Differentiation in fitness-related traits in response to elevated temperatures between leading and trailing edge populations of marine macrophytes

Abstract

The 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.

Fig 1. Photosynthetic temperature response of Fucus vesiculosus.

Normalised maximum quantum yield (Fv/Fm) for the alga Fucus vesiculosus from the leading (Greenland, blue) and rear (Portugal, red) edge of distribution, directly after a 3 hour exposure to control (10°C) or elevated temperatures (18, 24, 28 and 32°C) and after a 21 hour recovery at control temperatures. Boxplot horizontal lines show the median, boxes show the 50% quartiles, and the error bars display the range of the data (n = 5). Asterisks show significant pair-wise differences between edges (* p<0.05; ** p<0.01) and different letters indicate significant pair-wise differences (p<0.05) between temperatures.

Fig 2. Photosynthetic temperature response of Zostera marina.

Normalised maximum quantum yield (Fv/Fm) for the seagrass Zostera marina from the leading (Greenland, blue) and trailing (Portugal, red) edge of distribution, directly after a 3 hours exposure (HS) to control (10°C) or elevated temperatures (18, 24, 28 and 32°C) and after recovery at control temperatures. Boxplot horizontal lines show the median, boxes show the 50% quartiles, and the error bars display the range of the data (n = 5). Asterisks show significant pair-wise differences between edges (* p<0.05) and different letters indicate significant pairwise differences between temperatures (p<0.05).

Fig 3. Gene expression temperature response of Fucus vesiculosus.

Expression of seven heat shock transcripts in response to control (10°C) and elevated seawater temperatures (24, 28 and 32°C), after 3h of HS and after recovery in the alga Fucus vesiculosus from the leading (North) and trailing (South) edge of distribution. Relative gene expression values were normalised to geometrical average of three reference genes and to a reference condition (each control at 10°C). Bars show the mean, and error bars display the range of the data (n = 3 biological replicates). Asterisks show significant pair-wise differences between edges (* p<0.05; ** p<0.01; *** p<0.001) and different letters indicate significant pair-wise differences between temperatures (p<0.05).