Factors influencing the distribution of aquatic plant communities in Irish canals

Terrestrial tardigrades are often found in the lichens and mosses growing on trees and rocks. The assertion that tardigrades in these habitats are very patchy in their distribution has rarely been backed by quantitative sampling. This study assesses spatial variability in tardigrade populations inhabiting small patches (0.1 cm² to over 5 cm²) of moss and lichen on trees and rocks at three sites in the United States of America. Tardigrades were collected from four replicate rocks in the Ouachita Mountains of Arkansas, with 30 lichen patches collected on two adjacent boulders and 20 moss patches on a second pair of boulders. In Fort Myers and in Citrus Springs, Florida, 30 lichen patches per tree were collected from two pairs of trees. The tardigrades in each sample were extracted, mounted, identified, and counted. The variation in tardigrade abundance among lichen or moss patches within rocks or trees was very high; the only consistent pattern was that very small patches usually lacked tardigrades. Tardigrade diversity and abundance also varied greatly within sites when lichens and mosses of the same species from different rocks and trees were compared (in the most extreme case one tree had numerous individuals of two tardigrade species present while the other had almost no tardigrades). The results of this quantitative sampling support the assertion that tardigrades are very patchy in distribution. Given the considerable time investment required for the quantitative processing of tardigrade samples, this high spatial variability in tardigrade diversity and abundance requires that researches testing ecological hypotheses about tardigrade abundance check variability before deciding how many samples to take.     

The brain of Echiniscus viridissimus Peterfi, 1956 (Heterotardigrada): a key to understanding the phylogenetic position of tardigrades and the evolution of the arthropod head

The brain of Echiniscus viridissimus , Peterfi, 1956 is composed of a series of orthogonally arranged neuropils. The most anterior neuropils are rireumbuccal, positioned dorso-and ventrolateral to the buccal tube and are associated with ganglia for sensory receptors of the mouth cone. Posterior to these are neuropils and ganglia for the (1) internal cirri and (2) cephalic papillae, external cirri, cirri A and clavae. They are joined by two pairs of vertical tracts to neuropils lateral to the buccal tube. A model based on the postcephalic organization of the tardigrade nervous system is used to propose a transformation of segmental ganglia that gives an arrangement congruent with the pattern of neuropils in the brain. The analysis suggests that the brain is derived from nervous elements of four segments with the fourth segment having contributed paired dorsal ganglia and their connecting vertical tracts to the first trunk ganglia of the ventral chain. The organization of the head of tardigrades is compared with that of other lobopods and arthropods and several possible key evolutionary innovations are offered. In addition homologous characters for the heads of tardigrades and other lobopods and arthropods are proposed and the nomenclature for the tardigrade cephalic nervous system is discussed.     

Limb development in a primitive crustacean,Triops longicaudatus: subdivision of the early limb bud gives rise to multibranched limbs

 Recent advances in developmental genetics of Drosophila have uncovered some of the key molecules involved in the positioning and outgrowth of the leg primordia. Although expression patterns of these molecules have been analyzed in several arthropod species, broad comparisons of mechanisms of limb development among arthropods remain somewhat speculative since no detailed studies of limb development exist for crustaceans, the postulated sister group of insects. As a basis for such comparisons, we analysed limb development in a primitive branchiopod crustacean, Triops longicaudatus. Adults have a series of similar limbs with eight branches or lobes that project from the main shaft. Phalloidin staining of developing limbs buds shows the distal epithelial ridge of the early limb bud exhibits eight folds that extend in a dorsal ventral (D/V) arc across the body. These initial folds subsequently form the eight lobes of the adult limb. This study demonstrates that, in a primitive crustacean, branched limbs do not arise via sequential splitting. Current models of limb development based on Drosophila do not provide a mechanism for establishing eight branches along the D/V axis of a segment. Although the events that position limbs on a body segment appear to be conserved between insects and crustaceans, mechanisms of limb branching may not. Received: 28 February 1996/Accepted: 24 June 1996     

Are the supportive structures of the tardigrade pharynx homologous throughout the entire group?

Based on the differences in position, structure and function of the cuticular supportive structures in the pharynx of representatives of two major groups of tardigrades, the Heterotardigrada and Eutardigrada, it is postulated that these structures are not homologous throughout the Tardigrada. Hithero, these structures have been termed placoids in both classes (fused placoids in Heterotardigrada versus macro- and microplacoids in Eutardigrada). It is proposed here that there are two different kinds of fused placoids within the Heterotardigrada and that only one of these is homologous to the macro- and microplacoids of the Eutardigrada. It is furthermore proposed that the plesiomorphic condition for the tardigrade pharynx is without reinforced cuticular supportive structures and not the presence of encrusted fused placoids as found in Heterotardigrada.     

Brain anatomy of the marine tardigrade actinarctus doryphorus (arthrotardigrada)

Knowledge of tardigrade brain structure is important for resolving the phylogenetic relationships of Tardigrada. Here, we present new insight into the morphology of the brain in a marine arthrotardigrade, Actinarctus doryphorus, based on transmission electron microscopy, supported by scanning electron microscopy, conventional light microscopy as well as confocal laser scanning microscopy. Arthrotardigrades contain a large number of plesiomorphic characters and likely represent ancestral tardigrades. They often have segmented body outlines and each trunk segment, with its paired set of legs, may have up to five sensory appendages. Noticeably, the head carries numerous cephalic appendages that are structurally equivalent to the sensory appendages of the trunk segments. Our data reveal that the brain of A. doryphorus is partitioned into three paired lobes, and that these lobes exhibit a more pronounced separation as compared to that of eutardigrades. The first brain lobe in A. doryphorus is located anteriodorsally, with the second lobe just below it in an anterioventral position. Both of these two paired lobes are located anterior to the buccal tube. The third pair of brain lobes are situated posterioventrally to the first two lobes, and flank the buccal tube. In addition, A. doryphorus possesses a subpharyngeal ganglion, which is connected with the first of the four ventral trunk ganglia. The first and second brain lobes in A. doryphorus innervate the clavae and cirri of the head. The innervations of these structures indicate a homology between, respectively, the clavae and cirri of A. doryphorus and the temporalia and papilla cephalica of eutardigrades. The third brain lobes innervate the buccal lamella and the stylets as described for eutardigrades. Collectively, these findings suggest that the head region of extant tardigrades is the result of cephalization of multiple segments. Our results on the brain anatomy of Actinarctus doryphorus support the monophyly of Panarthropoda. J. Morphol. 275:173–190, 2014. © 2013 Wiley Periodicals, Inc.     

Segmentation,neurogenesis and formation of early axonal pathways in the centipede,Ethmostigmus rubripes (Brandt)

Summary We have examined the embryo of the centipedeEthmostigmus rubripes to determine the degree of evolutionary conservatism in the developmental processes of segmentation, neurogenesis and axon formation between the insects and the myriapods. A conspicuous feature of centipede embryogenesis is the early separation of the left and right sides of the ganglionic primordia by extra-embryonic ectoderm. An antibody to the protein encoded by theDrosophila segmentation geneengrailed binds to cells in the posterior margin of the limb buds in the centipede embryo, in common with insect and crustacean embryos. However, whereas in insects and crustaceans this protein is also expressed in a subset of cells in the neuroectoderm, the anti-engrailed antibody did not bind to cells in the ganglionic primordia of the centipede embryo. Use of the BrdU labelling technique to mark mitotically active cells revealed that neuroblasts, the ubiquitous neuron stem cell type in insects, are not present in the centipede. The earliest central axon pathways in the centipede embryo do not arise from segmentally repeated neurons, as is the case in insects, but rather by the posteriorly directed growth of axons originating from neurons located in the brain. Axonogenesis by segmental neurons begins later in development; the pattern of neurons involved is not obviously homologous to the conservative set of central pioneering neurons found in insects. Our observations point to considerable differences between the insects and the myriapods in mechanisms for neurogenesis and the formation of central axon pathways, suggesting that these developmental processes have not been strongly conserved during arthropod evolution.     

Interspecific Association and Substrate Specificity in Tardigrades from Florida, Southeastern United States

The distribution of terrestrial tardigrades in the Gulf Coast states of the United States is poorly known. Only one species has been reported in Florida. In this study moss, liverwort, lichen and fern samples (47 identified species) from trees and shrubs were collected from all 67 Florida counties. These samples contained 20 species of tardigrade. All possible pairs of tardigrade species and tardigrade and substrate were tested for interspecific association. Only three significant negative and one positive interspecific association between tardigrades were detected. Evidence for substrate specificity was weak. Although some tardigrade species were significantly associated with mosses or foliose lichens in general, no significant association between a tardigrade species and a substrate species was detected.     

Somatic musculature of Tardigrada: phylogenetic signal and metameric patterns

Although studies describing molecular‐based phylogenies within tardigrades are now frequently being published, this is not the case for studies combining molecular and morphological characters. Tardigrade phylogeny is still based, from a morphological point of view, almost exclusively on chitinous structures and little attention has been given to detecting and using novel morphological data. Consequently, we analysed the musculature of seven tardigrade species belonging to the main phyletic lines by confocal laser scanning microscopy and compared these morphological results with new molecular analyses (18S+28S rRNA genes). Finally, we analysed all the data with a total evidence approach. A consilience in the phylogenetic relationships among orders and superfamilies of tardigrades was obtained among the evolutionary trees obtained from morphological, molecular and total evidence approaches. Comparative analysis on the musculature allowed the identification of serial homologies and repeated metameric patterns along the longitudinal animal body axis. A phenomenon of mosaic evolution was detected in musculature anatomy, as dorsal musculature was found to be highly modified with respect to the other body muscle groups, probably related to the evolution of dorsal cuticular plates. An understanding of tardigrade musculature anatomy will give fundamental information to understand the evolution of segmental pattern within Panarthropoda. © 2013 The Linnean Society of London     

Graphical Presentation of the African Tardigrade FaunaUsing GIS with the Description of Isohypsibius malawiensis sp. n. (Eutardigrada: Hypsibiidae) from Lake Malawi

The tardigrade fauna of the African continent is reviewed and presented graphically using Worldmap, a Geographical Information System (GIS) developed for exploring geographical diversity patterns in large biological datasets. References to the African tardigrade fauna have been gathered from published literature and supplemented with unpublished species information from the collection of Prof. Reinhardt M. Kristensen (RMK), Zoological Museum, University of Copenhagen. 156 species belonging to 36 genera of tardigrades are present. They consist of 105 eutardigrade species and 51 heterotardigrade species, of which 42 species are semiterrestrial and 9 species are marine. The presence of tardigrades are reported from 20 countries, but of these 9 are represented by a single reference. Marine tardigrades in particular have been neglected, with only a single report from the shores of the African continent. Data from the RMK collection of samples from Egypt is included in the analysis.

The scattered and sparse knowledge of the African tardigrade distribution makes general conclusions difficult, but emphasises the large regions which require further investigation. The current distribution patterns corresponds with easy accessible or ‘tourist’ locations. Regions where GIS could be used to illustrate ecological preferences are also pointed out by the analysis.

Isohypsibius malawiensis sp. n. is described from Lake Malawi. It has a smooth body surface and lacks eyes. The heteronych claws are without lunulae, but below the claws a double arched cuticular bar is present. The dorsal and ventral apophyses of the buccal tube have a large drop-shaped swelling. The first macroplacoid is the largest followed by the second, which is only slightly larger than the third; microplacoid is absent. Males are smaller than females. Females lay only one to two very large eggs in the exuvium. The Malpighian tubules are clover-shaped. The new species is interstitial in coarse sediment and no other tardigrade species were present.     

The tardigrade Hypsibius dujardini, a new model for studying the evolution of development

Studying development in diverse taxa can address a central issue in evolutionary biology: how morphological diversity arises through the evolution of developmental mechanisms. Two of the best-studied developmental model organisms, the arthropod Drosophila and the nematode Caenorhabditis elegans, have been found to belong to a single protostome superclade, the Ecdysozoa. This finding suggests that a closely related ecdysozoan phylum could serve as a valuable model for studying how developmental mechanisms evolve in ways that can produce diverse body plans. Tardigrades, also called water bears, make up a phylum of microscopic ecdysozoan animals. Tardigrades share many characteristics with C. elegans and Drosophila that could make them useful laboratory models, but long-term culturing of tardigrades historically has been a challenge, and there have been few studies of tardigrade development. Here, we show that the tardigrade Hypsibius dujardini can be cultured continuously for decades and can be cryopreserved. We report that H. dujardini has a compact genome, a little smaller than that of C. elegans or Drosophila, and that sequence evolution has occurred at a typical rate. H. dujardini has a short generation time, 13–14 days at room temperature. We have found that the embryos of H. dujardini have a stereotyped cleavage pattern with asymmetric cell divisions, nuclear migrations, and cell migrations occurring in reproducible patterns. We present a cell lineage of the early embryo and an embryonic staging series. We expect that these data can serve as a platform for using H. dujardini as a model for studying the evolution of developmental mechanisms.     

The Mouth Cone and Mouth Ring of Echiniscus viridissimus Peterfi, 1956 (Heterotardigrada) with Comparisons to Corresponding Structures in Other Tardigrades

The mouth cone and mouth ring of the tardigrade Echiniscus viridissimus are described and their primary homology is assessed by comparing them to structures considered to be homologous in other heterotardigrades, eutardigrades, and ecdysozoans. In E. viridissimus the mouth cone is divided into anterior and posterior regions that differ in the ultrastructure of their cuticle. The mouth ring of the buccopharyngeal apparatus lies just anterior to the buccal tube. It is connected to it by a narrow flange or ridge of cuticle that allows the rostral end of the mouth tube to telescope part way into the mouth ring. Sensilla innervating the mouth cone and mouth ring in E. viridissimus correspond to sensilla of the circumoral and suboral sensory regions described for eutardigrades. The sensory fields, of both the mouth cone and mouth ring, exhibit two planes of mirror image or biradial symmetry. Although the relative dimensions of mouth ring differ markedly between E. viridissimus and eutardigrades, homologous areas can be identified easily. For example, the supporting rods of the mouth ring in E. viridissimus appear to be homologous to the supporting plates in the lamellae of eutardigrades. The widespread occurrence of the mouth cone and mouth ring in hetero- and eutardigrades suggests that these structures were present in the last common ancestor of all tardigrades. Expression of these characters in other ecdysozoans is more problematic. Tentatively the mouth cone and mouth ring are considered to be homologous to the circumoral plates of onychophorans and the ring of radiating plates found in the ‘Peytoia’ apparatus of Cambrian arthropods. These structures are not thought to be homologous to the introverts of scalidophorans or the circumoral structures of nematodes or nematomorphs, and are considered to be absent in these taxa. Thus, the mouth cone and mouth ring are potentially key synapomorphies for Panarthropoda.     

Isolated microspore culture of Chinese flowering cabbage (Brassica campestris ssp. parachinensis)

Microspores of several genotypes of Brassica campestris ssp. parachinensis have been cultured in vitro and induced to undergo embryogenesis and plant formation. Conditions favourable for embryogenesis in this species include a bud size of 2–2.9 mm, NLN-13 culture medium (Nitsch and Nitsch 1967; Lichter 1981, 1982; Swanson 1990), and an induction through exposure to 32°C for a period of 48 h. Longer periods of an elevated temperature for induction of embryogenesis resulted in embryo abortion at early developmental stages. With the protocol developed here, microspores of 60–80% of donor plants could be induced to produce embryos, although embryo yields were low, i.e. 2–5 embryos per 10 buds. Some genotypes responded to culture conditions with high numbers of embryo formation (100–150 embryos per 10 buds) but most of these subsequently failed to mature. The pattern of cell division and morphological changes of the microspores in culture were studied using various microscopic techniques.     

External morphology of limb development in the amphipod <Emphasis Type="Italic">Orchestia cavimana</Emphasis> (Crustacea,Malacostraca, Peracarida)

The external morphology of limb development in Orchestia cavimana is examined by scanning electron microscopy and fluorescence staining from the appearance of the first limb buds until hatching. As other amphipods, O. cavimana undergoes direct development and the degree of segmental differentiation shows a more or less continual decrease in anteroposterior direction. Limbs form ventrally as small buds, which elongate and divide into podomeres early in development. This early subdivision largely corresponds with the limb segmentation of the hatchling. When the post-naupliar limbs start to develop, the germ band begins to split into two halves along the midline, so that the trunk limbs transiently occupy a very dorsolateral position. After the germ band has closed again, the differentiation into the characteristic amphipodan tagmata (cephalothorax, pereon, pleon) takes place and the limb podomeres lose their round-shape. The late embryo is covered by a so-called intermediate cuticle, which is formed after an embryonic moult and shed after hatching. The early development of O. cavimana reveals the Anlage of a vestigial seventh pleonic segment that is assumed to belong to the ground pattern of malacostracans, but is retained as a free, limbless segment only in adult Leptostraca. A transient subdivision of the proximal segment of the pleopods suggests the occurrence of a coxa and a basis in these limbs. The mandible attains its upright, adult position via a characteristic bending process that is strikingly similar to that in Archaeognatha (Insecta).     

Tardigrade Fauna of the Region between Oka and Volga Rivers (Russia) and Patterns of their Distribution in Substrates and Habitats

The region between the Oka and Volga rivers has been largely neglected with respect to tardigrade biodiversity. In the present study a total of 21 species from 7 genera, 3 families, 2 orders and only 1 class of tardigrades were present, and some of their ecological preferences were noted. The tardigrades in the studied region were mainly cosmopolitan. Eurytopic, hygrophilic and xerophilic species were common while obligate freshwater species were not found. There is a marked similarity in the tardigrade fauna within similar habitats from different locations within the region.     

Soil-inhabiting Tardigrade Communities in Forests of Central Japan

This study was carried out for the purpose of detecting the relationship between soil-inhabiting tardigrade communities and environmental factors of various forests. Nine forests in the southern part of Kanagawa Prefecture, Central Japan, were selected for this study. Four vegetation types were designated; broadleaved (evergreen/deciduous), coniferous and orchard. In these sites, dry weight of leaf litter, soil pH, soil hardness, soil moisture content and porosity ratio were measured. Wet soil faunal frequencies were also described. The Baermann funnel method was adopted for collecting tardigrades, and DIC microscopy was used for specific identification. To clarify the correlation between environmental variables and tardigrade faunae, multivariate analysis was applied. The tardigrade fauna occurred as two distinct groups. The first group primarily contained Macrobiotus species. The second group contained the genus Diphascon (e.g. D. nobilei, D. patanei, D. prorsirostre). An exception to this was D. pingue, included in the former. Most of the Diphascon species were concentrated in Nebu (coniferous forest site), while, Macrobiotus species were dominant in other sites. Distinct environmental factors could not be identified, but the nematodal frequency was recognized as the main factor in forming these groups. The uniqueness of Nebu, which does not correspond to large-scaled vegetational classification, was substantiated by statistical data. Nebu’s coniferous forest apparently has created a unique environment for sustaining rare species (Diphascon species). This study concluded that forests should be evaluated not only by the macroscopic factors, such as landscape, but also by the microscopic communities, such as those including the tardigrades.     

High-frequency embryogenesis inBrassica campestris microspore culture

Microspore culture is a very important and useful tool in plant breeding for haploid production and has been developed for many years.Brassica campestris (Brassica rapa L. ssp.oleifera) is an important oilseed crop, but it is relatively recalcitrant in tissue culture including microspore culture. The microspore culture in our laboratory is based on the Canadian protocol. Thirty genotypes ofB. campestris were included in this study; twenty produced embryos. The highest yield was 5930 embryos per 100 buds from Canadian genotype Cv-2, this result was one of the best that had been reported in microspore culture inB. campestris. The buds measuring 2.0 mm to 3.9 mm in length responded best to produce embryos, the optimum timing for microspore culture was confirmed to be during the mid-late to very-late uninucleate stage. The buds could be removed from either the main raceme or lateral racemes. Activated charcoal (150 mg l^-1) was added to the liquid NLN medium, it promoted embryogenesis significantly; embryo development was faster and the embryo yield was significantly higher than those cultures without activated charcoal. The donor plant condition was considered an important factor influencing embryogenesis; older donor plants (older than five weeks) and a cold treatment are recommended.     

Induction of triploid Citrus plants from endosperm calli in vitro

Summary Triploid hybrid Citrus plants were regenerated by somatic embryogenesis in vitro from endosperm derived calli. A sequence of media formulations was used to induce and support proliferation of primary callus from endosperm, to induce embryogenesis from primary callus, and to allow embryo development leading to viable plantlets. Calli were induced from cellular endosperm of Citrus sinensis (sweet orange), C. Xparadisi (grapefruit), and C. grandis (pummelo) excised 12–14 weeks post-anthesis. Induction of embryogenesis from sweet orange and pummelo primary calli required gibberellic acid and double mineral nutrient concentrations. Embryogenesis was not induced from grapefruit calli in these experiments. Only sweet orange embryos developed sufficiently to allow plant regeneration. Triploid axillary buds were minigrafted onto etiolated diploid rootstock seedlings in vitro in order to transfer triploid regenerants to soil and the external environment. Triploidy (2n = 3x = 27) was observed consistently in all phases of regeneration and in recovered plants. These results demonstrate that triploid hybrid plant recovery from Citrus endosperm can overcome barriers to sexual hybridization resulting from apomixis.Florida Agricultural Experiment Station Journal Series No. R-00627     

Population density and species composition of moss-living tardigrades in a boreo-nemoral forest

This study investigates for the first time the tardigrade fauna in a variety of different mosses from a coniferous forest and an adjacent clear-cut area in southern Sweden. Tardigrades were found in a majority of the samples. Sixteen species were recorded, of which the cosmopolitan species Macrobiotus hufelandi was the far most common. Some mosses, particularly species with "wefts" growth form, contained more tardigrades than other mosses, indicating that growth form may have an impact on tardigrade abundance. Mosses of the same species collected from a forest and from a clear-cut, respectively, did not show a general trend in the overall abundance of tardigrades, but the forest tended to contain more species. Five species of tardigrades ( Murrayon dianae, Isohypsibius sattleri, Platicrista angustata, Diphascon belgicae and Diphascon pingue ) never previously reported from Sweden were recorded.