Le Cortex de la Vacuole Contractile et son Ultrastructure chez les Ciliés

SYNOPSIS. In various ciliates the contractile vacuole is a permanent organelle, delimited by a differentiated cortex.
The cortex is made up of a dense reticulum of anastomosing tubules limited by a smooth membrane, and vesicles. This "spongiome" can be considered as a localized and specialized condensation of the endoplasmic reticulum.     

Carbon and water fluxes in a calcareous grassland under elevated CO2

1. As part of a long-term study of the effects of elevated CO₂ on biodiversity and ecosystem function in a calcareous grassland, we measured ecosystem carbon dioxide and water-vapour fluxes over 24-h periods during the 1994 and 1995 growing seasons. Data were used to derive CO₂ and H₂O gas-exchange response functions to quantum flux density (QFD).
2. The relative increase in net ecosystem CO₂ flux (NEC) owing to CO₂ enrichment increased as QFD rose. Daytime NEC at high QFD under elevated CO₂ increased by 25% to 60%, with the greatest increases in the spring and after mowing in June when above-ground biomass was lowest. There was much less stimulation of NEC in early June and again in October when the canopy was fully developed. Night-time NEC was not significantly altered under elevated CO₂.
3. Short-term reversal of CO₂ concentrations between treatments after two seasons of CO₂ exposure provided evidence for a 50% downward adjustment of NEC expressed per unit above-ground plant dry weight. However, when expressed on a land area basis, this difference disappeared because of a c. 20% increase in above-ground biomass under elevated CO₂.
4. Ecosystem evapotranspiration (ET) was not significantly altered by elevated CO₂ when averaged over all measurement dates and positions. However, ET was reduced 3–18% at high QFD in plots at the top of the slope at our study site. In summary, CO₂ enrichment resulted in a large stimulation of ecosystem CO₂ capture, especially during periods of a large demand of carbon in relationship to its supply, and resulted in a relatively small and variable effect on ecosystem water consumption.     

Reproductive patterns of the Pacific oyster Crassostrea gigas in France

Summary

In France, national management programs focus research on understanding reproductive factors in Crassostrea gigas to confront problems of the oyster industry. However, little information has been documented in which reproductive patterns include sexual changes. The reproductive cycle of oysters at three sites of the Atlantic coast of France was examined from 1996 to 1998, and the seasonal variations in oocyte size-frequencies, and sex ratio were described. The results showed a synchronism within the population concerning reproductive behavior. Young oocytes are generated after spawning and show no apparent changes during winter. Growth of oocytes begins in spring and cells reach maturity in April-May and are ready for a single spawning season in June-July. Oocytes that were not released during spawning are reabsorbed within the gonad. The significant difference between sites is that spawning occurred 1 month later in the southern area. A modal analysis showed that oocyte populations in the sample individuals are primordially bimodal, but with polymodal occurrences in June-July, in some cases. Irregular alternative sexuality was detected at all sites, and hermaphrodites appear to be a transition phase that allows changes from male to female during early spring. Previous observations, together with the study of the development of oocyte cohorts over time, permit a hypothetical model concerning the kinetics of gametogenesis in C. gigas. The model suggests that primary oocytes are generated from energy supplied from degenerating, as well as young oocytes that do not reach the mature stage within the gonad during autumn-winter. It seems that, during vitellogenesis, there is disintegration of smaller cells coupled with transfer of energy to the larger oocytes, which continue to grow and mature.     

Biotic filtering and mass effects in small shrub patches: is arthropod community structure predictable based on the quality of the vegetation?

1. Community assembly is affected by four processes: dispersal, filtering effects (selection), ecological drift and evolution. The role of filtering relative to dispersal and drift should decline with patch size, hampering possibilities to predict which organisms will be observed within small‐sized patches. However, vegetation structure is known to have a marked impact on species assemblages, and plant quality may act as a biotic filter. This challenges the assumption of unpredictable species assemblages in small‐sized vegetation patches. 2. Using 32 stands of five shrub species in south‐west Finland, this study investigated whether biotic filtering effects caused by patch‐forming plants are strong enough to overcome the mixing of mobile arthropod assemblages across small patches. 3. Stochastic variation did not hide the signals of biotic filtering and dispersal in the small shrub patches. Habitat richness around the patches explained a three times larger share of variation in the species composition than did the identity of the patch‐forming plant, but it had less effect on the abundance of arthropods. A radius of 50–100 m around a patch explained the species composition best. 4. Abundance patterns varied between the feeding guilds; the patch‐forming shrub influenced the abundances of detritivores and leaf‐feeding herbivores, whereas the abundances of flower‐visiting herbivores appeared to track the flowering phenology of the plants. Shrub identity had little effect on omnivores or predators. Predator abundances were correlated with the abundance of potential prey. 5. The results of this study suggest that community composition within a vegetation patch may be predictable even if dispersal overrides local filtering effects, as suggested by the mass‐effects paradigm.     

CO2 responsiveness of plants: a possible link to phloem loading

Of the many responses of plants to elevated CO₂, accumulation of total non-structural carbohydrates (TNC in % dry weight) in leaves is one of the most consistent. Insufficient sink activity or transport capacity may explain this obvious disparity between CO₂ assimilation and carbohydrate dissipation and structural investment. If transport capacity contributes to the problem, phloem loading may be the crucial step. It has been hypothesized that symplastic phloem loading is less efficient than apoplastic phloem loading, and hence plant species using the symplastic pathway and growing under high light and good water supply should accumulate more TNC at any given CO₂ level, but particularly under elevated CO₂. We tested this hypothesis by carrying out CO₂ enrichment experiments with 28 plant species known to belong to groups of contrasting phloem-loading type. Under current ambient CO₂ symplastic loaders were found to accumulate 36% TNC compared with only 19% in apoplastic loaders (P=0.0016). CO₂ enrichment to 600 μmol mol^?1 increased TNC in both groups by the same absolute amount, bringing the mean TNC level to 41% in symplastic loaders (compared to 25% in apoplastic loaders), which may be close to TNC saturation (coupled with chlornplast malfunction). Eight tree species, ranked as symplastic loaders by their minor vein companion cell configuration, showed TNC responses more similar to those of apoplastic herbaceous loaders. Similar results are obtained when TNC is expressed on a unit leaf area basis, since mean specific leaf areas of groups were not significantly different. We conclude that phloem loading has a surprisingly strong effect on leaf tissue composition, and thus may translate into alterations of food webs and ecosystem functioning, particularly under high CO₂.     

A portable system for collecting anatomical joint angles during stair ascent: a comparison with an optical tracking device

Background

Assessments of stair climbing in real-life situations using an optical tracking system are lacking, as it is difficult to adapt the system for use in and around full flights of stairs. Alternatively, a portable system that consists of inertial measurement units (IMUs) can be used to collect anatomical joint angles during stair ascent. The purpose of this study was to compare the anatomical joint angles obtained by IMUs to those calculated from position data of an optical tracking device.

Methods

Anatomical joint angles of the thigh, knee and ankle, obtained using IMUs and an optical tracking device, were compared for fourteen healthy subjects. Joint kinematics obtained with the two measurement devices were evaluated by calculating the root mean square error (RMSE) and by calculating a two-tailed Pearson product-moment correlation coefficient (r) between the two signals.

Results

Strong mean correlations (range 0.93 to 0.99) were found for the angles between the two measurement devices, as well as an average root mean square error (RMSE) of 4 degrees over all the joint angles, showing that the IMUs are a satisfactory system for measuring anatomical joint angles.

Conclusion

These highly portable body-worn inertial sensors can be used by clinicians and researchers alike, to accurately collect data during stair climbing in complex real-life situations.

     

Relation between temporal persistence of host plants and wing length in leafhoppers (Hemiptera: Auchenorrhyncha)

Abstract.

• 1 Wing form frequencies in 255 populations of 101 species of leafhoppers (Auchenorrhyncha) in temporary and permanent habitats were documented.
• 2 The proportion of brachypterous specimens in the leafhopper assemblages on ruderal host plants in temporary habitats (median 2%, range 0–8%) was significantly lower than that on permanent host plants in undisturbed habitats (median 22%, range 0–94%).
• 3 Leafhopper species typical of temporary habitats were either monomorphic, macropterous, or wing-dimorphic with macropterous forms prevailing in both sexes.
• 4 Among the eighty-nine species recorded in permanent habitats, forty-five species were wing dimorphic. In forty-one dimorphic species, a brachypterous form prevailed. This prevalence was found for both sexes in thirty-one species, for only females in nine species and for only males in one species.
• 5 The prevalence of brachypters in males, but not in females, found in Anoscopus flavostriatus, is probably the first such documented case in Auchenorrhyncha.
• 6 The hypothesis is proposed that in temporary habitats, density-dependent production of macropters in wing dimorphic species is an adaptation to frequent habitat deterioration caused by factors independent of the density of the species.
• 7 The predominance of brachypters in permanent habitats indicates that a density-dependent decrease in fitness usually does not offset the potential decrease in fitness connected with macroptery and dispersal. Because of this inability of leafhopper populations to decrease significantly the quality of their resources, a high population density cannot be used as a predictor of future quality of these resources, which is information essential for efficient dispersal behaviour.