Osteology and relationships of the temnospondyl genus Sclerocephalus

The temnospondyl Sclerocephalus from the Permo‐Carboniferous of Germany is one of the most completely preserved and most abundant Palaeozoic tetrapods. Here, we review the complete osteology of the genus, based on a range of fully grown specimens housed in public collections. Among the four valid species, Sclerocephalus haeuseri and Sclerocephalus nobilis reached an adult size of well beyond 1 m in length, and had robust postcranial skeletons. In the skull, the exoccipital and sphenethmoid bones were ossified, completing the well‐known ossification sequence in S. haeuseri. Large adults had an elongate trunk and a laterally compressed tail, and some individuals also retained lateral line sulci: features that taken together suggest an aquatic life. The coracoid, pubis, carpals, tarsals, and the bony tail are fully ossified in the largest specimens. The genus Sclerocephalus forms a weakly supported clade nesting firmly at the base of the Stereospondylomorpha, and the close resemblance between Sclerocephalus, Onchiodon, and Eryops is found to be partially based on shared derived states, but is mostly based on symplesiomorphies. Cladistic analysis of 54 characters and 18 taxa finds more support for the Eryopoidea hypothesis (Eryopidae + Stereospondylomorpha) than for the Euskelia hypothesis (Eryopidae + Zatracheidae + Dissorophoidea). This indicates that the large temnospondyls of the Permian and Mesozoic probably formed a natural group, and that the terrestrial adaptations of Eryops and the dissorophoids probably evolved by convergence.     

Interactions between fauna and sediment control the breakdown of plant matter in river sediments

1. A substantial portion of particulate organic matter (POM) is stored in the sediment of rivers and streams. Leaf litter breakdown as an ecosystem process mediated by microorganisms and invertebrates is well documented in surface waters. In contrast, this process and especially the implication for invertebrates in subsurface environments remain poorly studied. 2. In the hyporheic zone, sediment grain size distribution exerts a strong influence on hydrodynamics and habitability for invertebrates. We expected that the influence of shredders on organic matter breakdown in river sediments would be influenced strongly by the physical structure of the interstitial habitat. 3. To test this hypothesis, the influence of gammarids (shredders commonly encountered in the hyporheos) on degradation of buried leaf litter was measured in experimental systems (slow filtration columns). We manipulated the structure of the sedimentary habitat by addition of sand to a gravel‐based sediment column to reproduce three conditions of accessible pore volume. Ten gammarids were introduced in columns together with litter bags containing alder leaves at a depth of 8 cm in sediment. Leaves were collected after 28 days to determine leaf mass loss and associated microbial activity (fungal biomass, bacterial abundance and glucosidase, xylosidase and aminopeptidase activities). 4. As predicted, the consumption of buried leaf litter by shredders was strongly influenced by the sediment structure. Effective porosity of 35% and 25% allowed the access to buried leaf litter for gammarids, whereas a lower porosity (12%) did not. As a consequence, leaf litter breakdown rates in columns with 35% and 25% effective porosity were twice as high as in the 12% condition. Microbial activity was poorly stimulated by gammarids, suggesting a low microbial contribution to leaf mass loss and a direct effect of gammarids through feeding activity. 5. Our results show that breakdown of POM in subsurface waters depends on the accessibility of food patches to shredders.     

Natural variation in ozone sensitivity among Arabidopsis thaliana accessions and its relation to stomatal conductance

Genetic variation between naturally occurring populations provides a unique source to unravel the complex mechanisms of stress tolerance. Here, we have analysed O₃ sensitivity of 93 natural Arabidopsis thaliana accessions together with five O₃‐sensitive mutants to acute O₃ exposure. The variation in O₃ sensitivity among the natural accessions was much higher than among the O₃‐sensitive mutants and corresponding wild types. A subset of nine accessions with major variation in their O₃ responses was studied in more detail. Among the traits assayed, stomatal conductance (g_st) was an important factor determining O₃ sensitivity of the selected accessions. The most O₃‐sensitive accession, Cvi‐0, had constitutively high g_st, leading to high initial O₃ uptake rate and dose received during the first 30 min of exposure. Analyzing O₃‐induced changes in stress hormone concentrations indicated that jasmonate (JA) concentration was also positively correlated with leaf damage. Quantitative trait loci (QTL) mapping in a Col‐0 × Cvi‐0 recombinant inbred line (RIL) population identified three QTLs for O₃ sensitivity, and one for high water loss of Cvi‐0. The major O₃ QTL mapped to the same position as the water loss QTL further supporting the role of stomata in regulating O₃ entry and damage.     

Over‐expression of bacterial γ‐glutamylcysteine synthetase (GSH1) in plastids affects photosynthesis,growth and sulphur metabolism in poplar (Populus tremula × Populus alba) dependent on the resulting γ‐glutamylcysteine and glutathione levels

We compared three transgenic poplar lines over‐expressing the bacterial γ‐glutamylcysteine synthetase (GSH1) targeted to plastids. Lines Lggs6 and Lggs12 have two copies, while line Lggs20 has three copies of the transgene. The three lines differ in their expression levels of the transgene and in the accumulation of γ‐glutamylcysteine (γ‐EC) and glutathione (GSH) in leaves, roots and phloem exudates. The lowest transgene expression level was observed in line Lggs6 which showed an increased growth, an enhanced rate of photosynthesis and a decreased excitation pressure (1‐qP). The latter typically represents a lower reduction state of the plastoquinone pool, and thereby facilitates electron flow along the electron transport chain. Line Lggs12 showed the highest transgene expression level, highest γ‐EC accumulation in leaves and highest GSH enrichment in phloem exudates and roots. This line also exhibited a reduced growth, and after a prolonged growth of 4.5 months, symptoms of leaf injury. Decreased maximum quantum yield (F_v/F_m) indicated down‐regulation of photosystem II reaction centre (PSII RC), which correlates with decreased PSII RC protein D1 (PsbA) and diminished light‐harvesting complex (Lhcb1). Potential effects of changes in chloroplastic and cytosolic GSH contents on photosynthesis, growth and the whole‐plant sulphur nutrition are discussed for each line.     

Climate change affects colonization dynamics in a metacommunity of three Daphnia species

Climate change is expected to alter the range and abundance of many species by influencing habitat qualities. For species living in fragmented populations, not only the quality of the present patches but also access to new habitat patches may be affected. Here, we show that colonization in a metacommunity can be directly influenced by weather changes, and that these observed weather changes are consistent with global climate change models. Using a long‐term dataset from a rock pool metacommunity of the three species Daphnia magna, Daphnia longispina and Daphnia pulex with 507 monitored habitat patches, we correlated a four‐fold increase in colonization rate with warmer, drier weather for the period from 1982 to 2006. The higher colonization rate after warm and dry summers led to an increase in metacommunity dynamics over time. A mechanistic explanation for the increased colonization rate is that the resting stages have a higher exposure to animal and wind dispersal in desiccated rock pools. Although colonization rates reacted in the same direction in all three species, there were significant species‐specific effects that resulted in an overall change in the metacommunity composition. Increased local instability and colonization dynamics may even lead to higher global stability of the metacommunity. Thus, whereas climate change has been reported to cause a unidirectional change in species range for many other species, it changes the dynamics and composition of an entire community in this metacommunity, with winners and losers difficult to predict.     

Endogenous pararetroviruses of allotetraploid Nicotiana tabacum and its diploid progenitors, N. sylvestris and N. tomentosiformis

Endogenous pararetroviruses (EPRVs) represent a new class of dispersed repetitive DNA in plants. The genomes of many Nicotiana species and other solanaceous plants are rich in EPRVs. Distinct EPRV families are present in N. sylvestris ( Ns ) and in N. tomentosiformis ( Nto ), the two diploid progenitors of allotetraploid N. tabacum . Nicotiana EPRVs represent an interesting type of repetitive sequence to analyse in polyploids because of their potential impact on plant fitness and the epigenetic architecture of plant genomes. The Ns EPRV family appears identical in N. sylvestris and N. tabacum , indicating little change has occurred in either species since polyploid formation. By contrast, the Nto EPRV family is larger in N. tomentosiformis than in N. tabacum , suggesting either preferential elimination from the polyploid genome or specific accumulation in the diploid genome following polyploidization. The lability of Nto EPRVs might be enhanced by a frequent association with gypsy retrotransposons. Although some EPRVs are probably benign, others are potentially pathogenic or, conversely, determinants of virus resistance. Normally quiescent EPRVs can be reactivated and cause symptoms of infection in hybrids of species that differ in their EPRV content. EPRVs that furnish immunity to the free virus exemplify the selective value of so-called 'junk' DNA. Variation in the abundance and distribution of EPRVs among related species can be useful in taxonomic and evolutionary studies.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 627–638.     

Spider cues stimulate feeding,weight gain and survival of crickets

1. To avoid predation, prey often change their behaviour upon encountering cues of predator presence. Such behavioural changes should enhance individual survival, but are likely to be energy‐demanding. This should deplete energy reserves of the prey, unless it increases food intake. 2. These hypotheses were studied by conducting two microcosm experiments. In the first, crickets were kept on plants previously occupied by a spider or on control plants. After 3 days leaf consumption and weight gain of the crickets were quantified. In the second experiment, crickets were kept in the presence or absence of spider cues for 3 days. Spiders were then added and predation of the crickets was recorded during 24 h. 3. Crickets that had previously experienced spider cues were more successful in avoiding predation. Moreover, crickets under predation risk tended to increase foraging in the first microcosm experiment and gained more weight in the second microcosm experiment. 4. The results demonstrate that previous experience of predator cues decreases predation rate. Furthermore, they suggest that crickets are able to compensate for increased energy demands caused by antipredator behaviour. In more natural situations, moving to cue‐free plants may play an addition role.     

Braincase,palatoquadrate and ear region of the plagiosaurid Gerrothorax pulcherrimus from the Middle Triassic of Germany

Abstract: The complete neurocranium plus palatoquadrate of the plagiosaurid temnospondyl Gerrothorax pulcherrimus from the Middle Triassic of Germany is described for the first time, based on outer morphological observations and micro‐CT scanning. The exoccipitals are strong elements with paroccipital processes and well‐separated occipital condyles. Anterolaterally, the exoccipitals contact the otics, which are mediolaterally elongated and have massive lateral walls. The otics contact the basisphenoid, which shows well‐developed sellar processes. Anteriorly, the basisphenoid is continuous with the sphenethmoid region. In its posterior portion, the sphenethmoid gives rise to robust, laterally directed laterosphenoid walls, a unique morphology among basal tetrapods. The palatoquadrate is extensively ossified. The quadrate portion overlaps the descending lamina of squamosal and ascending lamina of pterygoid anteriorly, almost contacting the epipterygoid laterally. The epipterygoid is a complex element and may be co‐ossified with otics and laterosphenoid walls. It has a broad, sheet‐like footplate and a horizontally aligned ascending process that contacts the laterosphenoid walls. The degree of ossification of the epipterygoid, however, is subject to individual variation obviously independent from ontogenetic changes. The stapes of Gerrothorax is a large, blade‐like element that differs conspicuously from the plesiomorphic temnospondyl condition. It has a prominent anterolateral projection which has not been observed in other basal tetrapods. Morphology of neurocranium and palatoquadratum of Gerrothorax most closely resembles that of the Russian plagiosaurid Plagiosternum danilovi, although the elements are less ossified in the latter. The extensive endocranial ossification of Gerrothorax is consistent with the general high degree of ossification in the exo‐ and endoskeleton of this temnospondyl and supports the view that a strong endocranial ossification cannot be evaluated as a plesiomorphic character in basal tetrapods.