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- Frayed at the edges: selective pressure and adaptive response to abiotic stressors are mismatched in low diversity edge populationsPublication . Pearson, Gareth; Lago-Lestón, Asunción; Mota, CatarinaTheory predicts that population structure and dynamics affect a population's capacity for adaptation to environmental change. For isolated, small and fragmented populations at the trailing edge of species distributions, loss of genetic diversity through random genetic drift may reduce adaptive potential and fitness levels for complex traits. This has important consequences for understanding population responses to, for example changing climate, but has rarely been tested in natural populations. We measured the intertidal thermal environment and tidal exposure (emersion) times for natural populations of the intertidal seaweed Fucus serratus at the centre (southwest UK) and southern edge (northern Portugal) of its range in the Eastern Atlantic, and for a congener, F. vesiculosus, whose range extends further south to Morocco. Fitness-related traits of individuals at each location were measured in common garden experiments: physiological resilience to desiccation and heat shock (PSII quantum yield), and the molecular phenotype of the heat shock response (quantitative PCR of heat shock protein gene transcripts). The realized thermal environment experienced by F. serratus was similar at the centre and southern edge of its distribution because the maximum shore height (and emersion period) was reduced in southern populations. For F. vesiculosus, thermal maxima were higher and occurred more frequently in the south, although maximum vertical height (emersion time) remained similar to central populations. Edge populations of F. serratus were less resilient to desiccation and heat shock than central populations, and expression of heat shock genes was higher at the same temperature, suggesting greater cellular stress. In contrast, there was no evidence for physiological divergence in heat shock response in F. vesiculosus, and little variation in gene expression. Synthesis. We provide evidence that compared with range-centre populations upper intertidal limits of F. serratus at the southern edge are 'pruned back' by abiotic stressors. Rather than being locally adapted, these small populations are less resilient to abiotic stresses and experience greater cellular stress during heat shock. These results suggest that ongoing climate forcing factors may threaten small, fragmented rear edge populations because of inherently reduced fitness and lower adaptive capacity relative to larger central populations.
- Differentiation in fitness-related traits in response to elevated temperatures between leading and trailing edge populations of marine macrophytesPublication . Mota, Catarina; Engelen, Aschwin; Serrao, Ester; Coelho, Márcio; Marba, Nuria; Krause-Jensen, Dorte; Pearson, GarethThe nature of species distribution boundaries is a key subject in ecology and evolution. Edge populations are potentially more exposed to climate-related environmental pressures. Despite research efforts, little is known about variability in fitness-related traits in leading (i.e., colder, high latitude) versus trailing (i.e., warmer, low latitude) edge populations. We tested whether the resilience, i.e. the resistance and recovery, of key traits differs between a distributional cold (Greenland) and warm (Portugal) range edge population of two foundation marine macrophytes, the intertidal macroalga Fucus vesiculosus and the subtidal seagrass Zostera marina. The resistance and recovery of edge populations to elevated seawater temperatures was compared under common experimental conditions using photosynthetic efficiency and expression of heat shock proteins (HSP). Cold and warm edge populations differed in their response, but this was species specific. The warm edge population of F. vesiculosus showed higher thermal resistance and recovery whereas the cold leading edge was less tolerant. The opposite was observed in Z. marina, with reduced recovery at the warm edge, while the cold edge was not markedly affected by warming. Our results confirm that differentiation of thermal stress responses can occur between leading and trailing edges, but such responses depend on local population traits and are thus not predictable just based on thermal pressures.