ИСТИНА

Orthonectida and Dicyemida are marine invertebrate parasites with extremely simplified morphological organization. They were previously (in the XIX century) considered as Mesozoa - a transitional link between protists and multicellular animals. With the advent of molecular data, the orthonectids and dicyemids have been firmly placed within Spiralia or Lophotrochozoa; however, the exact phylogeny of the Mesozoan taxa remains difficult to solve even with genomic or transcriptomic data due to the extreme sequence divergence. Some reports recognize monophyly of the “Mesozoa”, others reject it. In our 450 gene phylogeny and mitochondrial genome analyses, we have found that Orthonectida and Dicyemida emerged independently within the Lophotrochozoa, with the orthonectids appearing within the crown annelids, and dicyemids as an isolated branch of lophotrochozoans, confirming earlier analyses. The site-heterogeneous substitution model (CAT) is necessary for resolving the orthonectids within the annelid assemblage. In mitochondrial genome analyses, Orthonectida groups with a crown annelid Lobatocerebrum sp. Orthonectids are among the taxa that were previously considered as separate phyla or classes (Echiura, Sipuncula, Pogonophora, Vestimentifera, Myzostomida, Lobatocerebridae, and Diurodrilus) but are now seen as deviant annelids. Dicyemida do not group with Orthonectida or other Annelida, which suggests that Mesozoa is a polyphyletic taxon.