Brian S. Leander

Microturbellarians are abundant and ubiquitous members of marine meiofaunal communities around the world. Because of their small body size, these microscopic animals are rarely considered as hosts for parasitic organisms. Indeed, many protists, both free-living and parasitic ones, equal or surpass meiofaunal animals in size. Despite several anecdotal records of “gregarines”, “sporozoans”, and “apicomplexans” parasitizing microturbellarians in the literature—some of them dating back to the nineteenth century—these single-celled parasites have never been identified and characterized. More recently, the sequencing of eukaryotic microbiomes in microscopic invertebrates have revealed a hidden diversity of protist parasites infecting microturbellarians and other meiofaunal animals. Here we show that apicomplexans isolated from twelve taxonomically diverse rhabdocoel taxa and one species of proseriate collected in four geographically distinct areas around the Pacific Ocean (Okinawa, Hokkaido, and British Columbia) and the Caribbean Sea (Curaçao) all belong to the apicomplexan genus Rhytidocystis. Based on comprehensive molecular phylogenies of Rhabdocoela and Proseriata inferred from both 18S and 28S rDNA sequences, as well as a molecular phylogeny of Marosporida inferred from 18S rDNA sequences, we determine the phylogenetic positions of the microturbellarian hosts and their parasites. Multiple lines of evidence, including morphological and molecular data, show that at least nine new species of Rhytidocystis infect the microturbellarian hosts collected in this study, more than doubling the number of previously recognized species of Rhytidocystis, all of which infect polychaete hosts. A cophylogenetic analysis examining patterns of phylosymbiosis between hosts and parasites suggests a complex picture of overall incongruence between host and parasite phylogenies, and varying degrees of geographic signals and taxon specificity.

Abstract

Rhabdocoels comprise a large group of flatworms that currently consists of two major subgroups: (1) Kalyptorhynchia with an anterior proboscis and (2) Dalytyphloplanida without a proboscis. Most genera of rhabdocoels can easily be classified into one of these two subgroups, except for the three monotypic representatives of the Mariplanellinae. Comparative morphological data and previous molecular phylogenetic analyses have so far been unable to determine the positions of these rarely encountered microturbellarians within the context of rhabdocoels. Here we describe a new species of Mariplanellinae, Mariplanella piscadera sp. nov. from Curaçao (Dutch Caribbean), and present an updated phylogeny inferred from 18S and 28S rDNA sequences, including data from M. piscadera sp. nov., M. frisia, and a selection of rhabdocoels, proseriates and other trepaxonematan flatworms. Our molecular phylogenetic trees reveal that Mariplanella belongs to a distinct higher-level group of rhabdocoels that forms the sister lineage to a clade uniting Kalyptorhynchia and Dalytyphloplanida. Therefore, we update the status of Mariplanellinae to Mariplanellida status novus, containing the family Mariplanellidae status novus, in order to better reflect our current understanding of rhabdocoel phylogeny and the establishment of now three major subgroups.

Read the full article here: https://link.springer.com/article/10.1007/s13127-022-00542-2