Differential regulation of hsp70 genes in the freshwater key species Gammarus pulex (Crustacea, Amphipoda) exposed to thermal stress: effects of latitude and ontogeny.

Temperature is one of the main abiotic factors influencing the distribution and abundance of organisms. In the Rhône River Valley, populations of the crustacean Gammarus pulex are distributed along a 5 °C thermal gradient from the North to the South of the valley. In this present work, we investigated the heat shock response of G. pulex according to latitudinal distribution (northern vs. southern populations) and ontogeny (adults vs. embryos from early stages). We isolated two isoforms (one constitutive hsc70 and one inducible hsp70) of heat shock proteins 70 (HSP70) and quantitatively compared their amounts of mRNA after heat shocks, using real-time PCR. Whereas the hsc70 (constitutive) gene did not vary between the two populations, a significant effect of the population was observed on the expression of the hsp70 (inducible) gene in adult specimens. The northern population of amphipods showed a greater magnitude of induction and a 2 °C lower onset temperature when compared to the southern population, suggesting that the northern population is more affected by elevated temperature than the southern one. We demonstrated that the expression of hsp70 may play a crucial role in the persistence of biogeographical patterns of G. pulex, since it reflects the natural distribution of this species along the latitudinal thermal gradient. A differential regulation of hsc70 gene was also observed according to the ontogenetic stage, with a switch from heat inducible in early life stages to constitutively and highly expressed in adults. These findings demonstrate the importance of considering the entire life cycle to better understand the adaptive response to thermal stress.

Effect of climate-related change in vegetation on leaf litter consumption and energy storage by Gammarus pulex from Continental or Mediterranean populations.

As a consequence of global warming, it is important to characterise the potential changes occurring for some functional processes through the intra-specific study of key species. Changes in species distribution, particularly when key or engineer species are affected, should contribute to global changes in ecosystem functioning. In this study, we examined the potential consequences induced by global warming on ecosystem functioning in term of organic matter recycling. We compared consumption of leaf litter by some shredder populations (Gammarus pulex) between five tree species inhabiting continental (i.e., the northern region of the Rhône River Valley) and/or Mediterranean (i.e., the southern region of the Rhône River Valley) conditions. To consider any potential adaptation of the gammarid population to vegetation in the same climate conditions, three populations of the key shredder Gammarus pulex from the northern region and three from the southern region of the Rhône River Valley were used. We experimentally compared the effects of the geographical origin of both the gammarid populations and the leaf litter species on the shredding activity and the physiological state of animals (through body triglyceride content). This study demonstrated that leaf toughness is more important than geographical origin for determining shredder leaf litter consumption. The overall consumption rate of the gammarid populations from the southern region of Rhône Valley was much higher than that of the populations from the northern region, but no clear differences between the origins of the leaf litter (i.e., continental vs. Mediterranean) were observed. The northwards shift of G. pulex populations adapted to warmer conditions might significantly modify organic matter recycling in continental streams. As gammarid populations can demonstrate local adaptations to certain leaf species as a trophic resource, changes in riparian vegetation associated with climate change might locally affect the leaf litter degradation process by this shredder.

Effects of diet and salinity on the survival, egg laying and metabolic fingerprints of the ground-dwelling spider Arctosa fulvolineata (Araneae, Lycosidae).

Soil salinity and the salinity of trophic resources may alter the osmoregulatory processes of arthropod, challenging the smooth regulation of body water, and, ultimately, survival. The intra and extracellular build-up of osmolytes represent a common strategy to attenuate acute hyperosmotic stress in several arthropod species. In the present study, we aimed to determine the impact of substrate and trophic resource salinities on salt tolerance in the female wolf spider, Arctosa fulvolineata, which is considered a specialist salt marsh species. We evaluated adult female survival and egg laying, and quantified the osmo-induced accumulation of compatible solutes (GC-MS). Three concentrations of substrate salinity were tested (0‰, 35‰ and 70‰) under three trophic conditions (starved spiders, spiders fed with salt prey [intertidal amphipods] and spiders fed with unsalted prey [freshwater amphipods]). We found no support for diet preferences in female A. fulvolineata, which exhibited similar predation rates on freshwater and marine amphipods. Survival and egg-laying were significantly impaired when female A. fulvolineata were exposed to hypersaline conditions for 12 days. Our results showed an increase in the level of several compatible solutes when spiders were exposed for 12 days to saline conditions. For instance, α-alanine, ß-alanine, arginine, asparagine, aspartate, homoserine, glutamine, glycine, proline and serine levels were 4-10 times higher under hypersaline conditions. The osmo-induced accumulation of amino acids may increase the osmolality of body fluids, thus enhancing the smooth regulation of body fluids and survival ability of wolf spider under extreme saline conditions.

Disentangling the effects of local and regional factors on the thermal tolerance of freshwater crustaceans.

In the global warming context, we compared the thermal tolerance of several populations of the crustacean Gammarus pulex (Amphipoda: Gammaridae) along a latitudinal thermal gradient in the Rhône Valley. To disentangle the effect of regional (North vs. South) and local (site-specific) factors, the ecophysiological responses of populations were investigated at two levels of biological organisation: whole organism level considering body size [critical thermal maximum (CTmax), mean speed of locomotion (MS), time mobile (TM)] and organelle function level [mitochondrial respiratory control ratios (RCRs)]. CTmax and RCRs, but not MS and TM, revealed a significantly higher thermal tolerance in southern populations compared to northern ones. Nevertheless, temperatures ≥ 30°C were deleterious for all populations, suggesting that populations located in the warmer limit of the species distribution will be more threatened by climate change as they live closer to their upper thermal limits. The strong differences observed between populations indicate that the species-level thermal tolerance used in predictive models may not be informative enough to study the impact of global warming on species distributions. This work also reveals that an appropriate choice of indicators is essential to study the consequences of global warming since the response of organisms at the whole body level can be influenced by local conditions.