A robust molecular phylogeny of the Tricladida (Platyhelminthes: Seriata) with a discussion on morphological synapomorphies

The invasive land flatworm Arthurdendyus triangulatus has repeated sequences in the mitogenome, extra-long cox2 gene and paralogous nuclear rRNA clusters

Using a combination of short- and long-reads sequencing, we were able to sequence the complete mitochondrial genome of the invasive 'New Zealand flatworm' Arthurdendyus triangulatus (Geoplanidae, Rhynchodeminae, Caenoplanini) and its two complete paralogous nuclear rRNA gene clusters. The mitogenome has a total length of 20,309 bp and contains repetitions that includes two types of tandem-repeats that could not be solved by short-reads sequencing. We also sequenced for the first time the mitogenomes of four species of Caenoplana (Caenoplanini). A maximum likelihood phylogeny associated A. triangulatus with the other Caenoplanini but Parakontikia ventrolineata and Australopacifica atrata were rejected from the Caenoplanini and associated instead with the Rhynchodemini, with Platydemus manokwari. It was found that the mitogenomes of all species of the subfamily Rhynchodeminae share several unusual structural features, including a very long cox2 gene. This is the first time that the complete paralogous rRNA clusters, which differ in length, sequence and seemingly number of copies, were obtained for a Geoplanidae.

Phylogenetic relationships of the Geoplaninae land planarians (Platyhelminthes, Tricladida) assessed with a total evidence approach, with the description of a new species of Gigantea

Several studies have focused on the phylogenetic relationships within the Geoplaninae land planarians (Tricladida). In those studies, ancient phylogenetic relationships remained obscure. In this work, the phylogeny of Geoplaninae is assessed through three different datasets, namely morphological, molecular, and both datasets combined, i.e, a total evidence approach (TE). The data matrix consisted of six DNA regions, including a newly developed marker (DOM5), and a morphological matrix with 37 characters. The study produced the best-resolved hypothesis so far for the phylogeny of Geoplaninae, although ancient clades still remain elusive. The effect of the morphological data on the TE tree topology and clade support is seemingly negligible. The phylogenetic tree also suggests that most of the diagnostic morphological characters of the genera are homoplastic, while unambiguous unique synapomorphies can characterize some supra-generic informal groupings.

The discovery of new Chilean taxa revolutionizes the systematics of Geoplaninae Neotropical land planarians (Platyhelminthes: Tricladida)

Two subfamilies of land planarians (Geoplanidae) are endemic to the Neotropical region, namely Geoplaninae (with 29 genera and 346 nominal species, most of which are from Brazil) and its sister-group Timyminae, with only two Chilean species. The systematics of these groups through morphology and molecular data (COI and 28S rDNA genes), including nine new Chilean species, is re-assessed in this study. The great morphological diversity of the Chilean species is congruent with the molecular trees and, accordingly, five new genera (Adinoplana, Harana, Myoplana, Sarcoplana and Transandiplana) are proposed, each characterized by putative synapomorphies. Seven new tribes are also erected (Adinoplanini, Gusanini, Haranini, Inakayaliini, Myoplanini, Polycladini and Sarcoplanini), each one monogeneric, except Geoplanini (which includes all genera under the current concept of Geoplaninae plus the Chilean Transandiplana) and Sarcoplanini (with Sarcoplana and the already known Mapuplana, Pichidamas and Wallamapuplana). Re-diagnoses of Geoplaninae, Timymini, Gusana, Inakayalia, Polycladus and Pichidamas are proposed and biogeographic remarks on Transandiplana are provided

Hammerhead flatworms (Platyhelminthes, Geoplanidae, Bipaliinae): mitochondrial genomes and description of two new species from France, Italy, and Mayotte

BACKGROUND

New records of alien land planarians are regularly reported worldwide, and some correspond to undescribed species of unknown geographic origin. The description of new species of land planarians (Geoplanidae) should classically be based on both external morphology and histology of anatomical structures, especially the copulatory organs, ideally with the addition of molecular data.

METHODS

Here, we describe the morphology and reproductive anatomy of a species previously reported as Diversibipalium "black", and the morphology of a species previously reported as Diversibipalium "blue". Based on next generation sequencing, we obtained the complete mitogenome of five species of Bipaliinae, including these two species.

RESULTS

The new species Humbertium covidum n. sp. (syn: Diversibipalium "black" of Justine et al., 2018) is formally described on the basis of morphology, histology and mitogenome, and is assigned to Humbertium on the basis of its reproductive anatomy. The type-locality is Casier, Italy, and other localities are in the Department of Pyrénées-Atlantiques, France; some published or unpublished records suggest that this species might also be present in Russia, China, and Japan. The mitogenomic polymorphism of two geographically distinct specimens (Italy vs France) is described; the cox1 gene displayed 2.25% difference. The new species Diversibipalium mayottensis n. sp. (syn: Diversibipalium "blue" of Justine et al., 2018) is formally described on the basis of external morphology and complete mitogenome and is assigned to Diversibipalium on the basis of an absence of information on its reproductive anatomy. The type- and only known locality is the island of Mayotte in the Mozambique Channel off Africa. Phylogenies of bipaliine geoplanids were constructed on the basis of SSU, LSU, mitochondrial proteins and concatenated sequences of cox1, SSU and LSU. In all four phylogenies, D. mayottensis was the sister-group to all the other bipaliines. With the exception of D. multilineatum which could not be circularised, the complete mitogenomes of B. kewense, B. vagum, B. adventitium, H. covidum and D. mayottensis were colinear. The 16S gene in all bipaliine species was problematic because usual tools were unable to locate its exact position.

CONCLUSION

Next generation sequencing, which can provide complete mitochondrial genomes as well as traditionally used genes such as SSU, LSU and cox1, is a powerful tool for delineating and describing species of Bipaliinae when the reproductive structure cannot be studied, which is sometimes the case of asexually reproducing invasive species. The unexpected position of the new species D. mayottensis as sister-group to all other Bipaliinae in all phylogenetic analyses suggests that the species could belong to a new genus, yet to be described.

A new species of alien terrestrial planarian in Spain: Caenoplana decolorata

Terrestrial planarians found in a plant nursery in Spain in 2012 are described as a new species, Caenoplana decolorata. Dorsally they are mahogany brown with a cream median line. Ventrally they are pastel turquoise fading to brown laterally. Molecular data indicate that they are a member of the genus Caenoplana, but that they differ from other Caenoplana species found in Europe. One mature specimen has been partially sectioned, and the musculature and copulatory apparatus is described, confirming the generic placement but distinguishing the species from other members of the genus. It is probable that the species originates from Australia.

The evolutionary terrestrialization of planarian flatworms (Platyhelminthes, Tricladida, Geoplanidae): a review and research programme

The terrestrialization of animal life from aquatic ancestors is a key transition during the history of life. Planarian flatworms form an ideal group of model organisms to study this colonization of the land because they have freshwater, marine, and terrestrial representatives. The widespread occurrence of terrestrial flatworms is a testament to their remarkable success occupying a new niche on land. This lineage of terrestrial worms provides a unique glimpse of an evolutionary pathway by which a group of early divergent aquatic, invertebrate metazoans has moved onto land. Land flatworms are among the first groups of animals to have evolved terrestrial adaptations and to have extensively radiated. Study of this terrestrialization process and the anatomical key innovations facilitating their colonization of the land will contribute greatly to our understanding of this important step in Metazoan history. The context and scientific background are reviewed regarding the evolutionary terrestrialization of land flatworms. Furthermore, a framework of a research programme is sketched, which has as its main objective to test hypotheses on the evolution of land planarians, specifically whether particular anatomical and physiological key innovations have contributed to their evolutionary successful terrestrial colonization and radiation. In this context special attention is paid to the respiration in aquatic and terrestrial planarians. The research programme depends on a comprehensive phylogenetic analysis of all major taxa of the land flatworms on the basis of both molecular and anatomical data. The data sets should be analyzed phylogenetically with a suite of phylogenetic inference methods. Building on such robust reconstructions, it will be possible to study associations between key innovations and the evolutionary terrestrialization process.

First record of the New Guinea flatworm Platydemus manokwari (Platyhelminthes, Geoplanidae) as an alien species in Hong Kong Island, China

The New Guinea flatworm (Platydemus manokwari) caused extinction of the native land snails on several Pacific island in past decades, and therefore it has been listed among the top 100 of the world's worst invasive alien species. Using morphological and molecular methods, New Guinea flatworms were discovered and identified for the first time in Hong Kong Island during a field investigation in July and August 2018. The flatworms were 32-60 mm long, 3-5 mm wide, and 1-2 mm thick. The dorsal side of the flatworm was dark brown with a thin yellow central line, and its ventral side appeared pale grey. To further verify this species, both 18S rDNA and mitochondrial cytochrome c oxidase subunit I gene (COX1) obtained from three specimens of P. manokwari were sequenced and analysed. While comparing these sequences with those previously deposited in GenBank, these 18S rDNA sequences shared 100% identity with the single available 18S rDNA sequence of P. manokwari; and the obtained COX1 sequences were identical to those of P. manokwari world genotype. Two native snails, Criptosoma imperator and Bradybaena similaris, have been found to be the prey of this predator during this investigation. Therefore, the invasive New Guinea flatworm certainly will cause a serious impact on the biodiversity of native snail populations, and an economic and environmental risk assessment for P. manokwari need to be completed in the near future in Hong Kong.

The complete mitochondrial genome of the Antarctic marine triclad, Obrimoposthia Wandeli (Platyhelminthes, Tricladida, Maricola)

For the first time, we report the complete mitochondrial genome (mitogenome) sequence of the marine triclad species, Obrimoposthia wandeli. The complete mitogenome of O. wandeli was 15,185 bp in length, contains 12 protein-coding genes (PCGs), 22 transfer RNAs (tRNAs), and 2 ribosomal RNAs (rRNAs). Compared to previously reported gene orders from the order Tricladida, O. wandeli had unique gene order. We constructed a phylogenetic tree based on the mitogenomes belonging to Rhabdocoela, Polycladida, and Tricladida and confirmed that O. wandeli is located in the basal position in the Tricladida.

Dawn of astome ciliates in light of morphology and time-calibrated phylogeny of Haptophrya planariarum, an obligate endosymbiont of freshwater turbellarians

Morphology, systematic position and time-calibrated phylogeny of Haptophrya planariarum were investigated. This endosymbiont of freshwater turbellarians is characterized by: (i) a length of about 200-900 μm; (ii) a campanulate to truncate claviform body carrying an anterior adhesive sucker; (iii) an ellipsoidal macronucleus localized in the rear body end; (iv) a contractile canal extending along the dorsal margin; and (v) usually more than 150 meridional ciliary rows, a horseshoe-shaped suture line along the sucker, and two inconspicuous secant systems at lateral ends of the suture line. In 18S rRNA gene phylogenies, astomes were depicted as a non-monophyletic group within the scuticociliate clade, whereby H. planariarum clustered with the loxocephalid genus Dexiotricha. After considering morphological evidence, statistical tree topology tests and evolutionary distances, we find astomes as a distinct group that evolved from a free-living scuticociliate ancestor in the early Paleozoic. Molecular clock analyses indicated that astomes living in annelids diverged from those inhabiting turbellarians within about 50 Ma during the Late Cambrian and the Upper Ordovician. This comparatively short time span might have not sufficed for fixation of molecular synapomorphies in the 18S rRNA gene and/or they might have been erased by substitutions during the almost 500 Ma-long evolutionary history of astomes.

Planarian Diversity and Phylogeny

Hundreds of planarian species exist worldwide, representing a rich phenotypic diversity. This chapter presents an overview of the morphology and anatomy of various taxonomic groups of planarian flatworms, focusing on features enabling recognition and identification of the animals. The most recent view on the phylogenetic relationships of the planarians is presented, together with geographic distribution patterns of major groups of triclads. The chapter concludes with a brief methodological section outlining species identification on basis of anatomical features. In conjunction with the established laboratory model species, the phenotypic diversity of planarians provides rich opportunities for comparative studies, which this chapter aims to inspire.

Diversity of introduced terrestrial flatworms in the Iberian Peninsula: a cautionary tale

Many tropical terrestrial planarians (Platyhelminthes, Geoplanidae) have been introduced around the globe. One of these species is known to cause significant decline in earthworm populations, resulting in a reduction of ecological functions that earthworms provide. Flatworms, additionally, are a potential risk to other species that have the same dietary needs. Hence, the planarian invasion might cause significant economic losses in agriculture and damage to the ecosystem. In the Iberian Peninsula only Bipalium kewense Moseley, 1878 had been cited till 2007. From that year on, four more species have been cited, and several reports of the presence of these animals in particular gardens have been received. In the present study we have: (1) analyzed the animals sent by non-specialists and also the presence of terrestrial planarians in plant nurseries and garden centers; (2) identified their species through morphological and phylogenetic molecular analyses, including representatives of their areas of origin; (3) revised their dietary sources and (4) used Species Distribution Modeling (SDM) for one species to evaluate the risk of its introduction to natural areas. The results have shown the presence of at least ten species of alien terrestrial planarians, from all its phylogenetic range. International plant trade is the source of these animals, and many garden centers are acting as reservoirs. Also, landscape restoration to reintroduce autochthonous plants has facilitated their introduction close to natural forests and agricultural fields. In conclusion, there is a need to take measures on plant trade and to have special care in the treatment of restored habitats.

Phylogenetic relationships within the flatworm genus Choeradoplana Graff (Platyhelminthes : Tricladida) inferred from molecular data with the description of two new sympatric species from Araucaria moist forests

The genus Choeradoplana encompasses 11 species, nine of which have a restricted distribution and are only known from their type localities. Herein we describe two new species of Choeradoplana from Araucaria moist forests, C. minima, sp. nov. Lemos & Leal-Zanchet and C. benyai, sp. nov. Lemos & Leal-Zanchet, based on morphological and molecular data, and use two molecular markers to investigate their phylogenetic relationships with other species in the genus, including its type species. Both morphological and molecular analyses clearly distinguish C. minima, sp. nov., C. benyai, sp. nov. and C. iheringi. The analyses of the ITS-1, COI and sequence divergence data also indicated that C. benyai, sp. nov. is more closely related to the type species of the genus, C. iheringi, than to C. minima, sp. nov. The three species are sympatric in Araucaria moist forest areas of the São Francisco de Paula National Forest; C. minima, sp. nov. and C. benyai, sp. nov. seem to be endemic to their type localities. Regarding external morphology, C. benyai, sp. nov. and C. iheringi could be considered cryptic species, only distinguishable on the basis of the copulatory apparatus. However, immature specimens of C. benyai, sp. nov. and C. iheringi could only be identified based on molecular data. Our results demonstrate that COI should be used with caution for reconstructing phylogenies, and other slower-evolving nuclear genes are a feasible alternative for resolving some of the phylogenetic relationships.

The female gonad in two species of Microplana (Platyhelminthes, Tricladida, Rhynchodemidae): ultrastructural and cytochemical investigations

The female gonad of the land planarians Microplana scharffi and Microplana terrestris consists of two small germaria located ventrally in the anterior third of the body and of two ventro-lateral rows of oblong vitelline follicles distributed between the intestinal pouches. Both these structures are enveloped by a tunica composed of an outer extracellular lamina and an inner sheath of accessory cells. Oocyte maturation is characterized by the appearance of chromatoid bodies and the development of endoplasmic reticulum and Golgi complexes. These organelles appear to be correlated with the production of egg granules with a fenestrated/granular content of medium electron density, about 4-5 mum in diameter, which remain dispersed in the ooplasm of mature oocytes. On the basis of cytochemical tests showing their glycoprotein composition, and their localization in mature oocytes, these egg granules have been interpreted as yolk. In the vitelline follicles, vitellocytes show the typical features of secretory cells with well-developed rough endoplasmic reticulum and Golgi complexes involved in the production of eggshell globules and yolk. The eggshell globules, which appear to arise from repeated coalescences of two types of Golgi-derived vesicles, contain polyphenols and, when completely mature, they measure about 1-1,2 mum in diameter and show a meandering/concentric content pattern as is typical of the situation observed in most Proseriata and Tricladida. Mature vitellocytes also contain a large amount of glycogen and lipids as further reserve material. On the basis of the ultrastructural features of the female gonad and in relation to the current literature the two species of rhynchodemids investigated appear to be closely related to the freshwater planarians belonging to the family Dugesiidae.

Estado do conhecimento dos macroturbelários (Platyhelminthes) do Brasil

Nas últimas décadas tem crescido o esforço internacional para conhecer a biodiversidade mundial visando a criação de políticas públicas de conservação. O Brasil vem participando deste esforço, como evidenciam as recentes publicações sobre o estado do conhecimento de vários táxons de organismos. Os macroturbelários (Polycladida e Tricladida, Platyhelminthes) fazem parte da fauna mundialmente menos estudada. Neste trabalho inventariamos por regiões a diversidade da fauna brasileira conhecida de macroturbelários e comparamos esta diversidade com a do resto do mundo. Analisamos também a atividade dos taxonomistas brasileiros do grupo no contexto mundial. Os resultados mostram que praticamente a totalidade das 246 espécies nominais descritas procede das regiões Sudeste e Sul. O número de espécies deverá aumentar significativamente quando forem feitas novas coletas, tanto em biomas e regiões ainda pouco ou nada explorados quanto nas áreas já amostradas. O Brasil está, relativamente a outros países, bem provido de especialistas. No entanto, em face da grande diversidade estimada, as amostragens deveriam ocorrer concomitantemente com a formação de novos especialistas.

Systematics of Chaetognatha under the light of molecular data, using duplicated ribosomal 18S DNA sequences

While the phylogenetic position of Chaetognatha has became central to the question of early bilaterian evolution, the internal systematics of the phylum are still not clear. The phylogenetic relationships of the chaetognaths were investigated using newly obtained small subunit ribosomal RNA nuclear 18S (SSU rRNA) sequences from 16 species together with 3 sequences available in GenBank. As previously shown with the large subunit ribosomal RNA 28S gene, two classes of Chaetognatha SSU rRNA gene can be identified, suggesting a duplication of the whole ribosomal cluster; allowing the rooting of one class of genes by another in phylogenetic analyses. Maximum Parsimony, Maximum Likelihood and Bayesian analyses of the molecular data, and statistical tests showed (1) that there are three main monophyletic groups: Sagittidae/Krohnittidae, Spadellidae/Pterosagittidae, and Eukrohniidae/Heterokrohniidae, (2) that the group of Aphragmophora without Pterosagittidae (Sagittidae/Krohnittidae) is monophyletic, (3) the Spadellidae/Pterosagittidae and Eukrohniidae/Heterokrohniidae families are very likely clustered, (4) the Krohnittidae and Pterosagittidae groups should no longer be considered as families as they are included in other groups designated as families, (5) suborder Ctenodontina is not monophyletic and the Flabellodontina should no longer be considered as a suborder, and (6) the Syngonata/Chorismogonata and the Monophragmophora/Biphragmophora hypotheses are rejected. Such conclusions are considered in the light of morphological characters, several of which are shown to be prone to homoplasy.

Ultrastructural and cytochemical aspects of the female gonad ofGeoplana burmeisteri (Platyhelminthes, Tricladida, Terricola)

The ultrastructure of the female gonad of the land planarian Geoplana burmeisteri was investigated by means of electron microscopy and cytochemical techniques. It consists of two small germaria located ventral to the intestine and of two irregular, lateral rows of vitelline follicles, both enveloped by a tunica composed of an extracellular lamina and an inner sheath of accessory cells. Accessory cell projections completely surround developing oocytes and vitellocytes. The main feature of oocyte maturation is the appearance of chromatoid bodies and the development of the rough endoplasmic reticulum (RER) and Golgi complexes. These organelles appear to be correlated with the production of egg inclusions of medium electron density, about 1.5-1.8 microm in diameter, which remain scattered in the ooplasm of mature oocytes. On the basis of cytochemical tests demonstrating their glycoprotein composition, these inclusions were interpreted as residual yolk globules. Vitellocytes are typical secretory cells with well-developed RER and Golgi complexes that are mainly involved in the production of yolk globules and eggshell globules, respectively. Eggshell globules appear to arise from repeated coalescence of small Golgi-derived vesicles and, at an intermediate stage of maturation, show a multigranular pattern. Later, after vesicle fusion, they reach a diameter of 1.3-1.6 microm when completely mature and show a meandering/concentric pattern, as is typical of the situation seen in most Proseriata and Tricladida. The content of yolk globules is completely digested by pronase, while the content of eggshell globules is unaffected. Mature vitellocytes contain, in addition, a large quantity of glycogen and lipid droplets as further reserve material. On the basis of the ultrastructural characteristics of the female gonad described above and in relation to the current literature, we conclude that G. burmeisteri appears to be more closely related to the freshwater triclads, in particular to members of the Dugesiidae, than to the marine triclads.

Should we be worried about long-branch attraction in real data sets? Investigations using metazoan 18S rDNA

Although long-branch attraction (LBA) is frequently cited as the cause of anomalous phylogenetic groupings, few examples of LBA involving real sequence data are known. We have found several cases of probable LBA by analyzing subsamples from an alignment of 18S rDNA sequences for 133 metazoans. In one example, maximum parsimony analysis of sequences from two rotifers, a ctenophore, and a polychaete annelid resulted in strong support for a tree grouping two "long-branch taxa" (a rotifer and the ctenophore). Maximum-likelihood analysis of the same sequences yielded strong support for a more biologically reasonable "rotifer monophyly" tree. Attempts to break up long branches for problematic subsamples through increased taxon sampling reduced, but did not eliminate, LBA problems. Exhaustive analyses of all quartets for a subset of 50 sequences were performed in order to compare the performance of maximum likelihood, equal-weights parsimony, and two additional variants of parsimony; these methods do differ substantially in their rates of failure to recover trees consistent with well established, but highly unresolved phylogenies. Power analyses using simulations suggest that some incorrect inferences by maximum parsimony are due to statistical inconsistency and that when estimates of central branch lengths for certain quartets are very low, maximum-likelihood analyses have difficulty recovering accepted phylogenies even with large amounts of data. These examples demonstrate that LBA problems can occur in real data sets, and they provide an opportunity to investigate causes of incorrect inferences.

Molecular phylogeny of the Platyhelminthes

The phylum Platyhelminthes has traditionally been considered the most basal bilaterian taxon. The main difficulty with this placement is the lack of convincing synapomorphies for all Platyhelminthes, which suggest that they are polyphyletic. Recent molecular findings based on 18S rDNA sequence data and number and type of Hox genes strongly suggest that the majority of Platyhelminthes are members of the lophotrochozoan protostomes, whereas the Acoelomorpha (Acoela + Nemertodermatida) fall outside of the Platyhelminthes as the most basal bilaterian taxon. Here we review phylum-wide analyses based on complete ribosomal and other nuclear genes addressed to answer the main issues facing systematics and phylogeny of Platyhelminthes. We present and discuss (i) new corroborative evidence for the polyphyly of the Platyhelminthes and the basal position of Acoelomorpha; (ii) a new consensus internal tree of the phylum; (iii) the nature of the sister group to the Neodermata and the hypotheses on the origin of parasitism; and (iv) the internal phylogeny of some rhabditophoran orders. Some methodological caveats are also introduced. The need to erect a new phylum, the Acoelomorpha, separate from the Platyhelminthes (now Catenulida + Rhabditophora) and based on present and new morphological and molecular characters is highlighted, and a proposal made.