature, pharynx innervation and nervous system in representatives of the two rhabdocoel sister-groups: Dalytyphloplanida (Castrada hofmanni) and Kalyptorhynchia (Gyratrix hermaphroditus). Both pharynges have a set of common characters (presence of a sphincter around the opening with a set of radial muscles; both the inner and outer walls of the pharynx are composed of two layers: inner circular muscles and outer longitudinal ones; septal (radial) muscles interconnect the inner and the outer pharynx walls; presence of retractor muscles; innervation of the pharynx by inner and outer layers of FMRFamide IR nerve fibers associated with 1–2 pairs of pharynx neurons). Both species studied also have similar nervous system patterns classified under concentrated orthogon type. We conclude that the rosulate pharynges are indeed homologous within Rhabdocoela. The possibility of pharynx rosulatus serving as an apomorphy for Rhabdocoela is discussed. Also we provide some evidence that pharynx rosulatus may have evolved directly from pharynx simplex, bypassing the pharynx plicatus type. How to cite this article: Raikova O.I., Kotikova E.A., Korgina E.M., Drobysheva I.M. 2019. Time to re-assess pharynx evolution in flatworms. Homology of pharynx rosulatus in rhabdocoel flatworms Dalytyphloplanida and Kalyptorhynchia // Invert. Zool. Vol.16. No.1. P.57–70. doi: 10.15298/invertzool. 16.1.07