A common origin of complex life cycles in parasitic flatworms: evidence from the complete mitochondrial genome of <Emphasis Type="Italic">Microcotyle sebastis</Emphasis> (Monogenea: Platyhelminthes)

Background  

The parasitic Platyhelminthes (Neodermata) contains three parasitic groups of flatworms, each having a unique morphology, and life style: Monogenea (primarily ectoparasitic), Trematoda (endoparasitic flukes), and Cestoda (endoparasitic tapeworms). The evolutionary origin of complex life cyles (multiple obligate hosts, as found in Trematoda and Cestoda) and of endo-/ecto-parasitism in these groups is still under debate and these questions can be resolved, only if the phylogenetic position of the Monogenea within the Neodermata clade is correctly estimated.     

Mitochondrial genomes of parasitic flatworms

Complete or near-complete mitochondrial genomes are now available for 11 species or strains of parasitic flatworms belonging to the Trematoda and the Cestoda. The organization of these genomes is not strikingly different from those of other eumetazoans, although one gene (atp8) commonly found in other phyla is absent from flatworms. The gene order in most flatworms has similarities to those seen in higher protostomes such as annelids. However, the gene order has been drastically altered in Schistosoma mansoni, which obscures this possible relationship. Among the sequenced taxa, base composition varies considerably, creating potential difficulties for phylogeny reconstruction. Long non-coding regions are present in all taxa, but these vary in length from only a few hundred to approximately 10000 nucleotides. Among Schistosoma spp., the long non-coding regions are rich in repeats and length variation among individuals is known. Data from mitochondrial genomes are valuable for studies on species identification, phylogenies and biogeography.     

Towards a theoretical basis of paleoecology: concepts of community dynamics

The science of paleoecology suffers from a lack of conceptual frameworks. Paleoecologists have been overconcerned with the inadequacies of the fossil record: as a result, community palmecology has historically developed very slowly. At the community ecosystem level, the need for a theoretical framework is so great that paleoecology must 'borrow' the hypotheses of modem ecology. Consideration of the stability-time hypothesis of Sanders in conjunction with the physical setting of transgression and regression has permitted the structuring of three community types and the interpretation of their behavior under variations in the physical environment. These community types (opportunistic. stable mature, relict mature) are recognizable in the fossil record and examples are given from the Upper Pennsylvanian of the Appalachian Basin.     

Vitellogenesis of basal trematode Aspidogaster limacoides(Aspidogastrea: Aspidogastridae)

The vitellogenesis of the trematode Aspidogaster limacoides (Aspidogastrea: Aspidogastridae), a parasite of cyprinid fishes, is described here using transmission electron microscopy. Four different stages of vitellocytes are differentiated: immature vitellocytes, early maturing vitellocytes, advanced maturing vitellocytes and mature vitellocytes. The process follows the same general pattern already described in other free-living neoophorans and parasitic flatworms (i.e. Trematoda, Monogenea and Cestoda): differentiation into mature vitelline cells involves the development of mitochondria, granular endoplasmic reticulum, Golgi complexes, lipid droplets and shell-globules. Mature vitellocytes of A. limacoides are composed of numerous shell-globule clusters, few lipid droplets and glycogen granules. They differ from those of another aspidogastrean Rugogaster hydrolagi in that they possess numerous globules tightly packed and by the presence of only one type of vitelline material. The interstitial tissue of vitelline follicles of A. limacoides contains a peripheral nucleus and long cytoplasmic projections extending between vitelline cells. Since aspidogastreans are considered as an archaic group of parasitic flatworms and thus have a strategic phylogenetic position, future works needs to pay special attention to the ultrastructural and chemical composition of mature vitellocytes within this basal group of trematodes.     

QUALITY OF THE TRIASSIC–JURASSIC BIVALVE FOSSIL RECORD IN NORTHWEST EUROPE

Abstract: The quality of the Triassic–Jurassic bivalve fossil record in northwest Europe has been measured using the Simple Completeness Metric (SCM). The SCM has been applied to the fossil record of total bivalve diversity and to the records of different ecological guilds. The Westbury and Lilstock Formations record high SCM values for most ecological groups. The ‘Pre‐Planorbis Beds’ of the lower Lias Group, however, witness a precipitous decline in the completeness of most guilds and emigration of taxa due to localized marine anoxia is a likely cause. Neither variation in lithofacies, shell mineralogy, sedimentary rock outcrop area, nor sequence architecture can convincingly explain the observed patterns of completeness. Our SCM data reveal that the Early Jurassic fossil record of infaunal suspension‐feeding bivalves is significantly poorer than that of epifaunal bivalves. Any differences in the apparent Rhaetian extinction rates between these two guilds should therefore be viewed with caution. Analyses of selectivity during the Late Triassic mass extinction based on studies of global databases appear robust in light of our SCM data. Nevertheless, future investigations of the Triassic–Jurassic benthic marine ecosystem undertaken at a finer‐resolution, may need to account for the poor quality of the Early Jurassic fossil records of certain ecological guilds, such as the infaunal suspension‐feeding taxa.     

Hox genes and the parasitic flatworms: new opportunities, challenges and lessons from the free-living

Research into the roles played by Hox and related homeotic gene families in the diverse and complex developmental programmes exhibited by parasitic flatworms (Platyhelminthes) can hardly be said to have begun, and thus presents considerable opportunity for new research. Although featured in some of the earliest screens for homeotic genes outside Drosophila and mice, surveys in parasitic flatworms are few in number and almost nothing is yet known of where or when the genes are expressed during ontogeny. This contrasts sharply with a significant body of literature concerning Hox genes in free-living flatworms which have long served as models for the study of regeneration and the maintenance of omnipotent cell lines. Nevertheless, available information suggests that the complement of Hox genes and other classes of homeobox-containing genes in parasitic flatworms is typical of their free-living cousins and of other members of the Lophotrochozoa. Recent work on Schistosoma combined with information on Hox gene expression in planarians indicates that at least some disruption of the clustered genomic arrangement of the genes, as well as of the strict spatial and temporal colinear patterns of expression typical in other groups, may be characteristic of flatworms. However, available data on the genomic arrangement and expression of flatworm Hox genes is so limited at present that such generalities are highly tenuous. Moreover, a basic underlying pattern of colinearity is still observed in their spatial expression patterns making them suitable as cell or region-specific markers. I discuss a number of fundamental developmental questions and some of the challenges to addressing them in relation to each of the major parasitic lineages. In addition, I present newly characterized Hox genes from the model tapeworm Hymenolepis and analyze these by Bayesian inference together with >100 Hox and ParaHox homeodomains of flatworms and select lophotrochozoan taxa, providing a phylogenetic scaffold for their identification.     

Uniformity in marine invertebrate ichnology

Relationships between ichnology and uniformitarianism are perhaps less complicated than those of their sister subdisciplines, paleontology and paleoecology. Trace fossils are manifestations of benthic behavior; and these traits, although evolving in significant ways, have remained stable over longer spans of time than individual species of invertebrates. The fossil record of behavior in fact originated earlier than the fossil record of invertebrate body parts. Although macroinvertebrates and their traces exhibit tremendous diversity of form and function, these fit into a relatively small number of behavioral patterns. The patterns, in turn, may correlate with prevailing environmental conditions, resulting in gradients among trace fossil assemblages, or ichnofacies. Behavioral patterns and characteristic ichnofacies therefore constitute the main basis for uniformity in ichnology. Thus, the most fundamental questions are: What is the specific function represented by the trace? How will it change as the tracemaker is influenced by other genetic, physiologic, or ecologic stimuli? In which facies will it likely occur? and what preservational biases are apt to modify the fossil record of this behavior and its environmental distribution? Approached from this standpoint, the present is indeed a key to the ichnologic past, and vice versa. In practice, however, the present has been studied considerably less than its importance would dictate.     

Increase of body size in sixgill sharks with change in diet as a possible background of their evolution

Body size variation of a predator is a simple way to follow the main changes in its food source during its life history or along its evolution in ecology and paleoecology, respectively. Here, we present possible first evidence of such predator-prey co-evolution through the study of the body size evolution among sixgill sharks (genus Hexanchus) inferred from their fossil record and by comparison to the life history of its two recent species. As for the observed ontogenic diet change of the living bluntnose sixgill shark (H. griseus), its ancestors appear to have developed a similar penchant for dining on marine mammals at the end of the Paleogene with a remarkably well-correlated timing.     

Invasion success: does size really matter?

The recent paper by Roy et al . (2001) presents a compelling relationship between range limit shifts, climatic fluctuations, and body size for marine bivalves in the fossil record. However, their extension of body size as a correlate for contemporary marine bivalve introductions is problematic and requires further scrutiny. Unlike their analysis of the fossil assemblage, the approach used for contemporary invasions does not adequately control for dispersal mechanism (vector) or source region. First, their analysis included mariculture species, intentionally introduced because of their large size, creating a vector-specific bias. Second, successful invaders from multiple source regions (Northern Hemisphere) were compared with potential invaders from a single source region (north-eastern Pacific), leaving both source and vector as uncontrolled variables. We present an analysis of body size for bivalve introductions from a single vector and source region, indicating no correlation between body size and invasion success when eliminating intentional introduction, source region and transport vector as confounding factors.     

Carnivore mollusks as natural enemies of invasive land flatworms

There are several records of the carnivorous behaviour of land flatworms, considered to be top‐predators in their micro‐habitats, by preying upon various species of invertebrates. However, there is little knowledge of predators on land‐flatworms. The possible impact of invasive land flatworms on prey populations has caused widespread concern, when considering their predatory behaviour, combined with recent human influence on the distribution of certain species. This work is the first record of predation on land flatworms by a carnivorous snail. Various‐sized land flatworms of 10 native species of the subfamily Geoplaninae, as well as the exotic species Bipalium kewense (subfamily Bipaliinae), were offered to Rectartemon depressus (Gastropoda, Streptaxidae), which accepted all. The predator also fed on the snail Bradybaena similaris. The snails were maintained in laboratory for an average period of 12 months based on a mixed diet of flatworms and B. similaris, suggesting that the snail is a polyphagous predator. Because certain land‐flatworm species have been described as invasive species which may have a potential impact on prey populations in native and man‐made ecosystems, it is proposed that carnivorous snails of other native species, as potential predators of flatworms, should be tested for possible use in biological control programmes of these invasive planarians.     

Boring Late Cambrian organisms

Specimens of inarticulate brachiopods (family Acrotrctidae) with boreholes were found in Upper Cambrian carbonates in the southern Great Basin of the United States. Some morphologic features and preferred orientation of the borings are similar to those made by predators and previously reported in the fossil and Holocene record. Such predatory activity on brachiopods is previously unknown in Cambrian rocks. Taxa associated with these specimens are not known to have been predators and identity of the predatory organism is unknown. Cambrian brachiopods, predation, paleoecology.     

Evolutionary Analysis of Mitogenomes from Parasitic and Free-Living Flatworms

Mitochondrial genomes (mitogenomes) are useful and relatively accessible sources of molecular data to explore and understand the evolutionary history and relationships of eukaryotic organisms across diverse taxonomic levels. The availability of complete mitogenomes from Platyhelminthes is limited; of the 40 or so published most are from parasitic flatworms (Neodermata). Here, we present the mitogenomes of two free-living flatworms (Tricladida): the complete genome of the freshwater species Crenobia alpina (Planariidae) and a nearly complete genome of the land planarian Obama sp. (Geoplanidae). Moreover, we have reanotated the published mitogenome of the species Dugesia japonica (Dugesiidae). This contribution almost doubles the total number of mtDNAs published for Tricladida, a species-rich group including model organisms and economically important invasive species. We took the opportunity to conduct comparative mitogenomic analyses between available free-living and selected parasitic flatworms in order to gain insights into the putative effect of life cycle on nucleotide composition through mutation and natural selection. Unexpectedly, we did not find any molecular hallmark of a selective relaxation in mitogenomes of parasitic flatworms; on the contrary, three out of the four studied free-living triclad mitogenomes exhibit higher A+T content and selective relaxation levels. Additionally, we provide new and valuable molecular data to develop markers for future phylogenetic studies on planariids and geoplanids.     

Phylogeny of Veneridae (Bivalvia) based on mitochondrial genomes

Veneridae is one of the most diverse families of bivalve molluscs. However, their phylogenetic relationships among subfamilies have been debated for years. To explore phylogenetic relationships of Veneridae, we sequenced 13 complete mitochondrial genome sequences from eight subfamilies and compared with available complete mitochondrial genome of other Veneridae taxa (18 previously reported sequences). Phylogenetic analyses using probabilistic methods recovered two highly supported clades. In addition, the protein‐coding gene order revealed a highly conserved pattern among the same subclade lineages. According to our molecular analyses, Tapetinae should be recognized as a valid subfamily, but the genera formed para‐polyphyletic clades. Chioninae was recovered not monophyletic that differs from a previously molecular phylogeny. Furthermore, the reconstructed chronogram calibrated with fossils recovered the Veneridae have originated during the early Permian (about 290 million years ago). Noticeably, programmed frameshift was found in the nad4 gene of Leukoma jedoensis, Anomalodiscus squamosus and Antigona lamellaris and cob gene of L. jedoensis. This is the first time that the presence of the programmed frameshift has been found in the protein‐coding genes of Heterodonta species. Our results improved the phylogenetic resolution within Veneridae, and a more taxonomic sampling analysis of the subfamily Chioninae is supposed to construct.     

Rangeomorphs,Thectardis (Porifera?) and dissolved organic carbon in the Ediacaran oceans

The mid-Ediacaran Mistaken Point biota of Newfoundland represents the first morphologically complex organisms in the fossil record. At the classic Mistaken Point localities the biota is dominated by the enigmatic group of "fractally" branching organisms called rangeomorphs. One of the few exceptions to the rangeomorph body plan is the fossil Thectardis avalonensis, which has been reconstructed as an upright, open cone with its apex in the sediment. No biological affinity has been suggested for this fossil, but here we show that its body plan is consistent with the hydrodynamics of the sponge water-canal system. Further, given the habitat of Thectardis beneath the photic zone, and the apparent absence of an archenteron, movement, or a fractally designed body plan, we suggest that it is a sponge. The recognition of sponges in the Mistaken Point biota provides some of the earliest body fossil evidence for this group, which must have ranged through the Ediacaran based on biomarkers, molecular clocks, and their position on the metazoan tree of life, in spite of their sparse macroscopic fossil record. Should our interpretation be correct, it would imply that the paleoecology of the Mistaken Point biota was dominated by sponges and rangeomorphs, organisms that are either known or hypothesized to feed in large part on dissolved organic carbon (DOC). The biology of these two clades gives insight into the structure of the Ediacaran ocean, and indicates that a non-uniformitarian mechanism delivered labile DOC to the Mistaken Point seafloor.     

The ecostratigraphic paradigm

What can distinguish ecostratigraphy from other stratigraphic concepts stems not so much from its specific methods as from the theoretical background proposed here and called the ecostratigraphic paradigm. This theory claims that the fossil record of evolution at the community, taxocoene, and/or species group level displays a more or less discrete pattern, with discontinuities imposed mostly by extrinsic, geologic events. This pattern may constitute a base for natural stratigraphic classification of rocks using coenozonal schemes. The ecostratigraphic paradigm may or may not refer to communities as distinct and real biologic units. The greatest advantage of the ecostratigraphic approach is that the resultant set of time-planes is much more informative about the course of geobiologic evolution than the traditionally used chronostratigraphic one. Stratigraphial theory. ecostratigraphy. chronostratigraphy. paleoecology. coenozones. communities, evolution .     

Selective solution of macroinvertebrate calcareous hard parts: a laboratory study

Ten suites of 16 common types of invertebrate hard parts were placed in acid baths for 24 hours to determine relative rates and common styles of dissolution. Skeletal mineralogies included aragonite and both high-magnesium and low-magnesium calcite. Hard parts included barnacle cxoskelctons, cchinoid tests, gastropod opercula and gastropod and bivalve shells. Calcitic barnacle plates dissolved most rapidly, aragonitic and high magnesium calcitic hard parts showed intermediate rates, and the calcitic shells of the oyster dissolved at the lowest rate. The surface area to weight ratio of the hard parts correlated ( r =0.650) significantly with the hard part's rate of dissolution. Skeletal remains with a high surface area to weight ratio dissolved faster than those with a low surface area to weight ratio. Skeletal porosity and mineralogy appeared to be responsible for additional variation in the rate of dissolution. The effect of the surface area to weight ratio is sufficient to overcome the effect of mineralogy. Dense, compact aragonitic hard parts can persist longer than porous, thin calcitic remains. Typical features associated with skeletal degradation include development of chalky textures, thinning of distal margins, surface etching and formation of holes in bivalve muscle scars. Such features may aid in the recognition of partial dissolution of skeletal remains in the rock record. □ Taphonomy, paleoecology, fossil-diagenesis.     

The fossil record of Limopsis (Bivalvia: Limopsidae) in Antarctica and the southern high latitudes

Abstract: Limopsis is one of the most speciose and widespread bivalve genera in the Southern Ocean at the present day. However, the fossil record of the genus is poorly known from the southern high latitudes. Here, we review the fossil record in this region to help understand the evolutionary origins of the genus. Limopsis infericola sp. nov. and additional specimens of a previously described species are added to the fossil record of Antarctica. The globally distributed limopsid clade had its earliest occurrences in the Early Cretaceous of Europe and New Zealand, then radiated during the Late Cretaceous (Maastrichtian, 70.6–65.5 Ma). Fossil evidence shows that the genus underwent a second, Cenozoic, radiation related to the isolation of Antarctica and the onset of cooling in the southern hemisphere. The genus has persisted in Antarctica for the last 50 myr, adapting to extreme changes in the environmental conditions, including surviving the last glacial maximum in marine refugia.     

No news on the flatworm front! Nitric oxide synthase in parasitic and free-living flatworms

The free radical nitric oxide (NO) has emerged as a simple and unique signalling molecule that can serve as neurotransmitter, paracrine substance or hormone. NO is a gas, formed by various neuronal cells, both centrally and peripherally. NO regulates cyclic GMP synthesis. The production of NO can be detected using the NADPH diaphorase (NADPH-d) histochemical stain for nitric oxide synthase (NOS). NOS was detected in two parasitic flatworms, Diphyllobothrium dendriticum and Hymenolepis diminuta, and two free-living flatworms, Planaria torva and Girardia tigrina. The staining for NOS was very strong in the nervous system of both parasitic worms. The main nerve cords, the transverse ring commmissures, nerves in association with the musculature, especially the cirrus musculature and sensory nerve endings showed NADPH-d staining. The NADPH-d staining in the free-living flatworms was much weaker. Still NOS activity was found in the neuropile of the brain and in association with the pharynx musculature. The demonstration of NOS in flatworms, indicates that NO is an old signal molecule in evolutionary terms. This revised version was published online in July 2006 with corrections to the Cover Date.     

A NEW METHOD FOR RECORDING THE IN SITU PUMPING ACTIVITY OF INFAUNAL BIVALVES

A new method for measuring the pumping activity of infaunalbivalves is described. The method is based on the simultaneousrecording of three temperatures: (1) the temperature of thesea water above the bivalve, (2) the temperature of the sedimentand (3) the internal temperature of the bivalve. When thermicshifts between sediment and sea water are induced by the sunand/or the ground water, comparison of the three temperaturesprovides information on the pumping activity of the bivalve.The method was applied during the day to Tapes decussatus andRuditapes philippinarum (Veneridae). Their pumping activityappears continuous except during emersion at ebb. Sestonic concentrationsas high as 325 mg 1^–1 induced no detectable interruption.Analysis of the data suggests that in situ pumping rate is notconstant but fluctuates. (Received 14 February 1996; accepted 15 May 1996)