Genetic assimilation of ancestral plasticity during parallel adaptation to zinc contamination in Silene uniflora.

Phenotypic plasticity in ancestral populations is hypothesized to facilitate adaptation, but evidence is piecemeal and often contradictory. Further, whether ancestral plasticity increases the probability of parallel adaptive changes has not been explored. The most general finding is that ancestral responses to a new environment are reversed following adaptation (known as reversion). We investigated the contribution of ancestral plasticity to adaptive evolution of gene expression in two independently evolved lineages of zinc-tolerant Silene uniflora. We found that the general pattern of reversion is driven by the absence of a widespread stress response in zinc-adapted plants compared with zinc-sensitive plants. We show that ancestral plasticity that moves expression closer to the optimum value in the new environment influences the evolution of gene expression among genes that are likely to be involved in adaptation and increases the chance that genes are recruited repeatedly during adaptation. However, despite convergence in gene expression levels between independently adapted lineages, ancestral plasticity does not influence how similar expression values of adaptive genes become. Surprisingly, we also observed that ancestral plasticity that increases fitness often becomes genetically determined and fixed, that is, genetically assimilated. These results emphasize the important role of ancestral plasticity in parallel adaptation.

Nitrification represents the bottle-neck of sheep urine patch N2O emissions from extensively grazed organic soils.

Extensively grazed grasslands are understudied in terms of their contribution to greenhouse gas (GHG) emissions from livestock production. Mountains, moorlands and heath occupy 18% of the UK land area, however, in situ studies providing high frequency N2O emissions from sheep urine deposited to such areas are lacking. Organic soils typical of these regions may provide substrates for denitrification-related N2O emissions, however, acidic and anoxic conditions may inhibit nitrification (and associated emissions from nitrification and denitrification). We hypothesised urine N2O-N emission factors (EFs) would be lower than the UK country-specific and IPCC default value for urine, which is based on lowland measurements. Using automated GHG sampling chambers, N2O emissions were determined from real sheep urine (930 kg N ha-1) and artificial urine (920 kg N ha-1) applied in summer, and from an artificial urine treatment (1120 kg N ha-1) and a combined NO3- and glucose treatment (106 kg N ha-1; 213 kg C ha-1) in autumn. The latter treatment provided an assessment of the soils capacity for denitrification under non-substrate limiting conditions. The artificial urine-N2O EF was 0.01 ±â€¯0.00% of the N applied in summer and 0.00 ±â€¯0.00% of the N applied in autumn. The N2O EF for real sheep urine applied in summer was 0.01 ±â€¯0.02%. A higher flux was observed in only one replicate of the real urine treatment, relating to one chamber where an increase in soil solution NO3- was observed. No lag phase in N2O emission was evident following application of the NO3- and glucose treatment, which emitted 0.69 ±â€¯0.15% of the N applied. This indicates nitrification rates are the bottle-neck for N2O emissions in upland organic soils. We calculated the potential impact of using hill-grazing specific urine N2O EFs on the UK inventory of N2O emissions from sheep excreta, and found a reduction of ca. 43% in comparison to the use of a country-specific excretal EF.