Polycladida phylogeny and evolution: integrating evidence from 28S rDNA and morphology

The ultrastructure of the apical organ of Curini-Galletti's larva, a new polyclad larval type

Polycladida are the only free-living flatworms with a planktonic larval stage in some species. Currently, it is not clear if a larval stage is ancestral in polyclads, and which type of larva that would be. Known polyclad larvae are Müller's larva, Kato's larva and Goette's larva, differing by body shape and the number of lobes and eyes. A valuable character for the comparison and characterisation of polyclad larval types is the ultrastructural composition of the apical organ. This organ is situated at the anterior pole of the larva and is associated with at least one ciliary tuft. The larval apical organ of Theama mediterranea features two multiciliated apical tuft sensory cells. Six unfurcated apical tuft gland cell necks are sandwiched between the apical tuft sensory cells and two anchor cells and have their cell bodies located lateral to the brain. Another type of apical gland cell necks is embedded in the anchor cells. Ventral to the apical tuft, ciliated sensory neurons are present, which are neighbouring the cell necks of two furcated apical tuft gland cells. Based on the ultrastructural organisation of the apical organ and other morphological features, like a laterally flattened wedge-shaped body and three very small lobes, we recognise the larva of T. mediterranea as a new larval type, which we name Curini-Galletti's larva after its first discoverer. The ultrastructural similarities of the apical organ in different polyclad larvae support their possible homology, that is, all polyclad larvae have likely evolved from a common larva.

A new species of Cycloporus from the Adriatic Sea, with an updated phylogeny of the families Euryleptidae and Stylostomidae (Polycladida, Platyhelminthes)

We describe Cycloporus pinkipus sp. n., a new polyclad flatworm species from the Adriatic coast of Croatia using live images, histological sections, and a molecular marker. It is the fifteenth described species of Cycloporus Lang, 1884 and the second described congener in the Mediterranean. The genus Cycloporus is characterised by a small oval body, tentacular bumps and the name-giving marginal pores. Cycloporus pinkipus sp. n. has a smooth dorsal surface, which is transparent creamy white with light brown to yellow spots, covered with prominent serial pink spots on the inner rim of the body margin. There is little variation of the genital organs between different species of the genus, therefore we recognise C. pinkipus sp. n. as a new species in particular due to its unique coloration, and a unique partial large nuclear ribosomal subunit (28S) sequence. In recent years the family Euryleptidae Stimpson, 1857, which also contains the genus Cycloporus, was discussed and revised in several molecular studies. In an updated molecular phylogeny of the Polycladida based on partial 18S and 28S rDNA marker genes, C. pinkipus sp. n. was recovered in a clade of many other Cycloporus species within Euryleptidae.

A phylogenomic approach to resolving interrelationships of polyclad flatworms, with implications for life-history evolution

Platyhelminthes (flatworms) are a diverse invertebrate phylum useful for exploring life-history evolution. Within Platyhelminthes, only two clades develop through a larval stage: free-living polyclads and parasitic neodermatans. Neodermatan larvae are considered evolutionarily derived, whereas polyclad larvae are hypothesized to be ancestral due to ciliary band similarities among polyclad and other spiralian larvae. However, larval evolution has been challenging to investigate within polyclads due to low support for deeper phylogenetic relationships. To investigate polyclad life-history evolution, we generated transcriptomic data for 21 species of polyclads to build a well-supported phylogeny for the group. The resulting tree provides strong support for deeper nodes, and we recover a new monophyletic clade of early branching cotyleans. We then used ancestral state reconstructions to investigate ancestral modes of development within Polycladida and more broadly within flatworms. In polyclads, we were unable to reconstruct the ancestral state of deeper nodes with significant support because early branching clades show diverse modes of development. This suggests a complex history of larval evolution in polyclads that likely includes multiple losses and/or multiple gains. However, our ancestral state reconstruction across a previously published platyhelminth phylogeny supports a direct developing prorhynchid/polyclad ancestor, which suggests that a larval stage in the life cycle evolved along the polyclad stem lineage or within polyclads.

The first report of a parasitic 'turbellarian' from a cephalopod mollusc, with description of Octopoxenus antarcticus gen. nov., sp. nov. (Platyhelminthes: Fecampiida: Notenteridae)

Parasitic 'turbellarians' are known from various animals such as echinoderms, crustaceans, annelids, bivalve and gastropod molluscs. So far, however, no 'turbellarians' have been reported from cephalopods. In this paper we report a parasitic 'turbellarian' from the giant Antarctic octopus, Megaleledone setebos. We dissected two specimens of M. setebos caught in the Ross Sea (Antarctica) and found numerous worms in their intestine and liver. The worms were spherical or oblong and had two morphologically different poles. The frontal pole bears a small conical protrusion containing large elongated pear-shaped frontal glands and large polygonal cells. The ducts of the frontal glands open terminally to form the frontal organ. The caudal pole has an opening shaped as a folded tube connected by the genital pore with a common genital atrium, which continues into a canal with a muscular sheath. The worms were identified as 'turbellarians' from the family Notenteridae (Fecampiida). This family contains only one species, Notentera ivanovi, reported from the gut of a polychaete at the White Sea. The worms that we found in the gastrointestinal tract of the octopuses were morphologically similar to N. ivanovi but differed from it in several important respects. Phylogenetic analysis based on 28S rDNA gene showed that the newly found worm clustered together with other fecampiids in a highly supported clade and was closely related to N. ivanovi. On the basis of these morphological and molecular data, we described a new species, Octopoxenus antarcticus gen. nov., sp. nov. (Fecampiida: Notenteridae), establishing a new genus to accommodate it and provided an updated diagnosis of the family Notenteridae. This is the first report of a parasitic 'turbellarian' from a cephalopod mollusc.

Descriptions of two new species of Armatoplana (Polycladida: Stylochoplanidae) from the coasts of Japan, with their phylogenetic positions in Leptoplanoidea

We describe two new species of Armatoplana Faubel, 1983, namely, A. albomaculata sp. nov. and A. kaburakii sp. nov., from Japan. This is the first record of the genus from the West Pacific. Armatoplana albomaculata sp. nov. has the following characteristics: i) no nuchal tentacles; ii) white spots on the dorsal surface of the body; iii) an elongated oval prostatic vesicle directing posteriorly but curving dorsally in the distal part; iv) a long, curved penis stylet; and v) a small, oval Lang’s vesicle without accessory vesicles. Armatoplana kaburakii sp. nov. is distinguished from other congeners by having i) no nuchal tentacles; ii) a large, elongated Lang’s vesicle without accessory vesicles; and iii) gonopores opening closely to each other. We propose to re-circumscribe Armatoplana so that it would not become a junior synonym of Candimboides Prudhoe, 1982 and Phylloplana Laidlaw, 1903. We provide partial sequences of the mitochondrial cytochrome c oxidase subunit I (COI) gene as DNA barcodes for the two new species. Our phylogenetic analyses based on concatenated sequences of the 16S, 18S, and 28S ribosomal DNA and COI revealed that A. albomaculata sp. nov. and A. kaburakii sp. nov. were sister taxa; however, they did not form a monophyletic clade with Armatoplana divae (Marcus, 1947) and Armatoplana leptalea (Marcus, 1947).  

Molecular Phylogenetic Analysis of Acotylea (Platyhelminthes: Polycladida)

Acotylea is a suborder of Polycladida (Rhabditophora, Platyhelminthes) characterized by lack of a cotyl (sucker-like structure) on the ventral surface of the body. We newly determined partial sequences of two mitochondrial (16S ribosomal RNA and cytochrome c oxidase subunit I) and two nuclear (18S and 28S ribosomal RNA) genes from 24 acotylean species (12 families and 14 genera). Based on these sequences in addition to those available in public databases, we inferred the phylogeny of 16 families and 27 genera of Acotylea from molecular phylogenetic analyses (maximum likelihood and Bayesian inference) based on concatenated gene sequences. Our analyses supported three clades corresponding to Discoceloidea, Leptoplanoidea, and Stylochoidea. The phylogenetic position of Callioplanidae remains unclear. Among family- or genus-level taxa, Gnesiocerotidae, Stylochoplanidae, and Comoplana were not monophyletic. We discuss the validities of Notocomplanidae and Koinostylochus, and the family-level assignment of Mirostylochus.

A giant new species of Enchiridium (Polycladida, Prosthiostomidae) from southwestern Japan

We describe a new species of polyclad flatworm, Enchiridiumdaidaisp. nov., from the rocky subtidal zone in the East China Sea along the coasts of the Kyushu and Okinawa Islands, Japan. Enchiridiumdaidaisp. nov. is characterized by i) the entire periphery of the dorsal surface narrowly fringed with orange, ii) a marginal-eyespot band extending to the position of the mouth (about anterior one-eighth of body), and iii) two prostatic vesicles covered by a common muscle sheath, which is penetrated by the ejaculatory duct. We performed a molecular phylogenetic analysis based on 945-bp 28S rDNA sequences of 16 species of Prosthiostomidae currently available in public databases in addition to those of E.daidaisp. nov. and Prosthiostomumtorquatum Tsuyuki et al., 2019. In the resulting tree, our new species was nested in a clade composed of Enchiridium species. The tree topology was in favor of a taxonomic view that Enchiridium should be defined by having i) a common muscle sheath that encloses two prostatic vesicles and ii) marginal eyespots that may or may not surround the periphery of the dorsal surface.

Polyclad phylogeny persists to be problematic

Two conflicting morphological approaches to polyclad systematics highlight the relevance of molecular data for resolving the interrelationships of Polycladida. In the present study, phylogenetic trees were reconstructed based on a short alignment of the 28S rDNA marker gene with 118 polyclad terminals (24 new) including 100 different polyclad species from 44 genera and 22 families, as well as on a combined dataset using 18S and 28S rDNA genes with 27 polyclad terminals (19 new) covering 26 different polyclad species. In both approaches, Theamatidae and Cestoplanidae were included, two families that have previously been shown to switch from Acotylea to Cotylea. Three different alignment methods were used, both with and without alignment curation by Gblocks, and all alignments were subjected to Bayesian inference and maximum likelihood tree calculations. Over all trees of the combined dataset, an extended majority-rule consensus tree had weak support for Theamatidae and Cestoplanidae as acotyleans, and also the cotylean genera Boninia, Chromyella and Pericelis appeared as acotyleans. With the most inclusive short 28S dataset, on the other hand, there is good support for the aforementioned taxa as cotyleans. Especially with the short 28S matrix, taxon sampling, outgroup selection, alignment method and curation, as well as model choice were all decisive for tree topology. Well-supported parts of the phylogeny over all trees include Pseudocerotoidea, Prosthiostomoidea, Stylochoidea, Leptoplanoidea and Cryptoceloidea, the latter three with new definitions. Unstable positions in the tree were found not only for Theamatidae, Cestoplanidae, Boninia, Chromyella and Pericelis, but also for Anonymus, Chromoplana and Cycloporus.

No head regeneration here: regeneration capacity and stem cell dynamics of Theama mediterranea (Polycladida, Platyhelminthes)

Research on the regeneration potential of flatworms (Platyhelminthes) has been mainly undertaken with planarians (Tricladida), where most species can regenerate a head and no proliferation takes place in the blastema, i.e. the early undifferentiated regenerative tissue. Only few studies are available for an early-branching group within the Platyhelminthes, the Polycladida. Head regeneration in polyclads is not possible, with a single exception from a study performed more than 100 years ago: Cestoplana was reported to be able to regenerate a head if cut a short distance behind the brain. Here, we show that 'Cestoplana' was misdetermined and most likely was the small interstitial polyclad Theama mediterranea. We revisited regeneration capacity and dynamics of T. mediterranea with live observations and stainings of musculature, nervous system, and proliferating and differentiating stem cells. In our experiments, after transversal amputation, only animals retaining more than half of the brain could fully restore the head including the brain. If completely removed, the brain was never found to regenerate to any extent. Different from planarians, but comparable to other free-living flatworms we detected cell proliferation within the posterior regeneration blastema in T. mediterranea. Similar to other free-living flatworms, proliferation did not occur within, but only outside, the differentiating organ primordia. Our results strongly imply that brain regeneration in the absence of the latter is not possible in any polyclad studied so far. Also, it appears that proliferation of stem cells within the regeneration blastema is a plesiomorphy in flatworms and that planarians are derived in this character.

Description of a new species of Paraplehnia (Polycladida, Stylochoidea) from Japan, with inference on the phylogenetic position of Plehniidae

We describe a new species of polyclad flatworm, Paraplehniaseisuiaesp. nov., from 298–310 m depths in the Sea of Kumano, West Pacific, Japan. Paraplehniaseisuiaesp. nov. is characterized by i) a developed muscular wall proximally occupying about one-third of the prostatic vesicle, ii) no common duct between the spermiducal bulbs and the prostatic vesicle, and iii) a genital pit between the male and female gonopores. We provide a partial sequence (712 bp) of the mitochondrial cytochrome c oxidase subunit I gene as a DNA barcode for the species. Our phylogenetic analyses based on 603-bp 28S rDNA sequences indicate that P.seisuiaesp. nov. is nested in a clade consisting of stylochoid species along with unidentified species of Stylochus. It suggests that Plehniidae belongs to Stylochoidea, although this should be confirmed by future studies that contain Plehniaarctica (Plehn, 1896), the type species of the type genus of the family. The interfamily relationship among the superfamily Stylochoidea remains poorly resolved.

Systematic congruence in Polycladida (Platyhelminthes, Rhabditophora): are DNA and morphology telling the same story?

Using 28S ribosomal DNA sequences, we inferred the internal relationships of the order Polycladida. We identified morphological characters for clade support when possible. Monophyletic Acotylea and Cotylea were consistently recovered. In Acotylea, the superfamilies Stylochoidea, Cryptoceloidea and Leptoplanoidea were supported, with Stylochoidea representing the most basal acotylean lineage. In Leptoplanoidea, we united genera lacking a penis armature into the new family Notocomplanidae. Gnesiocerotidae was recovered as the most basal leptoplanoid lineage, and Stylochoplanidae and Notoplanidae were paraphyletic. Among cotyleans, Cestoplanidae, Diposthus popeae + Pericelis spp., Boniniidae, Pseudocerotidae and Prosthiostomidae formed clades. Genera in Euryleptidae were monophyletic, but the family itself was recovered with low support only. The established superfamilies Pseudocerotoidea, Euryleptoidea, Periceloidea and Chromoplanoidea are not supported. Pericelis has been moved to Diposthidae and Pericelidae has been abolished. A clade of Boniniidae + Theama spp. + Chromyella sp. was supported. In Pseudocerotidae, the number of male reproductive structures unites Pseudobiceros and Thysanozoon. Tytthosoceros has been abolished, with all currently described species now placed in Phrikoceros. Our results support several additional synonymies and taxonomic corrections. This new phylogeny provides an increased understanding of relationships in the order and offers a framework for future testing of hypotheses of character evolution and life-history strategies.

Probing recalcitrant problems in polyclad evolution and systematics with novel mitochondrial genome resources

For their apparent morphological simplicity, the Platyhelminthes or "flatworms" are a diverse clade found in a broad range of habitats. Their body plans have however made them difficult to robustly classify. Molecular evidence is only beginning to uncover the true evolutionary history of this clade. Here we present nine novel mitochondrial genomes from the still undersampled orders Polycladida and Rhabdocoela, assembled from short Illumina reads. In particular we present for the first time in the literature the mitochondrial sequence of a Rhabdocoel, Bothromesostoma personatum (Typhloplanidae, Mesostominae). The novel mitochondrial genomes examined generally contained the 36 genes expected in the Platyhelminthes, with all possessing 12 of the 13 protein-coding genes normally found in metazoan mitochondrial genomes (ATP8 being absent from all Platyhelminth mtDNA sequenced to date), along with two ribosomal RNA genes. The majority presented possess 22 transfer RNA genes, and a single tRNA gene was absent from two of the nine assembled genomes. By comparison of mitochondrial gene order and phylogenetic analysis of the protein coding and ribosomal RNA genes contained within these sequences with those of previously sequenced species we are able to gain a firm molecular phylogeny for the inter-relationships within this clade. Our phylogenetic reconstructions, using both nucleotide and amino acid sequences under several models and both Bayesian and Maximum Likelihood methods, strongly support the monophyly of Polycladida, and the monophyly of Acotylea and Cotylea within that clade. They also allow us to speculate on the early emergence of Macrostomida, the monophyly of a "Turbellarian-like" clade, the placement of Rhabditophora, and that of Platyhelminthes relative to the Lophotrochozoa (=Spiralia). The data presented here therefore represent a significant advance in our understanding of platyhelminth phylogeny, and will form the basis of a range of future research in the still-disputed classifications within this taxon.