bluuebunny/crossref_metadata_2025 huggingface dataset:
Export of invertebrates and detritus from fishless headwater streams in southeastern Alaska: implications for downstream salmonid production
Authors: WIPFLI MARK S. , GREGOVICH DAVID P.
Published: March 2003

1. We examined the export of invertebrates (aquatic and terrestrial) and coarse organic detritus from forested headwaters to aquatic habitats downstream in the coastal mountains of southeast Alaska, U.S.A. Fifty‐two small streams (mean discharge range: 1.2–3.6 L s−1), representing a geographic range throughout southeast Alaska, were sampled with 250‐μm nets either seasonally (April, July, September) or every 2 weeks throughout the year. Samples were used to assess the potential subsidy of energy from fishless headwaters to downstream systems containing fish.

2. Invertebrates of aquatic and terrestrial origin were both captured, with aquatic taxa making up 65–92% of the total. Baetidae, Chironomidae and Ostracoda were most numerous of the aquatic taxa (34, 16 and 8%, respectively), although Coleoptera (mostly Amphizoidae) contributed the greatest biomass (30%). Mites (Acarina) were the most numerous terrestrial taxon, while terrestrial Coleoptera accounted for most of the terrestrial invertebrate biomass.

3. Invertebrates and detritus were exported from headwaters throughout the year, averaging 163 mg invertebrate dry mass stream−1 day−1 and 10.4 g detritus stream−1 day−1, respectively. The amount of export was highly variable among streams and seasons (5–6000 individuals stream−1 day−1 and <1–22 individuals m−3 water; <1–286 g detritus stream−1 day−1 and <0.1–1.7 g detritus m−3 water). Delivery of invertebrates from headwaters to habitats with fish was estimated at 0.44 g dry mass m−2 year−1. We estimate that every kilometre of salmonid‐bearing stream could receive enough energy (prey and detritus) from fishless headwaters to support 100–2000 young‐of‐the‐year (YOY) salmonids. These results illustrate that headwaters are source areas of aquatic and terrestrial invertebrates and detritus, linking upland ecosystems with habitats lower in the catchment.
Coping with warmer, large rivers: a field experiment on potential range expansion of northern quagga mussels (Dreissena bugensis)
Authors: Thorp James H. , Alexander James E. , Cobbs Gary A.
Published: March 2003

SUMMARY 1. Zebra mussels (Dreissena polymorpha) have established a much greater range in North America and Europe than quagga mussels (D. bugensis), which occupy a very similar niche.

2. We hypothesised that quaggas are physiologically capable of sustaining populations in warmer rivers currently occupied only by zebra mussels and that unidentified, non‐physiological factors account for their more limited distribution.

3. Growth and survival of individually tagged mussels (976 D. bugensis from Lake Erie; 2625 D. polymorpha from Lake Erie and the Ohio River) were recorded monthly for up to 15 months in an outdoor stream mesocosm receiving unfiltered water from the Ohio River.

4. Extreme temperatures stressed both species; but in contrast to several previous laboratory studies, quaggas survived high temperatures better than zebra mussels. We suspect this was the result of species‐specific differences in their ability to obtain, assimilate and/or catabolise food at high, sublethal temperatures.

5. A unimodal growth pattern was observed in both species, with the highest growth rates from late spring to early autumn.

6. Our survival and growth data suggest that quaggas are not physiologically limited from expanding southward.

7. While lacking definitive proof that dreissenid populations in rivers are ecologically sustainable without upstream lentic ecosystems and/or unintended human intervention, we suggest that complex river currents and upstream retentive and highly productive slackwater habitats in rivers may help sustain downstream populations of these meroplanktonic, dreissenid mussels.
Comparing grazing by Dreissena polymorpha on phytoplankton in the presence of toxic and non‐toxic cyanobacteria
Authors: Dionisio Pires L. M. , Van Donk E.
Published: March 2003

SUMMARY 1. The feeding behaviour of the zebra mussel (Dreissena polymorpha) was studied in the laboratory on different combinations of food, including a green alga (Chlamydomonas reinhardtii) and toxic and non‐toxic strains of the cyanobacterium Microcystis aeruginosa.

2. The highest clearance rate of phytoplankton by zebra mussels was found when the mussels were feeding on a mixture of Chlamydomonas and non‐toxic Microcystis, the lowest on a mixture of Chlamydomonas and toxic Microcystis.

3. The differences found in the clearance rates between food combinations can be partly explained by the production of pseudofaeces containing live phytoplankton cells. Zebra mussels expelled significantly more live phytoplankton cells in the presence of toxic Microcystis than in the presence of non‐toxic Microcystis. The pseudofaeces contained predominantly live Chlamydomonas cells. Proportionately much less live Microcystis cells were encountered in the pseudofaeces.

4. Consequently, grazing of zebra mussels on a combination of Chlamydomonas and Microcystis may finally result in a dominance of Chlamydomonas over Microcystis. The presence of toxic Microcystis may even strengthen this shift.
Population dynamics and production of Jesogammarus annandalei, an endemic amphipod, in Lake Biwa, Japan
Authors: Ishikawa Toshiyuki , Urabe Jotaro
Published: March 2003

SUMMARY 1. Population dynamics and production of Jesogammarus annandalei, an endemic amphipod in Lake Biwa, were examined from April 1997 to June 1998. The life cycle of this species was 1 year with the new generation beginning in early autumn. They preferred low temperature (<12 °C) and their spatial distribution varied seasonally and accordingly.

2. In deep water, the abundance of J. annandalei ranged from 200 to 63 000 m−2 and decreased towards summer and the biomass (0.01∼3.6 g C m−2) was on average comparable that of zooplankton. The density was much higher than that recorded by a study conducted 35 years ago.

3. Individual growth rate of this amphipod was high in winter and spring but decreased in summer. Annual production of J. annandalei (6.2 g C m−2 year−1) was only 2% of primary production but was at the higher end of the range reported for amphipods in oligo‐ and mesotrophic lakes.

4. These results are consistent with the view that Lake Biwa is becoming more eutrophic, with a consequent decrease in the abundance of predatory fish in the profundal zone.
Differential responses to UVR by bacterioplankton and phytoplankton from the surface and the base of the mixed layer
Authors: Xenopoulos Marguerite A. , Schindler David W.
Published: March 2003

SUMMARY 1. We tested the influence of ultraviolet radiation (UVR) and shallow stratification on phytoplankton and bacterioplankton from the surface and the base of the mixed layer in two boreal lakes in north‐western Ontario, Canada.

2. We measured phytoplankton biomass and production, bacterioplankton production and plankton respiration after transplantation under three solar radiation treatments: ambient radiation (Photosynthetically active radiation (PAR) + ultraviolet‐A (UVA) + ultraviolet‐B (UVB)), minus UVB (PAR + UVA) and PAR only. We repeated this experiment on three occasions in each lake during the summer.

3. Solar stress (measured as reduced growth and photoinhibition) was generally only found in the ‘base phytoplankton’ (i.e. originating from the base of the mixed layer). No inhibition of photosynthesis by UVB exposure was found in near‐surface phytoplankton. On the other hand, production of near‐surface bacterioplankton was reduced following a 4‐h UVR exposure but had increased after a 48‐h exposure to both UVA and UVB compared with the PAR only treatment.

4. Negative effects of UVR on phytoplankton and bacterioplankton were not ubiquitous. We emphasise the importance of conducting experiments repeatedly, particularly those which test the effects of UVR on different community assemblages from different lakes.
Reaction distance of Bythotrephes longimanus, encounter rate and index of prey risk for Harp Lake, Ontario
Authors: Muirhead Jim , Sprules W. Gary
Published: March 2003

SUMMARY 1. Some predatory zooplankton, such as the large cladoceran Bythotrephes longimanus (Leydig), have a large compound eye, hence vision may play an important role in prey detection and encounter. Our objectives were to quantify, under different light regimes, the reaction distance of Bythotrephes to an assortment of zooplankton prey and to model encounter rate with prey from Harp Lake, Ontario. Reaction distance to prey increased at higher light intensity.

2. Results from the encounter model show that small, slow‐moving prey faced the greatest risk from Bythotrephes and most encounters occurred in the upper 10 m of the water column throughout the 24‐h period. The model was highly sensitive to ambient light. Encounter rate and prey risk were two to three orders of magnitude more sensitive to swimming velocity of the predator than to that of the prey.
Measurement of three‐dimensional structure of plants with a simple device and estimation of light capture of individual leaves
Authors: Takenaka A. , Inui Y. , Osawa A.
Published: March 2003

1. A simple device called a ‘pocometer’ (POlar COordinate METER) was developed to measure three‐dimensional structure of plants. It consists of a tape‐measure to measure distance and two protractors to measure zenith angle and azimuth angle.

2. The pocometer can determine locations of points within a few metres distance with a resolution of less than 1cm. Location of any point on a plant can be measured in 10 to 30s depending on the ease of pulling the tape measure to the point of interest.

3. A system to use data obtained with the pocometer to calculate plant light capture was developed. The degree of shading at any point on a plant is estimated by checking obstruction by other plant parts of the view toward the sky at that point.

4. Photon flux density (PFD) on leaf surfaces was estimated for Aucuba japonica, a broad‐leaved evergreen shrub, using the above system. The estimated PFDs for individual leaves of a plant corresponded to the sensor‐measured PFDs with correlation coefficients of 0·67 to 0·92.
Economics of spider orb‐webs: the benefits of producing adhesive capture thread and of recycling silk
Authors: Opell B. D.
Published: March 2003

1. The replacement of dry, fuzzy cribellar prey capture thread by viscous, adhesive capture thread was a major event in the evolution of orb‐weaving spiders. Over 95% of all orb‐weaving species now produce adhesive threads.

2. Adhesive thread achieves its stickiness with a much greater material economy than does cribellar thread.

3. Transformational analyses show that, relative to spider mass, adhesive orb‐weavers invest less material per mm of capture thread and produce stickier capture threads than do cribellate orb‐weavers.

4. The total cost of producing an orb‐web that contains cribellar thread is reduced by 32% when a spider recycles its silk and another 34% when these capture threads are replaced by adhesive threads of equal stickiness.

5. The increased economy with which adhesive capture thread achieves its stickiness may have been an important factor that favoured the origin and success of modern orb‐weaving spiders that produce adhesive capture threads.
Photoinhibition in tropical forest understorey species with short‐ and long‐lived leaves
Authors: Lovelock C. E. , Kursar T. A. , Skillman J. B. , Winter K.
Published: March 2003

1. Shade‐tolerant species that inhabit the understorey have a range of leaf lifetimes (from 1 to 8 years), which may indicate a variety of strategies for dealing with increases in light associated with tree‐fall gaps. We hypothesized that species with long‐lived leaves should be more tolerant of an increase in light levels than species with short‐lived leaves.

2. In understorey plants of 12 shade‐tolerant rain‐forest species, photoinhibition, measured as a reduction in the chlorophyll fluorescence parameter Fv/Fm when leaf discs were exposed to 1h at 1000μmol m–2s–1, was greater in species with short‐lived leaves than species with long‐lived leaves.

3. Less photoinhibition in species with long‐lived leaves was not associated with higher levels of non‐photochemical dissipation (NPQ) of absorbed light, but may be the result of a higher yield of photosystem II compared with short‐lived leaves.

4. Thus, species with long‐lived leaves are more tolerant of abrupt increases in light that occur when tree‐fall gaps are formed than species with short‐lived leaves.

5. Discs from leaves of all species growing in tree‐fall gaps had higher levels of NPQ, yield of photosystem II and more rapid recovery from photoinhibition than leaves developed in the understorey; however, there were no differences among species with short‐ and long‐lived leaves.
Contrasting digestive strategies of fruit‐eating birds
Authors: Witmer M. C. , Van Soest P. J.
Published: March 2003

1. Sugary fruits dominate the annual diet of Cedar Waxwings (Bombycilla cedrorum), whereas lipid‐rich foods dominate the diets of frugivorous thrushes. Nutrient utilization and preferences of Cedar Waxwings and thrushes fed sugary and lipid‐rich natural fruits were assessed to ascertain the potential for nutrient‐based specializations that could explain diet selection in nature.

2. Compared allometrically, Cedar Waxwings showed higher rates of sugar assimilation from sugary fruits than did thrushes, by virtue of higher intake rates, and achieved the highest energy assimilation rates when eating a sugary fruit. A trade‐off to this digestive strategy appears to be less efficient utilization of lipids when eating exclusively lipid‐rich fruits.

3. Thrushes digested lipids more efficiently than Cedar Waxwings as a function of intake rate or estimated retention time, and thrushes achieved the highest energy assimilation rates when eating a lipid‐rich fruit. Specialization to a diet rich in lipids appears to limit the rate at which sugary fruits can be processed.

4. Within each bird species, sugar digestive efficiencies were high and were not reduced at higher intake rates. Lipids were consumed at lower rates than sugars, and lipid digestive efficiencies declined with increasing intake rates. Frugivorous birds modulated digestive processing of fruits according to the time needed for efficient digestion and/or absorption of sugars and lipids, respectively.

5. Rate of nitrogen intake, not sugar assimilation, positively influenced body mass changes of birds. Sugary fruits appear nutritionally rich in energy, but limited in protein for avian frugivores. Relatively high intake rates and low protein requirements of Cedar Waxwings suggest that specialization to sugary, low‐protein diets involves traits that facilitate acquisition and conservation of protein/amino acids.

6. Cedar Waxwings and thrushes show digestive specialization to the utilization of sugars and lipids, respectively, as dominant dietary nutrients. These traits explain patterns of food selection by these birds in the laboratory and in nature. Variation in the value of particular fruit nutrients to different birds renders the dietary descriptor of ‘frugivore’ ambiguous in a nutritional context.
Limits to water transport inJuniperus osteospermaandPinus edulis: implications for drought tolerance and regulation of transpiration
Authors: Linton M. J. , Sperry J. S. , Williams D. G.
Published: March 2003

1.An air‐injection method was used to study loss of water transport capacity caused by xylem cavitation in roots and branches ofPinus edulis(Colorado Pinyon) andJuniperus osteosperma(Utah Juniper). These two species characterize the Pinyon–Juniper communities of the high deserts of the western United States.Juniperus osteospermacan grow in drier sites thanP. edulisand is considered the more drought tolerant.

2.Juniperus osteospermawas more resistant to xylem cavitation thanP. edulisin both branches and roots. Within a species, branches were more resistant to cavitation than roots forP. edulisbut no difference was seen between the two organs forJ. osteosperma. There was also no difference between juveniles and adults inJ. osteosperma; this comparison was not made forP. edulis.

3.Tracheid diameter was positively correlated with xylem cavitation pressure across roots and stems of both species. This relation suggests a trade‐off between xylem conductance and resistance to xylem cavitation in these species.

4.During summer drought,P. edulismaintained higher predawn xylem pressures and showed much greater stomatal restriction of transpiration, consistent with its greater vulnerability to cavitation, thanJ. osteosperma.

5.These results suggest that the relative drought tolerance ofP. edulisandJ. osteospermaresults in part from difference in their vulnerability to xylem cavitation.
Survival of colour‐polymorphicEristalis arbustorumhoverflies in semi‐field conditions
Authors: Ottenheim M. M. , Wertheim B. , Holloway G. J. , Brakefield P. M.
Published: March 2003

1.The theory on adaptive phenotypic plasticity assumes different fitness optima in different environments. To demonstrate these fitness differences in the field or under laboratory conditions can be difficult.

2.The size of the coloured patches on the abdomen ofEristalis arbustorumvaries with rearing temperature. At low temperatures small patches develop and at higher temperatures larger patches develop. Consequently, the colour patch size varies seasonally.

3.To study the relative fitness of two extreme phenotypes in abdominal coloration in the summer, a total of 591 laboratory‐reared flies were released in six large population cages. As a measure of fitness, survival was estimated.

4.In all replicate cages the survival of the pale phenotypes was higher than that of the dark phenotypes, indicating a higher fitness for the pale flies under summer conditions.

5.The fat content of flies reared under identical conditions to those released in the cages was measured. Pale flies had ≈ 0·7 mg more fat than dark flies (± 25% extra fat). However, it is argued that this difference in fat reserves could not have been the only cause of the observed difference in survival.

6.The results are discussed in relation to mimicry and thermoregulation.
Elemental stoichiometry of Drosophila and their hosts
Authors: Markow T. A. , Raphael B. , Dobberfuhl D. , Breitmeyer C. M. , Elser J. J. , Pfeiler E.
Published: March 2003

1. Nitrogen (N) and phosphorus (P) availabilities are important ecological determinants of resource use in nature. Despite the wide range of hosts used by species of the genus Drosophila, elemental composition of natural resources of these flies has never been investigated.

2. Total body N and P contents were determined in seven species of wild‐caught Drosophila, their natural hosts, and artificial diets routinely used to rear these flies in the laboratory. The flies tested included D. hydei, D. arizonae, D. simulans and D. pseudoobscura collected from rotting fruit (melons), and the cactophilic D. nigrospiracula, D. mojavensis and D. pachea collected from their specific host plants, Saguaro, Organpipe and Senita cactus, respectively.

3. Natural hosts varied in elemental composition, with fruit showing higher N (2·8–4·3% dry mass) and P (0·50–0·67%) levels compared with cacti (0·5–1·6% N; 0·01–0·29% P). No consistent differences in N and P levels were found between healthy and necrotic cactus tissue.

4. Total body N and P also varied among Drosophila species. This variation mirrored the levels of N and P found in the respective hosts and laboratory diets. N:P ratios were consistently lower in female flies compared with conspecific males suggesting phosphorus demands during oogenesis are high.

5. Potential mechanisms by which Drosophila deal with N or P limitation in nature are discussed.
Acclimation of photosynthesis to light: a mechanistic approach
Authors: Kull O. , Kruijt B.
Published: March 2003

1. We developed a mechanistic model to explain acclimation of photosynthesis to the radiation distribution within plant canopies. The model predicts the amount of photosynthetic apparatus in any leaf from the balance between supply of carbohydrates by photosynthesis and the demand for resources by enzyme synthesis and turnover and export to sinks elsewhere in the plant.

2. The turnover of the photosynthetic apparatus is assumed to depend on the amount of available resources: nitrogen and carbohydrates. Leaves export carbohydrates into a common pool of carbohydrates and take nitrogen from the common nitrogen pool. All leaves of one plant are assumed to share these common pools.

3. The model allows description of the dynamics of the acclimation process known to occur within weeks after changes in the environment. The model also adequately explains measured leaf nitrogen distributions and total leaf area in a multispecies canopy.

4. According to our model, differences between species in leaf nitrogen distribution with respect to PPFD can be explained with differences in plant common pools of carbohydrates and nitrogen.

5. Although canopies cannot be treated as a big leaf in a simple way because mechanisms predicting amount of photosynthetic apparatus in a single leaf and in entire canopy are different, total canopy nitrogen can be used as an estimate for canopy total photosynthesis.
Acclimation in dynamic models based on structural relationships
Authors: MÄKELÄ A.
Published: March 2003

1. A number of recent tree growth models have been based on the assumption that tree structure follows certain empirical rules and carbon allocation is performed so as to maintain these rules when growth and senescence occur simultaneously. This paper introduces a method of combining structural rules with adaptive regulation in a carbon balance model.

2. The method is based on regulating gross growth but situations are analysed where increased senescence of tissue functions as an alternative means of re‐establishing the balance.

3. The model does not aim to be mechanistic but merely to describe the process of regulation, or acclimation to changing environments and situations, as constrained by the carbon balance. The method incorporates parameters that describe the rate of returning to the balance after disturbance or after a change in the goal, which can be determined empirically.

4. Two models are given as examples of the use of the method, and the requirements and limitations of control in a carbon balance framework are discussed.

5. The method is best applied as a technical tool to describe variations and disturbance in balanced growth models when the variations are present but not very large. In addition, it can be used as a theoretical framework for the analysis of regulation as constrained by the conservation of mass.
Assessing the benefits of aggregation: thermal biology and water relations of anomalous Emperor Moth caterpillars
Authors: Klok C. J. , Chown S. L.
Published: March 2003

1.Three hypotheses have been proposed to account for the maintenance of aggregation behaviour in caterpillars. While the benefits of aggregation for defence and for the elevation of growth rates have been investigated in some species, both of these hypotheses have been criticized, and in many cases the alternative hypothesis regarding the benefits of physiological regulation has not been investigated.

2.In this paper the field thermal biology and water relations of solitary caterpillars (instars IV–V) and aggregations of caterpillars (instars I–III) are examined inImbrasia belina(Westwood) (Lepidoptera: Saturniidae), a species where an instar‐related change in aggregation behaviour is obligatory (instars I–III always aggregate, instars IV–V are always solitary), and where the benefits of defence and of overwhelming inducible host plant defences are unlikely to be significant.

3.Aggregations of instars II–III maintain body temperatures equivalent to those of larger solitary individuals of instars IV and V, but significantly higher than those of operative models of single caterpillars of instars II–III. In addition, behavioural thermoregulation is not used to elevate body temperatures, although hanging behaviour takes place when upper lethal temperature limits are approached. Nonetheless, solar radiation has a marked influence on the temperature of aggregations because of the accumulation of higher heat loads by the larger masses of large aggregations.

4.Instars IV–V can survive higher temperatures than instars I–III, and this may be because of their accumulation of very high heat loads in the field. Instars I and III can avoid these high heat loads by partial dispersal from the aggregation. In contrast, instars I–III have significantly lower critical thermal minima than the later instars, which may allow larvae to regain aggregations if they are separated from them.

5.Water loss rates of real aggregations, of a given size, within a particular instar are lower than the sum of the rates of the same number of individual caterpillars of the same instar. In addition, survival of water loss tends to be higher in individuals from aggregations than in single individuals.

6.Using this evidence in conjunction with studies that demonstrate significant predation in all instars, aggregation in the early instars irrespective of whether outbreaks take place or not, and the absence of inducible defences in the major host (Colophospermum mopane) ofI. belina, it is concluded that physiological regulation, of temperature and water balance, is the most likely explanation for the maintenance of aggregation behaviour in this species.
Host specificity in parasitic mistletoes (Loranthaceae) in New Zealand
Authors: Norton D. A. , De Lange P. J.
Published: March 2003

1.We quantify the degree of host specificity for the five extant New Zealand loranthaceous mistletoes (Alepis flavida, Ileostylus micranthus, Peraxilla colensoi, Peraxilla tetrapetalaandTupeia antarctica).

2.Host specificity is highest forA. flavida, P. colensoiandP. tetrapetalawhich primarily parasitize species ofNothofagus, and lowest forT. antarcticaand especiallyI. micranthuswhich parasitize a wide range of host species.

3.These patterns of host specificity support the suggestion that relative host abundance is a key factor determining the degree of host specialization in mistletoes (resource fragmentation hypothesis). While evolutionary history may be important in the specificity of the mistletoe–host relationship in some situations, our data suggest that for New Zealand mistletoes evolutionary history simply reflects the temporal component of relative host abundance.

4.We conclude that it is the stability of host availability through time and space which is the dominant factor determining host specificity patterns.
Interaction between ants, extrafloral nectaries and insect herbivores in Neotropical coastal sand dunes: herbivore deterrence by visiting ants increases fruit set inOpuntia stricta(Cactaceae)
Authors: Oliveira P. S. , Rico‐Gray V. , Castillo‐Guevara C. Díaz‐Castelazo† and C.
Published: March 2003

1.This study examines the anti‐herbivore effect of ants visiting the extrafloral nectaries (EFNs) ofOpuntia stricta(Cactaceae) and its possible influence on the plant’s reproductive output in Mexican coastal sand dunes.Opuntia’s EFNs are located in the areoles of the developing tissue of emerging cladodes and flower buds.

2.Ants visited the EFNs ofO.strictaon a round‐the‐clock basis. The associated ant assemblage was formed by nine species distributed in four subfamilies, and the species composition of the principal ant visitors changed markedly from day to night period.

3.Cladodes of control (ants present) and treatment (ants excluded) plants ofOpuntiawere equally infested by sucking bugs and mining dipterans. Damage to buds by a pyralid moth, however, was significantly higher on treatment than on control plants. Ant visitation toOpuntia’s EFNs translated into a 50% increase in the plant’s reproductive output, as expressed by the number of fruits produced by experimental control and treatment branches. Moreover, fruit production by ant‐visited branches was positively and significantly associated with the mean monthly rate of ant visitation to EFNs.

4.This is the first demonstration of ant protection leading to increased fruit set in the Cactaceae under natural conditions. Although the consequences of damage by sucking and mining insects remain unclear forOpuntia, the results show how the association of EFNs with vulnerable reproductive plant organs can result in a direct ant‐derived benefit to plant fitness.
Microbial processes and carbon‐isotope fractionation in tropical and temperate grassland soils
Authors: ŠantRůČková H. , Bird M. I. , Lloyd J.
Published: March 2003

1. The carbon content and δ13C value of soil organic carbon (SOC), microbial biomass (Cmic) and respired CO2 were measured in a range of grassland soils from tropical and temperate biomes to determine if isotope effect of microbial degradation can induce a shift in isotope composition of SOC and CO2. The soil from a depth of 0–2 cm was analysed. Cmic was measured using the chloroform fumigation extraction method, while CO2 was measured in a closed system after 3 and 10 days of incubation. Two soils, temperate and tropical, were used for a long‐term experiment, in which measurements were performed after 3, 10 and 40 days of incubation.

2. SOC and Cmic decrease exponentially with increasing mean annual temperature. Cmic decreases more slowly than SOC, resulting in a higher proportion of Cmic in the SOC of tropical soils relative to temperate soils.

3. The δ13C value of Cmic and respired CO2 reflects gross changes in the δ13C value of SOC in the corresponding sample. On average, Cmic is 13C‐enriched by c. 2‰ compared with SOC, while respired CO2 is 13C‐depleted by c. 2·2‰ compared with Cmic. Thus, the observed 13C‐enrichment in Cmic is balanced by a corresponding 13C‐depletion in respired CO2 resulting in the δ13C value of respired CO2 being approximately similar to the δ13C of SOC.

4. The isotope effect of microbial degradation is of importance in soil. It can be induced by selective utilization of SOC and isotope discrimination during metabolism. Metabolic isotopic discrimination is dependent on the growth stage of the soil microbial population.
Photosynthetic responses to a climate‐warming manipulation for contrasting meadow species in the Rocky Mountains, Colorado, USA
Authors: Loik M. E. , Redar S. P. , Harte J.
Published: March 2003

1. Microclimate was measured and photosynthetic responses to a climate warming manipulation were compared for the evergreen shrub Artemisia tridentata and the herbaceous forb Erigeron speciosus in the Rocky Mountains, Colorado, USA.

2. Soil was warmer and drier under infra‐red heaters compared with control plots.

3. Midday xylem pressure potential did not differ for A. tridentata on heated vs control plots but was lower for E. speciosus on heated plots compared with controls. Leaf temperatures did not vary for the two species on heated or control plots.

4. There were no significant treatment or species differences in the diurnal patterns of CO2 assimilation or stomatal conductance to water vapour. Also, there were no differences in responses to leaf temperature.

5. The quantum yield for CO2 assimilation over a range of PPFD was lower for plants on heated plots. There was a marked difference between species in the pattern of stomatal conductance to water vapour over a range of PPFD, but no differences as a result of the heating treatment.

6. The quantum efficiency of PSII electron transport was significantly affected by heating. Non‐radiative energy dissipation was greater for A. tridentata compared with E. speciosus. There was recovery of FV/FM for A. tridentata but not for E. speciosus.

7. Heating appears to affect plants via changes in soil water content rather than by increasing leaf temperature. The deciduous species E. speciosus appears to undergo some permanent closure of PSII on heated plots, in contrast to the evergreen shrub A. tridentata. Such differences may help explain the increase in above‐ground biomass accumulation in response to heating for shrubs, compared with the decrease observed for deciduous herbaceous species.
Experimental assessment of the functional morphology of inflorescences of Daucus carota (Apiaceae): testing the ‘fly catcher effect’
Authors: Lamborn E. , Ollerton J.
Published: March 2003

1. The functional significance of inflorescence morphology of Daucus carota L. (Apiaceae) was tested using an inflorescence manipulation experiment. Specifically, we sought to explain the role (if any) in pollination of the dark central floret of this species.

2. In central England, D. carota was found to be pollinated by a taxonomically wide range of insects that varied in their inflorescence visitation rates and in their pollen loads. There was also variation in abundance of some taxa between the 2 years of this study.

3. By removing the dark central floret and adding false florets we sought to affect pollinator visitation rates and seed set in a manner consistent with the ‘fly catcher effect’ hypothesis of Eisikowitch (1980). Taxa responded to these manipulations but not in ways that shed light on the adaptive role of the dark central floret. Seed set was likewise unaffected in any consistent way.

4. The role of the dark central floret remains an enigma. We speculate, following Darwin (1888), that the dark central floret may now be functionless and possibly represents a trait that has persisted long after its original function has been lost.
Interference competition and sexual selection promote polymorphism in Colias (Lepidoptera, Pieridae)
Authors: Nielsen M. G. , Watt W. B.
Published: March 2003

‘Alba’ is a genetic polymorphism in butterflies of the genus Colias (the sulphurs) which confers on its female carriers white (A–) rather than yellow (aa) wings. Previous studies of ‘alba’s’ resource‐based effects in Colias alexandra and Colias scudderi suggested the rare morph should be eliminated in each species. Here the selective regime acting on ‘alba’s’ wing colour effect is examined to determine if it can maintain polymorphism.

Field experiments and observations reveal species‐specific differences in morph fecundity resulting from selection on wing colour. ‘Alba’ female and male Colias lose flight time, an index of Colias fitness, to unproductive courtship with pierines (white butterflies in the subfamily Pieridae), a form of interference competition. Flight time losses are greater in C. alexandra, which co‐occur more extensively with pierines than C. scudderi. ‘Alba’ females also spent less time being harassed by courting male Colias than yellow females; this form of sexual selection on morph wing colour is particularly strong in C. alexandra.

Loss of male and ‘alba’ female Colias flight time and the degree of male Colias preference for the yellow morph correlate with the extent each Colias species co‐occurs with pierines, suggesting interference competition influences Colias courtship decisions and ‘alba’ frequency.

Both interference competition and sexual selection are density dependent in their effects; these effects were modelled using Holling’s equation. Modelling results show temporal variation in Colias and pierine densities over the past 20 years caused morph fitness to vary such that in at least one year each morph in each species had greater relative fitness with respect to colour‐based selective pressures. Although this selective regime does not maintain polymorphism under temporal models of fitness variation, trade‐offs between these colour‐based effects and ‘alba’s’ temperature and resource‐based effects in a spatially heterogeneous environment could maintain polymorphism.
Phosphorus acquisition by Bromus madritensis ssp. rubens from soil interspaces shared with Mojave Desert shrubs
Authors: Yoder C. K. , Nowak R. S.
Published: March 2003

Bromus madritensis ssp. rubens (L.) Husnot (Red Brome) is an invasive annual grass that is associated with increases in fire frequency and decreases in perennial plant diversity. The success of Red Brome in the Mojave Desert has been attributed to its competitive ability, but competition between Red Brome and native shrubs for below‐ground resources has not been investigated previously. In this study we present a modification of previous dual‐isotope methods that assesses competitive interactions and responses among plants under field conditions. We then use this method to (i) determine if direct competition for phosphorus (P) occurs between Red Brome and native shrubs, and (ii) evaluate the effectiveness of phosphate acquisition by Red Brome from soil interspaces shared with different Mojave Desert shrub species.

Clipping Red Brome to remove ≈85% of its foliage on the day prior to labelling soil interspaces with P isotopes did not have the desired effect of inhibiting phosphorus uptake by Red Brome. Therefore we were unable to verify that direct competition for P occurred between Red Brome and native shrubs.

Nonetheless, by sampling plant tissue from unclipped strips of Red Brome that were centred between two shrubs, we were able to evaluate the effectiveness of phosphate acquisition by Red Brome from interspaces shared with different shrubs. There were no differences in the amount of phosphorus acquired by Red Brome from soil interspaces shared with the evergreen shrub Larreatridentata (D.C.) Cov. versus soil interspaces shared with the drought deciduous shrub Lycium pallidum Miers. However, Red Brome acquired more than six times as much P from soil interspaces shared with Larrea than from soil interspaces shared with the drought deciduous shrub Ambrosia dumosa Payne. Less P uptake by Red Brome from soil interspaces shared with Ambrosia suggests greater effectiveness of Ambrosia compared with Larrea to deplete soil P.
Energy expenditure and water flux of free‐living Dune Larks in the Namib: a test of the reallocation hypothesis on a desert bird
Authors: Williams J. B.
Published: March 2003

1. Dune Larks (Mirafra erythroclamys) are the only avian species that has evolved the capability of surviving in the Namib Sand Sea, one of the driest regions in the world. Their field metabolic rate (FMR) and water flux, as measured by doubly labelled water, during the breeding and non‐breeding season are reported here.

2. FMR is relatively constant over the annual cycle, which conforms to the ‘reallocation hypothesis’. With data pooled for both seasons, males expended energy at a rate of 97·5 ± 20·4 kJ day−1 (±1 SD) and females at 87·5 ± 13·0 kJ day−1. The data supported the hypothesis that Dune Larks do not possess a reduced FMR, a result attributable to their elevated basal metabolism. In a previous laboratory investigation, Dune Larks had a low rate of evaporative water loss despite their elevated basal metabolic rate (BMR). The present study demonstrated that this low evaporative water loss translates into a low water influx in the field.

3. The FMR of Dune Lark females equalled 88·1 ± 6·9 kJ day−1 during incubation, and 88·5 ± 9·9 kJ day−1 when they were feeding 8–10‐day‐old nestlings. These data do not support the idea that the incubation period is a time of low energy demand.

4. Dune Larks had a relatively constant water influx throughout the year. For sexes combined, mass‐adjusted values of water influx averaged 0·59 ml H2O/(day mass0·682).

5. Calculations showed that neonates received 10 kJ day−1 whereas towards the end of the nestling period, chicks were fed 75 kJ day−1, the maximum gross energy intake for nestlings. During chick rearing, males increased the amount of food that they found by 57·7%, females by 64·3%, late in the nestling period.

6. Dune Larks foraged in the morning and late afternoon, but were constrained by their environment to seek shade at midday during the breeding season.

7. Metabolic water production plays a significant role in the water economy of Dune Larks. Assuming a mixed diet, 47% of their water intake comes from the oxidation of food stuffs.
Double‐strand break repair mediated by DNA end‐joining
Authors: Tsukamoto Yasumasa , Ikeda Hideo
Published: March 2003

DNA double‐strand breaks formed by ionizing irradiation or other stresses are repaired by homologous recombination or DNA end‐joining. This review focuses on the mechanism of double‐strand break repair mediated by DNA end‐joining, in which many factors have recently been identified. After DNA double‐strand breakage, DNA end‐joining takes place between the DNA ends that have nonhomologous sequences or very short regions of homology. The broken DNA is repaired if the DNA end‐joining occurs in the same molecule, while it causes chromosome aberrations such as deletions, insertions, translocations and inversions if it occurs between different molecules. Rad50 and its relatives, Ku‐proteins, DNA ligase VI and silencing factors, are involved in DNA end‐joining in yeast and mammalian cells. These findings led us to propose a model in which the formation of a heterochromatin‐like complex at broken ends is an important element in DNA end‐joining.
The resolvase encoded by Xanthomonas campestris transposable element ISXc5 constitutes a new subfamily closely related to DNA invertases
Authors: Liu Chia‐Chen , Hühne Rolf , Tu Jenn , Lorbach Elke , Dröge Peter
Published: March 2003

Background

Conservative site‐specific recombination is responsible for the resolution of cointegrates which result during the transposition of class II transposable elements. Resolution is catalysed by a transposon‐encoded recombinase, resolvase, that belongs to a large family of recombinases, including DNA invertases. Resolvases and the related invertases are likely to employ similar reaction mechanisms during recombination. There are important differences, however. Resolvases require two accessory DNA binding sites within each of the two directly repeated recombination sites. Invertases instead need a host factor, Fis, and an enhancer type DNA sequence, in addition to two inversely orientated recombination sites.

Results

The resolvase encoded by transposable element ISXc5 from the Gram‐negative phytopathogen Xanthomonas campestris shows two features which distinguish it from other known resolvases. First, it is more closely phylogenetically related to invertases than other resolvases. In particular, two functionally important regions seem highly conserved between this resolvase and members of the invertase subfamily. Second, the enzyme exhibits a large extension of its carboxy‐terminal domain with unknown function. We purified ISXc5 resolvase and analysed its resolution reaction in vitro. Our biochemical and DNA topological analysis reveals that critical features of resolution are similar, if not identical, to that carried out by γδ resolvase. However, despite its apparent similarity to invertases, we were unable to detect recombination on standard substrates for DNA inversion, in either the presence or absence of Fis.

Conclusions

ISXc5 resolvase employs a reaction mechanism which is common to members of the resolvase family. Its position near the evolutionary borderline to invertases and its high degree of identity within two functionally important regions with members of the DNA invertase subfamily suggest that only a few replacements of critical residues may suffice to convert this resolvase into a functional, possibly Fis‐dependent invertase.
Epigenetic reprogramming of the human H19 gene in mouse embryonic cells does not erase the primary parental imprint
Authors: Mitsuya Kohzoh , Meguro Makiko , Sui Hajime , Schulz Thomas C. , Kugoh Hiroyuki , Hamada Hiroshi , Oshimura Mitsuo
Published: March 2003

Background

Genomic imprinting in mammals is thought to result from epigenetic modifications to chromosomes during gametogenesis, which leads to differential allelic expression during development. There is a requirement for an appropriate experimental system to enable the analysis of the mechanisms of genomic imprinting during embryogenesis.

Results

To develop a novel in vitro system for studying the molecular basis of genomic imprinting, we constructed mouse cell lines containing either a paternal or maternal human chromosome 11, by microcell‐mediated chromosome transfer. Allele‐specific expression and DNA methylation studies revealed that the imprinting status of the human H19 gene was maintained in mouse A9 mono‐chromosomal hybrids. Each parental human chromosome was introduced independently into mouse near‐diploid immortal fibroblasts (m5S) and two embryonal carcinoma (EC) cell lines (OTF9‐63 and P19). The paternal allele of human H19 remained in a repressed state in m5S cells, but was de‐repressed in both EC cells. The paternal H19 allele was demethylated extensively in OTF9‐63 cells, whereas the only alteration in P19 hybrids was de novo methylation on both alleles in the 3′ region. Following in vitro differentiation, the expressed paternal H19 allele was selectively repressed in differentiated derivatives of EC hybrids.

Conclusion

These results indicated that human imprint marks could function effectively in mouse cells, and that the imprinting process was epigenetically reprogrammed in embryonal carcinoma cells, without erasure of the primary imprint that marked the parental origin. Therefore, these mono‐chromosomal hybrids could provide a valuable in vitro system to study the mechanisms involved in the regulation of imprinted gene expression.
Individual LCR hypersensitive sites cooperate to generate an open chromatin domain spanning the human β‐globin locus
Authors: Li Guolong , Lim Kim‐Chew , Douglas Engel James , Bungert Jörg
Published: March 2003

Background

The human β‐globin locus control region (LCR) is composed of five DNase I hypersensitive (HS) sites located 5′ to the multiple genes it regulates. The LCR has been shown to comprise, among other essential properties, an activity that is required for generating a chromatin structure which renders the entire β‐globin gene locus accessible to exogenous nucleases. This nuclease‐sensitive state is generally believed to be reflective of the chromatin environment that is permissive for transcriptional activation of the globin genes.

Results

Here we show, in mice bearing intact YAC transgenes that encompass the whole human β‐globin locus, that the deletion of individual core LCR HS sites negatively affects the ability of the LCR to confer this open chromatin conformation throughout the locus, and when analysed in concert with the effect that these same mutations have on transcription, the data show that the chromatin opening activity is a necessary, but not sufficient, prerequisite for globin gene expression. The results also show that after deletion of individual hypersensitive sites, the mutated LCR is no longer able to provide an accessible chromatin environment that is independent from the site of YAC transgene integration.

Conclusions

These experiments provide further evidence for the hypothesis that the HS sites must act cooperatively to fulfil the multiple functions that are attributable to the LCR.
Meiotic telomeres: a matchmaker for homologous chromosomes
Authors: Hiraoka Yasushi
Published: March 2003

Telomeres, with their special structures and special schemes of synthesis, are essential for protecting the ends of eukaryotic linear chromosomes during cell proliferation. In addition to this basic function, the meiosis‐specific functions of telomeres have long been inferred from the cytological observations of characteristic chromosome configurations in meiotic prophase. Recent studies in the fission yeast Schizosaccharomyces pombe have provided deeper insights into the role of meiotic telomeres in the pairing of homologous chromosomes. Here I have summarized our current understanding of the meiotic behaviour of telomeres in S. pombe, and discuss the role of telomeres in meiosis.