Adaptive divergence generates distinct plastic responses in two closely relatedSeneciospecies

Abstract

16Phenotypic plasticity can maintain population fitness in novel or changing environments if it allows the 17 phenotype to track the new trait optimum. Understanding how adaptation to contrasting environments 18 determines plastic responses can identify how plasticity evolves, and its potential to be adaptive in response 19 to environmental change. We sampled 79 genotypes from populations of two closely related but ecologically 20 divergent ragwort species (Senecio, Asteraceae), and transplanted multiple clones of each genotype into four 21 field sites along an elevational gradient representing each species' native range, the edge of their range, and 22 conditions outside their native range. At each transplant site, we quantified differences in survival, growth, 23 leaf morphology, chlorophyll fluorescence and gene expression for both species. Overall, the two species 24 differed in their sensitivity to the elevational gradient. As evidence of plasticity, leaf morphology changed 25 across the elevational gradient, with changes occurring in opposite directions for the two species. Differential 26 gene expression across the four field sites also revealed that the genetic pathways underlying plastic 27 responses were highly distinct in the two species. Despite the two species having diverged recently, 28 adaptation to contrasting habitats has resulted in the evolution of distinct sensitivities to environmental 29 variation, underlain by distinct forms of plasticity. 30 genotype-by-environment interactions, phenotypic plasticity, physiological plasticity, specialisation 32 33 62when the environment is predictable, leading to adaptive plasticity within the environmental limits 63 experienced during adaptation (Bradshaw 1965; Schlichting 1986; Baythavong and Stanton 2010). Whether 64 such plasticity will continue to be adaptive when exposed to novel conditions, such as those imposed by 65 4 climate change, remains an empirical issue (Ghalambor et al. 2007). Strong stabilising selection created by 66 predictable environments is expected to lead to specific plastic responses and reduce genetic variation for 67 plasticity (Oostra et al. 2018). By contrast, populations adapted to a wider range of habitats that are more 68 spatially and temporally variable are predicted to maintain genetic variation in plastic responses, increasing 69 the potential for selection on plasticity (Chevin et al. 2010). Detecting and characterising patterns of G×E for 70 a range of naturally occurring genotypes can help us understand whether evolutionary responses can occur 71 even if plasticity is constrained in certain directions (Via 1993; Chevin and Hoffmann 2017). 72 The genetic architecture underlying variation in plasticity is largely unknown (Fusco and Minelli 2010). 73 Plastic responses at the gene expression level are most likely controlled either by epiallelic control of the 74 genes themselves or allelic variation in the regulators of the genes (Rockman and Kruglyak 2006). If allelic 75 (sequence changes) or epiallelic (e.g. DNA me...

Adaptive plasticity in development rate and genetic variance in survival increase the potential for adapting to novel environments

“…Etna (2,000m and above) where it occurs on lava flows that are covered by snow each winter. A previous reciprocal transplant of cuttings of both species showed that the two species have evolved different forms of plasticity associated with adaptive divergence to habitats at these contrasting elevations (Walter et al 2020).…”

Vascular plant species diversity of Mt. Etna (Sicily): endemicity, insularity and spatial patterns along the altitudinal gradient of the highest active volcano in Europe

“…There is also support from phylogenetic studies for an increase in diversification with elevation (Merckx et al, 2015), particularly in high elevation 'island-like habitats' (Hughes & Eastwood, 2006). Moreover, phylogenetic evidence indicates that many high-elevation endemics across the globe are phylogenetically young taxa resulting from recent fast diversification (Winkworth et al, 2004;Merckx et al, 2015;Walter et al, 2020). This fact fits well with the geological evolution of Etna, which, although started almost 600,000 years ago on the sea floor, really a true stratovolcano (named ''Ellittico'') exceeding 3,000 m in altitude, is formed only from about 100,000 years ago (De Beni et al, 2011).…”

Hidden genetic variation in plasticity provides the potential for rapid adaptation to novel environments

“…). These leaf measurements represent leaf morphology and investment traits that show plastic responses to the abiotic environment in S. chrysanthemifolius (Walter et al 2020b), and in other plant systems, including sunflowers (Royer et al 2009). We also used a Dualex instrument (Force-A) to measure the chlorophyll, flavonol and anthocyanin pigment content of the leaf.…”

“…Etna (Table S1; also see Fig. 1 in Walter et al 2020b). Where possible, we sampled individuals that were at least ten meters apart from each other to minimize chances of sampling close relatives.…”

“…Soil at lower elevations soil is characterized as a sandy loam, but changes to volcanic sand above 1,500m (Walter et al 2020b).…”