The potential for adaptation in a natural Daphnia magna population: broad and narrow-sense heritability of net reproductive rate under Cd stress at two temperatures

Abstract

The existence of genetic variability is a key element of the adaptive potential of a natural population to stress. In this study we estimated the additive and non-additive components of the genetic variability of net reproductive rate (R₀) in a natural Daphnia magna population exposed to Cd stress at two different temperatures. To this end, life-table experiments were conducted with 20 parental and 39 offspring clonal lineages following a 2 × 2 design with Cd concentration (control vs. 3.7 μg Cd/L) and temperature (20 vs. 24 °C) as factors. Offspring lineages were obtained through inter-clonal crossing of the different parental lineages. The population mean, additive and non-additive genetic components of variation in each treatment were estimated by fitting an Animal Model to the observed R₀ values using restricted maximum likelihood estimation. From those estimates broad-sense heritabilities (H²), narrow-sense heritabilities (h²), total (CV_G) and additive genetic coefficients of variation (CV_A) of R₀ were calculated. The exposure to Cd imposed a considerable level of stress to the population, as shown by the fact that the population mean of R₀ exposed to Cd was significantly lower than in the control at the corresponding temperature, i.e. by 23 % at 20 °C and by 88 % at 24 °C. The latter difference indicates that increasing temperature increased the stress level imposed by Cd. The H² and CV_G were significantly greater than 0 in all treatments, suggesting that there is a considerable degree of genetic determination of R₀ in this population and that clonal selection could rapidly lead to increasing population mean fitness under all investigated conditions. More specifically, the H² was 0.392 at 20 °C+Cd and 0.563 at 24 °C+Cd; the CV_G was 30.0 % at 20 °C+Cd and was significantly higher (147.6 %) in the 24 °C+Cd treatment. Significant values of h² (= 0.23) and CV_A (= 89.7 %) were only found in the 24 °C+Cd treatment, suggesting that the ability to produce more offspring under this stressful condition may be inherited across sexual generations. In contrast, in the less stressful 20 °C+Cd treatment the h² (0.06) and CV_A (7.0 %) were very low and not significantly higher than zero. Collectively our data indicate that both the asexual and sexual reproduction phases in cyclic parthenogenetic D. magna populations may play a role in the long-term adaptive potential of Daphnia populations to chemical stress (with Cd as the current example) and that environmental variables which influence the stress level of that chemical may influence this adaptive potential (with temperature as current example).