Ultrastructural study of the epidermis in two free-living Platyhelminthes, Mesostoma viaregginum and M. productum (Rhabdocoela, Typhloplanida)

Abstract. The epidermis of the free-living typhloplanids Mesostoma viaregginum and M. productum (Mesostominae) is described. In both species, the epidermis has polarized cells with nuclei located at the basal part of the cell, whereas mitochondria are in the apical one. The epidermis is entirely covered by microvilli and locomotory cilia anchored in the cytoplasm by vertical and horizontal rootlets. Rootlets exhibit distinct length and periodic structure in the two species. Furthermore, in each species vertical and horizontal rootlets possess different periodic structure. The pattern of termination of microtubules in epidermal cilia is described for the first time in the Typhloplanida; central microtubules shift along one axonemal side, doublets 1 and 6–9 lose their microtubule B, and gradually peripheral doublets become singlets. Finally, an electron-dense material caps the tip of the cilia. This pattern of termination closely resembles that of Temnocephalida, Kalytorhynchia, and Dalyelliida examined so far, but differences exist.     

Meiosis in Mesostoma ehrenbergii ehrenbergii (Turbellaria,Rhabdocoela)

At metaphase I during spermatogenesis in Mesostoma ehrenbergii ehrenbergii (2 × = 10), 3 bivalents and 4 univalents form. The bivalents each have a single distally localised chiasma. Electron microscope serial section reconstructions have shown that 3 short pieces of synaptonemal complex (SC) are present in pachytene nuclei in the testis. All the SC is contained in a lobe which is separated by an invagination from the main body of the nucleus. The SC pieces vary in length from approximately 2 m to 5 m, and the greatest amount found in one nucleus was approximately 13 m. It is proposed that the pieces of SC correspond to the paired regions of the 3 bivalents present at metaphase I. This system is therefore an example of localised pairing leading to localised chiasmata. Later prophase stage nuclei have also been reconstructed, in which stacks of polycomplex are present but no SC has been found. The polycomplex is thought to be an association of discarded SC sub-units.     

Meiosis in Mesostoma ehrenbergii ehrenbergii (Turbellaria,Rhabdocoela)

Female meiosis in Mesostoma ehrenbergii ehrenbergii is achiasmate. Electron microscope serial section reconstructions of nuclei from the germarium region of the ovary have shown that synaptonemal complex (SC) is present during the early prophase stages. The greatest amounts present were in two nuclei containing 389 and 401 respectively. The reconstructions showed that discontinuities in the SC existed along the lengths of the bivalents in these two nuclei which were taken to represent the maximally paired stage, i.e., pachytene. — Reconstructions of post pachytene nuclei showed that SC was absent from the bivalents and therefore retention of SC until metaphase I is not the mechanism used by this achiasmate species to ensure homologous chromosome segregation. An alternative mechanism for this function is proposed based on observations made on the later stages of meiosis in maturing oocytes.     

Meiosis in Mesostoma ehrenbergii ehrenbergii (Turbellaria,Rhabdocoela)

At metaphase I of meiosis in spermatocytes of Mesostoma ehrenbergii ehrenbergii [2n=10] three bivalents and four univalents form. The same two chromosome pairs always form the univalents. Analysis of metaphase I, anaphase I and metaphase II configurations in fixed testis material suggested that the distribution of the four univalents is not a random process but the correct segregation of one member of each pair to each pole is actively achieved before the end of metaphase I. In live preparations of testis material univalents were observed to move between the poles of metaphase I cells, eventually reaching the correct segregation. All cells observed to enter anaphase I had the correct segregation of univalents. It is proposed that the univalent movement during metaphase I is directed towards obtaining the correct segregation of univalents before the cells enter anaphase.     

Zur fortpflanzungsbiologie von mesostoma ehrenbergii (Focke, 1836) (Turbellaria)

1. At temperature levels from 10 to 25°C animals from resting eggs produce subitaneous eggs independent on temperature. In contrast animals from subitaneous eggs produce subitaneous eggs dependent on temperature. At a high rate subitaneous eggs are only formed at temperature levels above 20°C.
2. Below 10°C no development occurs in the juveniles. At temperatures of 30/22°C (24.7°C) the first subitaneous eggs are formed after 6–9 days, at 14/9°C (10.7°C) they are formed after 34 days. At different temperature levels the developmental rate of the young is from 10.5 to 42 days. One generation extends over 16.5 (30/22°C) to 75 days (14/9°C). The average egg production is 10–20 subitaneous eggs or 30–60 resting eggs. The maximum egg production of one individual is 50 subitaneous eggs or 84 resting eggs. 50% of the animals have just formed resting eggs, before the juveniles are hatched. Resting eggs in the first egg-batch are formed 6–20 days later than subitaneous eggs. The duration of life is between 65 (30/22°C) and 140 days (19/13°C).
3. Young worms in resting eggs have a dormance period of at least 15–30 days.

At room temperatures (20°C) no juvenile in resting eggs hatches from water. By combining room and refrigerator (3.5°C) temperatures the hatching rate increases to a maximum of 85%. To reach a hatching rate of 50–65% the influence of low temperatures must be at least 30 days. At room temperatures 60% of the young in resting eggs hatch from mud covered with water. Combining high and low temperatures the hatching success is between 67 and 81%, where the highest percentage of the young may hatch at room temperature. Up to 90 days low temperatures cause a maximum hatching rate of 79%. It decreases to approximately 30% after 180 days. At high temperatures resting eggs preserved in 100% moist mud, survive for two months. By adding a period of low temperatures the hatching rate increases to a maximum of 52%. Low temperatures are survived for more than 6 months. Up to 30 days preservation at 3.5°C causes a maximum hatching rate of 61%, up to 12o days it decreases to 30%. At room temperature the young in resting eggs are not resistant against air-dried mud (30–40% rel. air moisture). Combining high and low temperatures air-dried mud is endured 1 month (hatching rate 5–14%). Preservation of 30–120 days at 3.5°C and 70% rel. air moisture result in a hatching rate of 43–61%. li]4. In the open air in Middle-Europe there occur 5–6 generations of M. ehrenbergii per life-cycle. The first generation hatches from resting eggs in May, where the production of subitaneous eggs is independent on temperature. All other generations up to October hatch from subitaneous eggs. The egg-production of those worms is dependent on environmental factors. In summer subitaneous egg production prevails, in autumn resting egg production. The abundance during the life-cycle is dependent on the number of animals which produce subitaneous eggs. Resting eggs are predestinated to endure periods of dryness and cold. The life-cycles of the species M. lingua and M. productum are different from those of M. ehrenbergii in length and in the number of generations. In both species 7 generations occur over 8 to 8.5 respectively 5.5 months. M. nigrirostrum only forms resting eggs. The life-cycle consists of one generation from February/March to May/June.     

Ultrastructural and cytochemical aspects of oogenesis in Castrada viridis (Platyhelminthes, Rhabdocoela).

The heterocellular female gonad of the typhloplanoid Castrada viridis consists of a single germarium and two rows of vitellaria. The germarium, composed of a germinative zone and a growth zone, is surrounded externally by a layer of accessory cells the function of which is hypothesized in this study. The main feature of oocyte differentiation is the synthesis of small electron-dense inclusions produced by the rough endoplasmic reticulum (R.E.R.) and Golgi complex. The electron-dense content of the egg inclusions reacts positively to the cytochemical test used to detect polyphenols and is only partially extracted following incubation in protease. The genesis, composition, and peripheral location of egg inclusions in mature oocytes suggest that they could represent residual eggshell granules. The presumed function of eggshell granules is discussed and their fine morphology is compared with that observed in other neoophoran Platyhelminthes.     

An allatotropin-like neuropeptide in <Emphasis Type="Italic">Mesostoma ehrenbergii</Emphasis> (Rhabdocoela,Platyhelminthes)

Allatotropin (AT), a neuropeptide isolated on the basis of its ability to stimulate the synthesis of juvenile hormones (JHs) in insects, has also been found in other groups. In addition to this function, AT has proved to be myotropic. In the present study, we analyze its expression in the free-living turbellarian Mesostoma ehrenbergii (Platyhelminthes: Typhloplanida) and its probable functional relationship with the muscle tissue. The results show the presence of an AT-like peptide in neurons located in different regions of the body of the flatworm. The analysis of the presence of the peptide together with phalloidin labeling suggests a functional relationship between the peptide and the muscle tissue. This is particularly evident at the level of the pharynx, where the peptide induces contractions at concentrations of 10⁻¹⁴ and 10⁻¹² M, suggesting that it is actually acting as a myoregulator. Detection of AT in several groups of protostomes but apparently not in deuterostomes suggests that this peptide could be a synapomorphic feature of protostomes. Indeed, the presence of AT in organisms that do not undergo metamorphosis could suggest that it was first involved in myotropic activities, being the induction of the synthesis of JHs a secondary function.     

Sperm morphology in the Pterastericolidae (Platyhelminthes, Rhabdocoela): phylogenetic implications

The sperm morphology of five species of the Pterastericolidae was studied with transmission electron microscopy. The spermatozoa of all species have two axoncmes, which are incorporated in the sperm cell body for most of their length. The axonemes arc of the'9 + 1'pattern characteristic of the flatworm taxon Trepaxonemata. Outer dynein arms are absent from the microtubule doublets in the axonemes. Dense bodies are few, and occur only in the distal part of the spermatozoa, but small electron dense granules are numerous. A sister group of the Neodermata consisting of the Pterastericolidae and the Fecampidae is proposed. The monophyly of the taxon Dalyellioida is discussed     

Ultrastructure of the female gonad of two temnocephalids (Platyhelminthes,Rhabdocoela)

The female gonad of Temnocephala dendyi and T. minor consists of a single germarium and two rows of vitellaria. It is enveloped by an extracellular lamina and accessory cells. Accessory cells are only peripherally located in the germarium while their cytoplasmic projections also fill the spaces between vitellocytes in the vitellarium. The main feature of oocyte maturation is the appearance of chromatoid bodies and the development of rough endoplasmic reticulum (R.E.R.) and Golgi complexes which appear to be correlated with the production of double-structured egg granules. The egg granules, which are localized in the cortical cytoplasm of mature oocytes, contain glycoproteins, are devoid of polyphenols and are similar in structure and composition to the cortical granules observed in some Digenea and Monogenea. Vitellocytes are typical secretory cells with well-developed R.E.R. and Golgi complexes which are involved in the production of shell globules and yolk. The multigranular pattern and the polyphenolic composition of the shell globules of the temnocephalids investigated are similar to those observed in other rhabdocoels, and in some Prolecithophora and Neodermata. This feature may represent a synapomorphy shared by these taxa. This revised version was published online in July 2006 with corrections to the Cover Date.     

The phylogeny of the cercomeria (Platyhelminthes: Rhabdocoela) and general evolutionary principles

The unified theory of evolution is an expansion of Darwinian theory that asserts that evolution is driven by entropic accumulation of genetic information that is constrained and organized primarily by the genealogical effects of phylogenetic history and developmental integration, and secondarily by ecological effects, or natural selection in its classical mode. Phylogenetic systematic analysis of the 8 major groups of parasitic rhabdocoelous platyhelminths permits empirical macroevolutionary evaluation of these postulates. Of the 131 characters considered, 127 are phylogenetically constrained, and 4 show evidence of 1 case of convergence each. Data from different developmental stages are phylogenetically congruent, despite differences in ecology among those stages. Ecological diversification, indicated by phylogenetic association of definitive hosts and parasites, and by changes in ecological components of life cycle patterns, is more conservative evolutionarily than diversification in developmental patterns, indicated by the appearance of unique larval stages, asexual proliferation of larvae, polyembryony, and heterochronic changes. These observations support the macroevolutionary postulates of the unified theory.     

Anatomy and ultrastructure of the proboscis in Mesorhynchus terminostylis (Platyhelminthes,Rhabdocoela)

The ultrastructural organization of the proboscis in Mesorhynchus terminostylis is distinctly different from that in other members of the Polycystididae in which it is currently classified. The sheath epithelium is formed by three belts, all with intra-epithelial nuclei. The apical belt of the bipartite cone epithelium has a single intrabulbar nucleus, and the basal belt possesses five insunk nucleiferous cell parts behind the bulb. Six types of glands surface through the epithelia; the three types emerging through the cone epithelium can be homologized with those described for Polycistis naegelii. Only uniciliary receptors are found in the epithelium. The musculature in the bulb has a very loose appearance, and the bulbar septum appears to be a bipartite basement membrane. The septum can be considered the basement membrane of the cone epithelium as if the contractile portion of the inner longitudinal muscles have invaded the epithelium and come to lie between the epithelial cells and the basement membrane. Thus the inner musculature of the bulb is entirely intraepithelial as is the case for Psammorhynchus tubulipenis and Cytocystis clitellatus. The systematic position of M. terminostylisremains uncertain but seems to lie between Psammorhynchus and Cytocystis on one hand and Koinocystididae and Polycystididae on the other.     

达氏科(扁形动物门,单肠目)涡虫二新种

本文记述淡水涡虫类单肠目Rhabdocoela达氏科Dalyelliidae小达氏涡虫属Microdalyellia一新种,广东小达氏涡虫Microdelyellia guangdongensis Wang et Rong,sp.nov.,模式标本由卢彦宏于2013年6月9日采自广东省深圳市石岩水库(22°40'58″N,113°53'42″E);杰氏涡虫属Gieysztoria一新种,七星岩杰氏涡虫Gieysztoria qixingyanensis Wang et Rong,sp.nov.,模式标本由刘旭坤于2014年8月6日采自广东省肇庆市七星岩风景区(23°04'32″N,112°28'46″E)。对2个涡虫新种的形态特征作了详细描述,并分别与小达氏涡虫属和杰氏涡虫属近似物种进行了比较。所有标本保存于中国科学院国家动物博物馆。     

Prey preference and utilization by Mesostoma lingua (Turbellaria,Rhabdocoela) at a low arctic site

The rhabdocoel Mesostoma lingua is a common inhabitant of rock bluff ponds in the area of Churchill, Manitoba. Unlike most flatworms it has the ability to prey actively on zooplankton. Among the zooplankton, Mesostoma prefers Daphnia pulex, Simocephalus vetulus, and Diaptomus tyrrelli. Larger calanoid copepods are difficult to capture and take much longer to consume. Growth in Mesostoma is maintained when individuals consume 0.5 Daphnia day^–1 and is maximal at 4 Daphnia day^–1. In comparison, Mesostoma maintained growth only when offered the copepod Diaptomus victoriaensis at the rate of 8–10 individuals-day^–1. Mesostoma may be an important agent in structuring pond zooplankton communities in the arctic, producing a shift in dominance away from cladocerans such as Daphnia toward larger calanoid copepods.     

New Pferastericola species (Platyhelminthes, Rhabdocoela) and a cladistic analysis of the Pterastericolidae

Two ncw species of the family Pterastericolidae which inhabit starfish arc dcscribcd. Pterastericola bergensis sp.n. from thc oesophagus and cardiac stomach of Psilaster andromecla has posterior gonopore and lateral vitellaria. Pterastericola asamushii sp.n. from the pyloric caeca of Astropecren scopurius has unciliated epithelium postcriorly and an anterior gonopore. A cladistic analysis of thc phylogcny of the Pterastcricolidae is presented. The withdrawal of the genus Trilohorhynchus is proposed.     

Molecular phylogeny of Kalyptorhynchia (Rhabdocoela,Platyhelminthes) inferred from ribosomal sequence data

With 556 species described to date, Kalyptorhynchia includes about one‐third of all species of rhabdocoel flatworms. In this study, we present the first molecular phylogenetic analysis of this taxon. The final analysis comprises 110 species. These represent 11 of the 17 known families. The largest family (241 known species), Polycystididae, is represented by nine of 10 subfamilies and 33 of the 58 genera. Sequence data from 18S rDNA and 28S rDNA were analysed using maximum likelihood and Bayesian approaches. Of the two taxa traditionally recognised within Kalyptorhynchia, Eukalyptorhynchia and Schizorhynchia, only Schizorhynchia is monophyletic. All eukalyptorhynch families, except Cicerinidae, are monophyletic. On the other hand, two of the three schizorhynch families included are not monophyletic. Within Polycystididae, the traditional taxonomy was not reflected in our phylogenetic analyses and most subfamilies are polyphyletic. These results suggest that current classification, mostly based on characters of the genital system, suffers from homoplasy. Where possible, a revised classification, taking into account these new findings, is given.     

Spermiogenesis and sperm in Mesostoma viaregginum (Plathelminthes, Rhabdocoela): an ultrastructural study to infer sperm orientation

Spermiogenesis in Mesostoma viaregginum begins with the formation of a zone of differentiation containing striated rootlets, two centrioles, and an intercentriolar body in-between. These centrioles generate two parallel free-flagella with the 9+“1” pattern of the Trepaxonemata growing out in opposite directions. Spermatid differentiation is characterised by a 90° latero-ventral rotation of flagella and a subsequent disto-proximal centriolar rotation, with a distal cytoplasmic projection. The former rotation involves the compression of a row of cortical microtubules and allows recognising a flagellar side and an aflagellar side in the late spermatid and in the mature spermatozoon. At the end of the differentiation, centrioles and microtubules lie parallel to the spermatid axis. The disto-proximal centriolar rotation is proposed as a synapomorphy for the Rhabdocoela. The modifications of the intercentriolar body during spermiogenesis and the migration of the nucleus and the centrioles towards the cytoplasmic distal projection are also described. The mature spermatozoon of M. viaregginum is filiform and tapered at both ends and presents many features found in the Rhabdocoela gametes. The nucleus disappears before the flagellar insertion and a density gradient of mitochondria is observed along the sperm axis. The anterior end of the spermatozoon of M. viaregginum is characterised by a tapering capped by a membrane expansion. This study has enabled us to describe precisely the orientation of spermatozoa in the Rhabdocoela in general: the centriolar extremity is proposed as the anterior one for the Rhabdocoela.     

A mechanism for self-inhibition of population growth in the flatworm Mesostoma ehrenbergii (Focke)

Summary By producing subitaneous eggs the rhabdocoel Mesostoma ehrenbergii may quickly increase the population size. The production of this kind of eggs by young worms is inhibited by the presence of adults in the culture medium. The inhibition is specific, and it is observed also if young and adult worms are not in direct contact, or if young worms are added to the water where adults had been previously kept: there is therefore evidence for a chemical mediator. The sensitivity to the inhibition occurs during the first five days after birth, with a maximum on the 2nd to 3rd day.     

Ultrastructure of spermiogenesis and the spermatozoon in Castrada cristatispina Papi, 1951 (Platyhelminthes, Rhabdocoela, Typhloplanida)

Spermiogenesis in Castrada cristatispina begins with the formation of a zone of differentiation containing two centrioles with associated striated rootlets and an intercentriolar body between them. The centrioles give rise to two parallel, free flagella of the Trepaxonemata 9 + '1' pattern, growing out in opposite directions. Spermatids undergo a latero-ventral rotation of the flagella and a subsequent disto-proximal rotation of centrioles, and a distal cytoplasmic projection appears. The former rotation involves the compression of a row of microtubules and allows the recognition of a ventral side and a dorsal side. At the end of the differentiation, the centrioles and cortical microtubules lie parallel to the sperm axis. The modifications of the intercentriolar body and the migration of the nucleus and the centrioles toward the distal projection are described. The mature spermatozoon of C. cristatispina is filiform, tapered at both ends and shares several features with the other Rhabdocoela gametes. Nevertheless, the posterior extremity is capped by an electron-dense material. A gradient between mitochondria and dense bodies exists along the sperm axis. This study has enable us a phylogenetic approach of the Rhabdocoela through a comparison of the ultrastructural features of C. cristatispina with the other Rhabdocoela taxa. We propose the disto-proximal rotation of centrioles as a synapomorphy of the Rhabdocoela.     

The Ultrastructure of the Proboscis in Psammorhynchus tubulipenis Karling, 1964 and Cytocystis clitellatus Karling, 1953 (Platyhelminthes,Rhabdocoela)

The ultrastructural organization of the proboscis in two species of free-living Platyhelminthes, Psammorhynchus tubulipenis and Cytocystis clitellatus is very alike but differs from previously described species. Both sheath and cone epithelium are composed of two circumferential belts. Only the basal cone epithelium is syncytial, while no nuclei were found in the distal belt of the sheath epithelium. The sheath epithelium is characterized by numerous infoldings of the basal plasma membrane. The nuclei present in the bulb belong to the proximal belt of the sheath epithelium and the apical cone epithelium. Nuclei of the basal cone epithelium are located insunk behind the proboscis bulb. The insunk cell parts pierce the septum of the bulb laterally near the proximal end. Different types of gland necks and sensory cells pierce the epithelia. Associated with the distal belt of the sheath epithelium, two sensory organs are found, containing multiciliary receptors with modified axonemata. Differences in organization of the proboscis musculature are described and compared with the organization in other species. The systematic position of both species is discussed in the light of the new findings.