Sperm competition favours intermediate sperm size in a hermaphrodite1.

Sperm competition is a potent mechanism of postcopulatory sexual selection that has been found to shape reproductive morphologies and behaviours in promiscuous animals. Especially sperm size has been argued to evolve in response to sperm competition through its effect on sperm longevity, sperm motility, the ability to displace competing sperm, and ultimately fertilization success. Additionally, sperm size has been observed to co-evolve with female reproductive morphology. Theoretical work predicts that sperm competition may select for longer sperm but may also favour shorter sperm if sperm size trades-off with number. In this study, we studied the relationship between sperm size and postmating success in the free-living flatworm, Macrostomum lignano. Specifically, we used inbred isolines of M. lignano that varied in sperm size to investigate how sperm size translated into the ability of worms to transfer and deposit sperm in a mating partner. Our results revealed a hump-shaped relationship with individuals producing sperm of intermediate size having the highest sperm competitiveness. This finding broadens our understanding of the evolution of sperm morphology by providing empirical support for stabilizing selection on sperm size under sperm competition.

The Abyssal Parasitic Flatworm Fecampia cf. abyssicola: New Records, Anatomy, and Molecular Phylogeny, with a Discussion on Its Systematic Position.

AbstractThe order Fecampiida, a group of parasitic turbellarians, has been poorly studied in terms of its species diversity, morphology, and ecology. Fecampiida is positioned within the monophyletic clade Adiaphanida, along with Tricladida and Prolecithophora, but their phylogenetic relationships are not well understood. Although the nervous and muscular systems of only two species in Fecampiida have been studied, recent research inferred morphological similarities between Fecampiida and Prolecithophora. In this study, we collected fecampiid cocoons and juveniles at depths of 1861-4438 m in Japanese waters. We identified the species on the basis of swimming juvenile specimens and by using histological and molecular methods, while we also examined its musculature and nervous system. Our study revealed a more complex nervous system than previously reported, with dorsal, lateral, and ventral pairs of longitudinal nerve cords connected through an anterior neuropile and posterior transverse commissures. While the nervous and muscular morphology suggested similarities with Prolecithophora, our phylogenetic analysis did not support a close relationship between Fecampiida and Prolecithophora.

Species composition and abundance of free-living turbellarians in three different wetlands in Southern Brazil.

Free-living turbellarians mostly live in marine and freshwater environments, but they have been little considered in ecological studies. The coastal plain in southern Brazil harbors a diverse fauna and flora, but only a few studies have related environmental factors to the abundance, richness, and distribution of turbellarians. Hence, we analyzed the structure of turbellarian communities in floating vegetation in three differently sized limnic environments. We aimed to determine differences in abundance, density, and richness of turbellarians among these habitats in relation to environmental factors. We found 1,225 turbellarians (34 species) distributed among 6 taxa. The most abundant was Tricladida with 49.4% relative abundance; the remaining (Catenulida, Dalytyphloplanida, Macrostomorpha, Kalyptorhynchia, and Prorhynchida) were less abundant. We found no significant differences among population parameters and environments. Small shallow lakes are a potentially rich environment, while water channels are the richest environment. Larger shallow lakes are not very diverse for turbellarians. Similarities regarding turbellarian communities among close limnic systems could be explained by the connectivity of bodies of water that presents similar aquatic macrophytes and hydrological subsystems, while larger shallow lakes have a different community because they have their own contributing basin and could be influenced by other factors.

New species of Temnocephala (Platyhelminthes, Temnocephalidae) ectosymbiont on decapod crustaceans Aegla jarai (Aeglidae) from Brazil and Samastacus spinifrons (Parastacidae) from Chile.

A new Neotropical temnocephalan species is described from Aegla jarai Bond-Buckup & Buckup, 1994 from Santa Catarina, Brazil and recorded on Samastacus spinifrons (Philippi, 1882) from five locations (Región de Biobío, Región de la Araucanía, and Región de Los Lagos) in Chile. Temnocephala catarinensis sp. nov. differs from 10 other species of Temnocephala Blanchard, 1849 epibionts on Aeglidae Dana, 1852 and/or Parastacidae Huxley, 1879 by having the following combination of characters: 1. a long and slightly curved cirrus; 2. an introvert with two distinct portions, a proximal one with grooves and no spines and a distal one with small spines with a strong base; 3. a single vaginal sphincter, well-developed and slightly asymmetrical; 4. a very short vagina with a weak muscular wall; 5. an almost triangular epidermal 'excretory' syncytial plates, with external margin in half circumference, sometimes reaching laterally the margin of body. The specimens of S. spinifrons have slightly larger measurements than specimens of A. jarai. The cirrus total length and introvert length, despite being larger, have almost the same number of spines in the distal portion. The shape of the EPs is similar in specimens from both hosts. The new species share some similarities with Temnocephala axenos Monticelli, 1898, Temnocephala mexicana Vayssière, 1898, Temnocephala kingsleyae Damborenea, 1994, and Temnocephala mertoni Volonterio, 2007, but the characters that differentiate them are presented and discussed. The new species has a wide geographic distribution represented by a large distance between the sampled locations (2,637 km) and separated by the Andes. This is not the only species with very distant records from each other as Temnocephala iheringi Haswell, 1893 and T. axenos or separated by the Andes as Temnocephala chilensis (Moquin-Tandon, 1846) and Temnocephala monticellii Ponce de León, Rudolph and Volonterio, 2020.

Acoel Single-Cell Transcriptomics: Cell Type Analysis of a Deep Branching Bilaterian.

Bilaterian animals display a wide variety of cell types, organized into defined anatomical structures and organ systems, which are mostly absent in prebilaterian animals. Xenacoelomorpha are an early-branching bilaterian phylum displaying an apparently relatively simple anatomical organization that have greatly diverged from other bilaterian clades. In this study, we use whole-body single-cell transcriptomics on the acoel Isodiametra pulchra to identify and characterize different cell types. Our analysis identifies the existence of ten major cell type categories in acoels all contributing to main biological functions of the organism: metabolism, locomotion and movements, behavior, defense, and development. Interestingly, although most cell clusters express core fate markers shared with other animal clades, we also describe a surprisingly large number of clade-specific marker genes, suggesting the emergence of clade-specific common molecular machineries functioning in distinct cell types. Together, these results provide novel insight into the evolution of bilaterian cell types and open the door to a better understanding of the origins of the bilaterian body plan and their constitutive cell types.

Temnocephalan epibionts (Platyhelminthes, Temnocephalida) on mollusks from Pantanal wetland, Brazil.

Temnocephala iheringi Haswell, 1893 was recorded in Marisa planogyra Pilsbry, 1933 and Pomacea maculata Perry, 1810. Also, Temnocephala amatoi sp. nov. was described from Pomacea scalaris (d'Orbigny, 1835). The mollusks were collected from temporary ponds at Ypiranga Farm, Poconé, Mato Grosso, Brazil. The new species differs from six other species of Temnocephala Blanchard, 1849 epibionts on molluks by having the following combination of characters: 1. a short and curved cirrus; 2. a narrow introvert with approximately 22 longitudinal rows of spines with 13 spines each; 3. internal spines with a narrow base, longer on the base of the introvert and smaller in the distal portion; 4. a single and asymmetrical vaginal sphincter and; 5. elliptical, longer than wide epidermal 'excretory' syncytial plates (EPs) with a excretory pore displaced to the anterior portion of the plate. Temnocephala lamothei Damborenea Brusa, 2008 has a curved cirrus, similar in size to the new species' cirrus. However, the introvert is not swollen with an oblique proximal margin, marked with a thickened oblique ring and two rows of 45-50 thin spines, differentiating the species from T. amatoi sp. nov.. When comparing the specimens of T. iheringi from Poconé with the redescription of the species from Southern Brazil previously published by the authors, we found significant intraspecific variation in the size of the adhesive disk, pharynx length, shaft and introvert length, spines of the introvert length, as well as differences in the number of paranephrocytes. The size of the EPs and, when comparing with published data from Uruguayan and Argentinean specimens, the body size also showed important differences. We hypothesize that the host species may be the cause of the intraspecific variations found. Overall, the record of two new hosts for T. iheringi, the intraspecific variation found in this species and its geographical distribution being expanded 418 km further to the north, together with the description of a new species, add acknowledgment to the Rhabdocoela fauna in the Brazilian Pantanal.

On a new Temnocephala (Platyhelminthes) from Southern Patagonia (Chile, Argentina), with a redescription of T. chilensis and some systematic and biogeographical insights.

To date, only one species of Temnocephala is known from Chile, Temnocephala chilensis, and three from southern Argentina (Patagonia), namely T. chilensis, Temnocephala dionii, and Temnocephala mexicana. Here we describe a new species of Temnocephala and provide an updated description of T. chilensis based on material found on an anomuran crab (Aeglidae) from southern Chile. Additional hosts and localities are reported for both species in southern Argentina and Chile, and a diagnostic key for all species of Temnocephala hosted on Aegla and Parastacidae is included as well. In southern Chile, both T. chilensis and the new species were found on the crayfish Samastacus spinifrons and on the anomuran crabs Aegla abtao and Aegla alacalufi; in addition, the new species was found on Aegla manni, and T. chilensis on Aegla rostrata. In southern Argentina, T. chilensis and the new species were found on Aegla riolimayana and S. spinifrons. Based on their shared traits (morphology of the penial stylet, host preferences and geographic distribution), the temnocephalans hosted in Aegla are tentatively gathered into two clusters, the Chilensis and Axenos groups.

Effects of two seminal fluid transcripts on post-mating behaviour in the simultaneously hermaphroditic flatworm Macrostomum lignano.

The seminal fluid proteins (SFPs) transferred to mating partners along with sperm often play crucial roles in mediating post-mating sexual selection. One way in which sperm donors can maximize their own reproductive success is by modifying the partner's (sperm recipient's) post-copulatory behaviour to prevent or delay re-mating, thereby decreasing the likelihood or intensity of sperm competition. Here, we adopted a quantitative genetic approach combining gene expression and behavioural data to identify candidates that could mediate such a response in the simultaneously hermaphroditic flatworm Macrostomum lignano. We identified two putative SFPs-Mlig-pro46 and Mlig-pro63-linked to both mating frequency and 'suck' frequency, a distinctive behaviour, in which, upon ejaculate receipt, the worm places its pharynx over its female genital opening and apparently attempts to remove the received ejaculate. We, therefore, performed a manipulative experiment using RNA interference-induced knockdown to ask how the loss of Mlig-pro46 and Mlig-pro63 expression, singly and in combination, affects mating frequency, partner suck propensity and sperm competitive ability. None of the knockdown treatments impacted strongly on the mating frequency or sperm competitive ability, but knockdown of Mlig-pro63 resulted in a significantly decreased suck propensity of mating partners. This suggests that Mlig-pro63 may normally act as a cue in the ejaculate to trigger recipient suck behaviour and-given that other proteins in the ejaculate have the opposite effect-could be one component of an ongoing arms race between donors and recipients over the control of ejaculate fate. However, the adaptive significance of Mlig-pro46 and Mlig-pro63 from a donor perspective remains enigmatic.

Immunostaining and In Situ Hybridization of the Developing Acoel Nervous System.

The study of acoel morphologies has been recently stimulated by the knowledge that this group of animals represents an early offshoot of the Bilateria. Understanding how organ systems and tissues develop and the molecular underpinnings of the processes involved has become an area of new research. The microscopic anatomy of these organisms is best understood through the systematic use of immunochemistry and in situ hybridization procedures. These methods allow us to map, in precise detail, the expression patterns of genes and proteins, in space and time. With the additional use of genomic resources, they provide us with insights on how a group of "early" bilaterians have diversified over time. As these animals are new to the world of molecular studies, the protocols have involved a lot of new and specific adaptations to their specific anatomical-histological characteristics. Here we explain some of these protocols in detail, with the aim that should prove useful in our much-needed understanding of the origins of bilaterian animals. An anatomical sketch is provided at the beginning as a necessary guide for those not familiar with the Acoela.

Seminal Fluid-Mediated Manipulation of Post-mating Behavior in a Simultaneous Hermaphrodite.

Seminal fluid proteins (SFPs) are uniquely positioned to mediate post-mating sexual selection and sexual conflict [1-3]. This role may be especially important in simultaneous hermaphrodites, in which individuals will often agree to receive sperm in order to be able to donate it, shifting the arena of sexual selection to post-mating reproductive interactions [4-7]. Nevertheless, as in separate-sexed organisms, identifying individual SFPs responsible for specific post-mating effects is difficult, owing to the complexity, rapid evolution, and functional redundancy of seminal fluid [8-11]. Here, we sought to identify SFPs that influence one striking post-mating behavior of the simultaneously hermaphroditic flatworm Macrostomum lignano, the so-called "suck behavior," in which worms respond to ejaculate receipt by placing their pharynx over their female genital opening and seemingly attempt to remove sperm and/or other ejaculate components [12-14]. We hypothesized that sucking is counter to the sperm donor's interests, potentially selecting for SFPs that reduce the suck propensity of mating partners. We tested this using a combination of quantitative genetics and RNA interference (RNAi) knockdown. As predicted, we found negative genetic correlations between the expression levels of six (out of 58) seminal fluid transcripts and partner suck propensity. RNAi knockdown confirmed that two of these transcripts, designated suckless-1 and suckless-2, indeed caused mating partners to suck less often. We suggest that these proteins are male counter-adaptations to recipient suck behavior, which itself is likely a female counter-adaptation in the ongoing evolutionary conflict to (re)gain control over ejaculate fate after mating in this hermaphroditic organism.

Flatworm mucus as the base of a food web.

BACKGROUND: By altering their habitats, engineering species can improve their own fitness. However, the effect of this strategy on the fitness of coexisting species or on the structure of the respective food web is poorly understood. In this study, bacteria and bacterivorous nematodes with short (Caenorhabditis elegans) and long (Plectus acuminatus) life cycles were exposed to the mucus secreted by the freshwater flatworm Polycelis tenuis. The growth, reproduction, and feeding preferences of the nematodes in the presence/absence of the mucus were then determined. In addition, confocal laser scanning microscopy (CLSM) was used to examine the structural footprint of the mucus and the mucus colonization dynamics of bacteria and protozoans. RESULTS: Mucus exposure resulted in a greater reproductive output in P. acuminatus than in C. elegans. In a cafeteria experiment, both nematode species were attracted by bacteria-rich patches and were not deterred by mucus. CLSM showed that the flatworms spread a layer of polysaccharide-rich mucus ca. 15 µm thick from their tails. Subsequent colonization of the mucus by bacteria and protozoans resulted in an architecture that progressively resembled a complex biofilm. The presence of protozoans reduced nematode reproduction, presumably due to competition for their bacterial food supply. CONCLUSION: Animal secretions such as mucus may have broader, community-level consequences and contribute to fueling microbial food webs.

A new species of Temnocephala (Platyhelminthes) with an unusual pharynx, including an amendment of the diagnosis of the genus.

Three species of Temnocephala have been reported on Aegla from Uruguay: Temnocephala axenos, Temnocephala mertoni and Temnocephala talicei. As part of a systematic review of the Uruguayan species of Temnocephala, a fourth species was found on Aegla platensis and Aegla uruguayana. This species, new for the genus, has the following diagnostic characters: Large temnocephalan; slightly curved penial stylet (225 µm in length) with a symmetrical introvert (27 long by 18 µm in diameter) that has at least 44 spine-like projections, each bearing 7 rows of internal spines; a seminal vesicle that opens sub-polarly into the contractile vesicle; a muscular vagina with a strong asymmetrical sphincter; preequatorial nephridiopores close to the internal borders of the elongate-oval excretory syncytia; a large pharynx with anterior and posterior sphincters of about the same diameter and a narrow pharyngeal lumen with nearly uniform width. To contemplate the characters of the pharynx, an amendment of the genus Temnocephala is given. The new species resembles Temnocephala cyanoglandula, which also has a very long, slightly curved penial stylet, but differs from it by having a shorter penial stylet, a lower number of rows of spines in the introvert, a larger, different type of pharynx, a smaller contractile vesicle and a larger sucker. The new species is carnivorous and feeds on whole preys, which are mainly annelids (Stratiodrilus) and other species of Temnocephala. The new species was found to be hyperparasitised by plerocercoid and nematode larvae, but hyperparasites were not found in the other sympatric species of Temnocephala. Based on the findings presented here, it is clear that detailed histological descriptions of the pharynx should be contemplated as part of the normal work in taxonomy of temnocephalans.

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.

Post-Embryonic Development and Genital-Complex Formation in Three Species of Polyclad Flatworms.

Without the establishment of effective culturing systems, little can be known about the late developmental stages of polyclad flatworms. Here, we report a laboratory culturing system for three polyclad species: Comoplana pusilla, Notocomplana koreana, and Pseudostylochus obscurus, and we describe changes in their morphology from hatching to reproductive maturity. These species hatch out as lobe-less larvae with four eyespots, but the number of eyespots increases in later development. Cross-like and triangularly shaped larvae are observed in N. koreana and P. obscurus, respectively. After settlement, a pale area appears on the body of juveniles and then develops into the copulatory complexes. All three species could be successfully reared on brine shrimp, but only C. pusilla and N. koreana achieved reproductive maturation in such a culturing system. In P. obscurus, switching the food to the gastropod Monodonta labio induced sexual maturation.

Bigger testes increase paternity in a simultaneous hermaphrodite, independently of the sperm competition level.

Hermaphroditic animals face the fundamental evolutionary optimization problem of allocating their resources to their male vs. female reproductive function (e.g. testes and sperm vs. ovaries and eggs), and this optimal sex allocation can be affected by both pre- and post-copulatory sexual selection. For example, local sperm competition (LSC) - the competition between related sperm for the fertilization of a partner's ova - occurs in small mating groups and can favour a female-biased sex allocation, because, under LSC, investment into sperm production is predicted to show diminishing fitness returns. Here, we test whether higher testis investment increases an individual's paternity success under sperm competition, and whether the strength of this effect diminishes when LSC is stronger, as predicted by sex allocation theory. We created two subsets of individuals of the simultaneously hermaphroditic flatworm Macrostomum lignano - by sampling worms from either the highest or lowest quartile of the testis investment distribution - and estimated their paternity success in group sizes of either three (strong LSC) or eight individuals (weak LSC). Specifically, using transgenic focal individuals expressing a dominant green-fluorescent protein marker, we showed that worms with high testis investment sired 22% more offspring relative to those with low investment, corroborating previous findings in M. lignano and other species. However, the strength of this effect was not significantly modulated by the experienced group size, contrasting theoretical expectations of more strongly diminishing fitness returns under strong LSC. We discuss the possible implications for the evolutionary maintenance of hermaphroditism in M. lignano.

Polyploidy as a chromosomal component of stochastic noise: variable scalar multiples of the diploid chromosome complement in the invertebrate species Girardia schubarti from Brazil / Poliploidia como componente cromossômico de ruído estocástico: variações escalares múltiplas do componente cromossômico diploide do invertebrado Girardia schubarti ocorrente no Brasil

Abstract Chromosome stoichiometry, a form of genetic plasticity, specifically refers to variation in the standard diploid genomic composition of an individual or species. In the present work, freshwater planarians (Girardia schubarti) were analyzed to recognize variations in chromosomal stoichiometry especially of complete ploidal change between specimens, within specimens and between cells within specimens and any relations they might have with selected components of phenotypic plasticity. Homoploid polyploids for the group reached rational scalar multiples (e.g. tetraploids) or irrational scalar multiples (e.g. triploids). Karyotypic mosaics emerged where individual cells presented polyploid multiples in arithmetic and geometric progressions. Ploidal multiplicity, a chromosomal component of stochastic noise, had positive phenotypic effects (increased dimensions) on morphologic criteria of body length, body width and dorsal surface reflecting a significant genotypic plasticity (GP) and robust phenotypic plasticity (PP). Variable but significant association of genotypic plasticity with robust phenotypic variance suggests kinetics of phenotypic homeostasis that is species-specific permitting phenotypic adaptability to environmental variables by means of GP. That association is diminished, deactivated or lost in more advanced and more complex organisms.

Resumo A estequiometria cromossômica, uma forma de plasticidade genotípica, representa variações na composição genômica diploide de um indivíduo ou espécie. Planárias límnicas (Girardia schubarti) foram analisadas para verificar a estequiometria cromossômica, especialmente alterações na ploidia entre espécimes, em cada espécime e entre células do mesmo espécime, além de relações dessas alterações com a plasticidade fenotípica. Espécimes poliploides homoploides apresentaram múltiplos escalares racionais ou irracionais, tais como triploides. Mosaicos cariotípicos ocorreram quando células apresentaram poliploides múltiplos em progressões aritméticas e geométricas. Nas planárias estudadas, a multiplicidade ploidal, um componente cromossômico de ruído estocástico, apresentou efeitos fenotípicos positivos, causando aumento das dimensões dos indivíduos, tais como comprimento corporal, largura do corpo e superfície dorsal, indicando plasticidade genotípica (GP) significativa e plasticidade fenotípica (PP) robusta. Associações significativas da plasticidade genotípica com variâncias fenotípicas robustas, embora variáveis, sugerem que a homeostase fenotípica, a qual é espécie-específica, possibilita adaptações a variáveis ambientais através da GP. Tal associação apresenta-se reduzida, desativada ou perdida em organismos mais complexos.

Syndesmis aethopharynx Westervelt & Kozloff, 1990 (Rhabdocoela: Umagillidae): a revisitation supported by scanning electron microscopy and molecular analyses.

Species of Syndesmis Francois, 1886 are rhabdocoel platyhelminths typically found in echinoids. Our knowledge of this group is based on old and insufficient studies, generally representing light microscopy-based species descriptions. Syndesmis aethopharynx Westervelt & Kozloff, 1990 is an understudied endosymbiont of Paracentrotus lividus (Lamarck), which is likely to have been confused with the type-species, i.e. Syndesmis echinorum François, 1886, in the literature. In this work, S. aethopharynx is revisited based on new data on surface morphology and phylogeny and basic ecological data are provided. Scanning electron microscopy analysis revealed that the whole ventral region of the worm is equipped with cilia, which supports the assumption that the unciliated epidermal area reported for some species of umagillids, likewise endosymbiotic in echinoderms, is an apomorphy. Following the results of the molecular phylogenetic analysis, species of Syndesmis are closely-related to symbionts of other echinoderms, i.e. holothurians, and like them, may have evolved from some free-living or symbiotic Provorticidae ancestor. Syndesmis spp. may stand for a key group in studying the evolution of feeding strategies in rhabdocoels, as their phylogenetic position is between intestinal and coelomic symbionts, and since both the digestive tube and perivisceral fluid were recorded as sites of infection. The infection levels were low, likely reflecting the aggregated distribution of the host and the fragile nature of the symbiont.

Nervous system and musculature of the parasitic turbellarian Notentera ivanovi (Plathelminthes, Fecampiida).

The article reports the first description of the architecture of the musculature and cholinergic nervous system of the parasitic turbellarian Notentera ivanovi from the White Sea using histochemistry and confocal scanning laser microscopy. It has been demonstrated that the body wall is composed of layers of circular and longitudinal muscles between which there are diagonal muscles described here for the first time. The nervous system is of the regular closely spaced orthogon type with pronounced radiality. In order to make it clear whether the orthogon of this type is a phylogenetic characteristic of this group of worms or its shape is associated with the flat rounded body, the study of other representatives of the Fecampiida group is required.

Syndesmis aethopharynx (Umagillidae, Rhabdocoela, Platyhelminthes) from the sea urchin Paracentrotus lividus: First record from the Eastern Mediterranean, phylogenetic position and intraspecific morphological variation.

Specimens of Syndesmis aethopharynx Westervelt & Kozloff, 1990 (Umagillidae, Rhabdocoela, Platyhelminthes) were collected from the intestine of several specimens of the sea urchin Paracentrotus lividus (Lamarck, 1816) [2], Hansson, 2001 at the Greek coast. This represents the first report of a species of Syndesmis from Greece. Our study has revealed several previously-unreported morphological details and intraspecific variation, which are added to the species description. The position of S. aethopharynx within Umagillidae is confirmed for the first time through molecular data (based on nuclear 18S rDNA), using both Bayesian and maximum likelihood analyses.

New insights into the karyotype evolution of the free-living flatworm Macrostomum lignano (Platyhelminthes, Turbellaria).

The free-living flatworm Macrostomum lignano is a model organism for evolutionary and developmental biology studies. Recently, an unusual karyotypic diversity was revealed in this species. Specifically, worms are either 'normal' 2n = 8, or they are aneuploid with one or two additional large chromosome(s) (i.e. 2n = 9 or 2n = 10, respectively). Aneuploid worms did not show visible behavioral or morphological abnormalities and were successful in reproduction. In this study, we generated microdissected DNA probes from chromosome 1 (further called MLI1), chromosome 2 (MLI2), and a pair of similar-sized smaller chromosomes (MLI3, MLI4). FISH using these probes revealed that MLI1 consists of contiguous regions homologous to MLI2-MLI4, suggesting that MLI1 arose due to the whole genome duplication and subsequent fusion of one full chromosome set into one large metacentric chromosome. Therefore, one presumably full haploid genome was packed into MLI1, leading to hidden tetraploidy in the M. lignano genome. The study of Macrostomum sp. 8 - a sibling species of M. lignano - revealed that it usually has one additional pair of large chromosomes (2n = 10) showing a high homology to MLI1, thus suggesting hidden hexaploidy in its genome. Possible evolutionary scenarios for the emergence of the M. lignano and Macrostomum sp. 8 genomes are discussed.