Ecological genetics of inbreeding, outbreeding and immunocompetence in Ranid frogs

Using artificial fertilization, I crossed frogs from different populations to evaluate fitness consequences for the offspring from an inbreeding-outbreeding perspective, and to evaluate quantitative genetic effects on immunocompetence against a fungal pathogen (Saprolegnia). Crosses between closely situated populations of different sizes generated contrasting results for the effects of outbreeding on offspring traits between populations and life history stages, emphasizing the importance of epistatic effects and the difficulties of relying on generalizations when making conservation decisions (e.g., regarding translocations). Experimental infection of frog eggs from six populations with Saprolegnia fungus showed a significant family effect on the degree of infection of eggs and embryos, in particular at lower fertilization success and with a significant temperature × population interaction effect. A paternal genetic effect on fungus resistance was found using a half-sib split design. Furthermore, relatively more eggs were infected when fertilized by sperm from the same, in contrast with a different population. However, there was no evidence for a stronger effect in isolated island populations. Although the mechanistic underpinnings remain unknown, these results suggest substantial levels of genetic variation in resistance to Saprolegnia in natural populations within and among populations. We also found that pre-hatching exposure to Saprolegnia dramatically reduced the size at metamorphosis in the absence of further exposure to the fungus, possible as a delayed effect of impaired embryonic development. However, in contrast to some other amphibians, induced hatching in response to Saprolegnia could not be confirmed. In conclusion, the results suggest that frog populations are genetically diverse even at small geographic scale with frequently strong and unpredictable consequences of in- and outbreeding for the response to stressors

Carbon isotopes in terrestrial ecosystem pools and CO 2

Stable carbon isotopes are used extensively to examine physiological, ecological, and biogeochemical processes related to ecosystem, regional, and global carbon cycles and provide information at a variety of temporal and spatial scales. Much is known about the processes that regulate the carbon isotopic composition (delta(13)C) of leaf, plant, and ecosystem carbon pools and of photosynthetic and respiratory carbon dioxide (CO(2)) fluxes. In this review, systematic patterns and mechanisms underlying variation in delta(13)C of plant and ecosystem carbon pools and fluxes are described. We examine the hypothesis that the delta(13)C of leaf biomass can be used as a reference point for other carbon pools and fluxes, which differ from the leaf in delta(13)C in a systematic fashion. Plant organs are typically enriched in (13)C relative to leaves, and most ecosystem pools and respiratory fluxes are enriched relative to sun leaves of dominant plants, with the notable exception of root respiration. Analysis of the chemical and isotopic composition of leaves and leaf respiration suggests that growth respiration has the potential to contribute substantially to the observed offset between the delta(13)C values of ecosystem respiration and the bulk leaf. We discuss the implications of systematic variations in delta(13)C of ecosystem pools and CO(2) fluxes for studies of carbon cycling within ecosystems, as well as for studies that use the delta(13)C of atmospheric CO(2) to diagnose changes in the terrestrial biosphere over annual to millennial time scales

Disentangling drought-induced variation in ecosystem and soil respiration using stable carbon isotopes

Unger S, Maguas C, Pereira JS, Aires LM, David TS, Werner C. Disentangling drought-induced variation in ecosystem and soil respiration using stable carbon isotopes. Oecologia. 2010;163(4):1043-1057.Combining C flux measurements with information on their isotopic composition can yield a process-based understanding of ecosystem C dynamics. We studied the variations in both respiratory fluxes and their stable C isotopic compositions (delta C-13) for all major components (trees, understory, roots and soil microorganisms) in a Mediterranean oak savannah during a period with increasing drought. We found large drought-induced and diurnal dynamics in isotopic compositions of soil, root and foliage respiration (delta C-13(res)). Soil respiration was the largest contributor to ecosystem respiration (R (eco)), exhibiting a depleted isotopic signature and no marked variations with increasing drought, similar to ecosystem respired delta(CO2)-C-13, providing evidence for a stable C-source and minor influence of recent photosynthate from plants. Short-term and diurnal variations in delta C-13(res) of foliage and roots (up to 8 and 4aEuro degrees, respectively) were in agreement with: (1) recent hypotheses on post-photosynthetic fractionation processes, (2) substrate changes with decreasing assimilation rates in combination with increased respiratory demand, and (3) decreased phosphoenolpyruvate carboxylase activity in drying roots, while altered photosynthetic discrimination was not responsible for the observed changes in delta C-13(res). We applied a flux-based and an isotopic flux-based mass balance, yielding good agreement at the soil scale, while the isotopic mass balance at the ecosystem scale was not conserved. This was mainly caused by uncertainties in Keeling plot intercepts at the ecosystem scale due to small CO2 gradients and large differences in delta C-13(res) of the different component fluxes. Overall, stable isotopes provided valuable new insights into the drought-related variations of ecosystem C dynamics, encouraging future studies but also highlighting the need of improved methodology to disentangle short-term dynamics of isotopic composition of R (eco)