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.     

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.     

DNA-Replikation und Chromosomenstruktur von Mesostoma (Turbellaria)

During meiosis in M. ehrenbergi (2n=10) and M. lingua (2n=8) male certain chromosomes never pair completely. In these bivalents only terminal pairing appears, crossing over could not be proved by ³H-thymidine autoradiography. DNA amounts of the M. ehrenbergi and M. lingua genomes are in a proportion of 10∶1. The mitotic S-phase of spermatogonia in M. ehrenbergi is twice as long as in M. lingua. In metaphase of spermatogonia a differentiated DNA replication pattern can be identified in M. ehrenbergi as late-pulse-replicating segments. After incorporation of ³H-thymidine X₂-metaphase chromosomes can be found, which show single chromatid labeling, terminal and intercalary isolabeling as well as kinds of chromosome labeling, which can only result from sister strand exchange. After treating the chromosomes with low temperature, colchicine or by hydrolysis (60° C) substructures of the chromatin become visible in both spezies which however are evaluated as artefacts. — Formation of the different isolabeling types is discussed on the basis of a two-strand model of the chromosome fibril. A hypothesis is formulated that the surplusage of DNA in M. ehrenbergi is distributed over all the length of the chromatids as small parts of heterochromatin. This hypothesis is supported by investigations of the DNA replication and the contractility of the chromosomes. Furthermore, a pattern of small DNA particles can be demonstrated after partial destruction of the DNA in metaphase chromosomes of M. ehrenbergi, which could represent this intercalary heterochromatin.     

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.     

Ultrastructure of the Stylet Protractor Muscle in Gyratrix hermaphroditus (Turbellaria,Rhabdocoela)

The subsarcolemmal cisternae of the sarcoplasmic reticulum form peripheral couplings with the sarcolemma. The junctional gap is crossed by periodic densities called junctional processes. Desmosomes provide mechanical coupling between the myofibres. Hemidesmosomes connect the myofibre with a well developed connective tissue sheath.     

Population growth in some Mesostoma species (Turbellaria) predatory on mosquitoes

SUMMARY 1. Turbellarian predators of the genus Mesostoma prey on the aquatic stages of mosquitoes. In order to evaluate their potential as control agents, a comparison of population parameters has been made on three species from Australia, Africa and Papua New Guinea: M. appinum , M. zariae and M. timbunke.
2. The life cycle of these species is relatively short at higher temperature ranges (22–30°C) and varies from 12 to 35 days, depending on the temperature. M. appinum does not increase in numbers at 30°C, white M. timbunke is unable to reach sexual maturity at 15°C. M. appinum , but not M. zariae , multiplies slowly at 15°C, although both species survive at this temperature. The fastest population growth was obtained for M. appinum at 22°C, and for M. zariae and M. timbunke at 30°C. The doubling time for the population is about 6–7 days in the multivoltine species, M. appinum and M. zariae , and about 18.5 days in univoltine M. timbunke .
3. A significant part of the growing population is composed of immature individuals, which corresponds to the high values of the reproductive parameters R_o and r_m. In M. appinum and M. zariae , individuals rarely produce dormant resting eggs except under deteriorating conditions. M. timbunke produces only dormant eggs, and in this respect resembles arctic and subarctic species. A convergence of the reproductive modes between this equatorial species and the high latitude species might be interpreted as adaptation to harsh environments.
4. The overall population growth of all Mesostoma spp. is several times faster than in Dugesia dorotocephala and D. trigrina , other flatworms studied as potential biological control agents of mosquitoes     

The geographical and bathymetrical distribution of the Fecampiidae (Turbellaria,Rhabdocoela)

All members of the Fecampiidae are endoparasites. Since 1964 when only four species were known, four new species and 11 cocoon types, made by as many unknown species, have been described.The Fecampiidae are distributed in all major oceans from shallow waters to more than 5000 m depth.     

Viruslike particles in Gyratrix hermaphroditus (Turbellaria: Rhabdocoela)

During an ultrastructural examination, viruslike particles were observed in the turbellarian Gyratrix hermaphroditus. This is the first time viruslike particles have been found in a noncultivated platyhelminth species. The particles are 70 nm in diameter and have a capsidlike outer layer and an inner core measuring 40–50 nm in diameter. They occur in a crystalline arrangement in the nucleus as well as in the cytoplasm. Numerous cytoplasmic abnormalities were seen in connection with the particles. The occurrence of the particles in different tissues and their significance for the host are discussed.     

The Ultrastructure of the Nervous System of Gyratrix hermaphroditus (Turbellaria, Rhabdocoela)

The peripheral nervous system and the synapses of G. hermaphroditus are studied with the electron microscope. There is a submuscular as well as a subepithelial plexus. The subepithelial plexus is found among the muscles and between the muscles and the basement membrane. It consists of fibres containing large lucent and lysosome-like vesicles and fibres with only small lucent (synaptic) vesicles. In the deeper lying submuscular plexus also dense and dense-cored vesicles occur in the fibres. Cell bodies are not observed in the plexuses. The separate nerve supplies of the pharynx and the gonads contain nerve cells of the neurosecretory type. Fibres of the same kinds as in the brain are also seen here. The synapses in the neuropile are of two kinds. 1. Symmetrical synapses with an additional presynaptic network are most common. 2. Synapses without thickenings of membranes are observed between lateral membranes of neurites. In the peripheral nervous system are two other kinds of synapses also observed. 1. Asymmertical synapses with a denser and wider postsynaptic thickening and 2. neuromuscular junctions. Neurites containing accumulations of small vesicles against the basement membrane are also described. The organization of the peripheral nervous system is described and discussed in relation to the systematic position of G. hermaphroditus.     

Die mechanismen der subitan- und dauereibildung bei Mesostoma lingua (Abildgaard, 1789) (Turbellaria,Neorhabdocoela)

Four populations of Mesostoma lingua from various geographical areas react in different ways according to certain environmental factors. The determination of the two eggtypes is particularly influenced by temperature changes and a crowding effect. The influence of other factors is of little consequence.     

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.     

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.     

The Eyes of Mesostoma ehrenbergi (Focke, 1836) (Platyhelminthes,Rhabdocoela). Fine Structure and Photoreceptor Membrane Turnover

Abstract Each pigment-cup eye of Mesostoma ehrenbergi consists of two photoreceptor cells, the anterior cell being bilobate. the posterior almost linear, and of a multicellular pigment cup. The nuclei of the photoreceptor cells are located inside the medial region of the brain. Thin cytoplasmic photoreceptor projections provided with neurosecretory-like granules are interposed between the inner surface of the eye cup and the distal extremity of the microvilli. The breakdown and renewal of microvillar membranes was analysed. Membrane turnover is a continuous process. At dusk and during the night abscission of photoreceptive membranes occurs. At dawn the membrane fragments are degraded to granular material, which is then endocytosed into the submicrovillar cytoplasm as coated vesicles. These vesicles form multivesicular bodies. The degradation of multivesicular body content occurs during the following light hours. The dark period is correlated with membrane synthesis for elongation of reticular membranes, which are converted into ellipsoid bodies. The formation of new microvillar membranes occurs at the base of the microvillar border, and involves the fusion with the old microvillar membranes of small vesicles detached from the tubular endoplasmic membranes and from the flattened concentric cisternae of ellipsoid bodies. The correlations with daily cycles of other invertebrates are discussed.     

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.     

Fine structure and functional role of the coverings of the eggs inMesostoma ehrenbergii (Focke) (Turbellaria,Neorhabdocoela)

Summary European populations of the turbellarianMesostoma ehrenbergii can produce two types of eggs according to a seasonal cycle. Subitaneous eggs (S eggs) are produced throughout the warm season and develop rapidly inside the parental uteri. They are meant to ensure a fast increase in the number of worms. Dormant eggs (D eggs) are produced during the warm season and are usually deposited in water at the death of the parent. They can hatch at the beginning of the next warm season and are, therefore, meant to ensure the survival of the population throughout the cold season, when adult worms are no longer present.S eggs are bounded by a thin, translucent covering which has a loose structure and is mainly proteinaceous in composition. These characteristics account for the functional role played by the S egg covering during embryonic development. The covering has, in fact, to be permeable to parental nutrients, and to keep pace, by stretching, with the enlargement of the embryo. By contrast, D eggs are bounded by a thick shell, presumably composed of a sclerotin, the precursors of which (polyphenols) are synthesized together with yolk in the vitelline cells. The shell has a smooth surface devoid of pores and has the function of isolating the developing D egg from the external environment.Supported by a grant from the Consiglio Nazionale della RicercheThe authors wish to thank Dr. P. Roach for his help in translating the paper     

An ecological study of Phaenocora unipunctata (Örsted) (Turbellaria; Rhabdocoela): predator-prey interrelationships

The diet of P. unipunctata living in an English pond was investigated by examination of the gut contents of field-collected specimens, by a serological method and by laboratory experiments. Tubificids, and to a lesser extent lumbriculids, are the preferred food of small and large flatworms. In the laboratory damaged Cladocera and Chironomidae were fed upon to a very limited extent and in the serological tests a few flatworms gave a positive reaction with anti-Cladocera and anti-Chironomidae sera, but it is unlikely that these taxa are preyed upon extensively in the field. Communal feeding occurs. Algae and Protozoa are not eaten. P. unipunctata has no competitors for the oligochaete resource in the pond.Predators of the flatworm were determined mainly by laboratory experiments. Only the larvae and adults of Dytiscus attacked a few specimens. It is unlikely that flatworm and its cocoons have any serious predators in the pond.     

Behavior of the rhabdocoel flatworm Mesostoma ehrenbergii in prey capture and feeding

The prey-capture and feeding behavior of the rhabdocoel flatworm Mesostoma ehrenbergii (Focke, 1836) was analyzed using a variety of live and dead prey, including Daphnia, mosquito larvae, and tubifex annelids. Prey-capture behavior was broken down into its individual components. Mesostoma could accommodate to and change its behavior depending on the size and type of prey. Mechanical rather than chemical cues were effective in inducing prey-capture behavior. No evidence for a special chemical paralysis as suggested by other workers was found. The apparent paralysis observed in cladocera such as Daphnia and mosquito larvae was, in part a behavioral response of the prey in ‘playing possum’ and also in part due to immobilization of the prey by the flatworm with mucous threads. This revised version was published online in July 2006 with corrections to the Cover Date.