A Transcriptomic-Phylogenomic Analysis of the Evolutionary Relationships of Flatworms

Abstract

SummaryThe interrelationships of the flatworms (phylum Platyhelminthes) are poorly resolved despite decades of morphological and molecular phylogenetic studies [1, 2]. The earliest-branching clades (Catenulida, Macrostomorpha, and Polycladida) share spiral cleavage and entolecithal eggs with other lophotrochozoans. Lecithoepitheliata have primitive spiral cleavage but derived ectolecithal eggs. Other orders (Rhabdocoela, Proseriata, Tricladida and relatives, and Bothrioplanida) all have derived ectolecithal eggs but have uncertain affinities to one another. The orders of parasitic Neodermata emerge from an uncertain position from within these ectolecithal classes. To tackle these problems, we have sequenced transcriptomes from 18 flatworms and 5 other metazoan groups. The addition of published data produces an alignment of >107,000 amino acids with less than 28% missing data from 27 flatworm taxa in 11 orders covering all major clades. Our phylogenetic analyses show that Platyhelminthes consist of the two clades Catenulida and Rhabditophora. Within Rhabditophora, we show the earliest-emerging branch is Macrostomorpha, not Polycladida. We show Lecithoepitheliata are not members of Neoophora but are sister group of Polycladida, implying independent origins of the ectolecithal eggs found in Lecithoepitheliata and Neoophora. We resolve Rhabdocoela as the most basally branching euneoophoran taxon. Tricladida, Bothrioplanida, and Neodermata constitute a group that appears to have lost both spiral cleavage and centrosomes. We identify Bothrioplanida as the long-sought closest free-living sister group of the parasitic Neodermata. Among parasitic orders, we show that Cestoda are closer to Trematoda than to Monogenea, rejecting the concept of the Cercomeromorpha. Our results have important implications for understanding the evolution of this major phylum.

Nemertean and phoronid genomes reveal lophotrochozoan evolution and the origin of bilaterian heads

“…15). To test these hypotheses, we retrieved deep RNA-seq reads from 26 taxa, including annelids 16 , molluscs 16 , nemerteans 7,19,20 , phoronids 7 , ectoprocts 5,21 and brachiopods 2,7,11 . After assembling the transcriptomes de novo, we retained those of high quality (Supplementary Fig.…”

Genome and transcriptome of the regeneration-competent flatworm, Macrostomum lignano

“…flatworm | regeneration | Macrostomum | neoblast | genome F latworms belong to the superphylum Lophotrochozoa, a vast assembly of protostome invertebrates (1,2) (Fig. 1A).…”

“…1B), a free-living, regenerating flatworm isolated from the coast of the Mediterranean Sea. M. lignano is an obligatorily cross-fertilizing simultaneous hermaphrodite (9) that belongs to Macrostomorpha, whereas the other often-studied freeliving flatworms and human parasitic flatworms all belong to clades that are potentially more derived (less ancestral) in comparison with Macrostomorpha (2) (Fig. 1C).…”

Two new families (Acari: Alicorhagiidae and Platyhelminthes: Prorhynchidae) reported for the Hungarian fauna From leaf litter in the Bükk Mountains

“…Prorhynchidae is shown to be a sister group of the marine Polycladida (Laumer & Giribet 2014, Egger et al 2015, Laumer et al 2015. Superficially they resemble minute freshwater Tricladida.…”

“…Slidemounted specimens are deposited in the Zoology Collection of the Hungarian Natural History Museum (HNHM) and The Ohio State University Acarology Laboratory (OSAL). Taxonomy follows Walter et al (2013) and Egger et al (2015).…”