Evolution of tolerance in an invasive weed after reassociation with its specialist herbivore

The interaction between the European wild parsnip Pastinaca sativa and its coevolved florivore the parsnip webworm Depressaria pastinacella, established in North America for over 150 years, has resulted in evolution of local chemical phenotype matching. The recent invasion of New Zealand by webworms, exposing parsnips there to florivore selection for the first time, provided an opportunity to assess rates of adaptive response in a real-time experiment. We planted reciprocal common gardens in the USA and NZ with seeds from (1) US populations with a long history of webworm association; (2) NZ populations that had never been infested and (3) NZ populations infested for 3 years (since 2007) or 6 years (since 2004). We measured impacts of florivory on realized fitness, reproductive effort and pollination success and measured phenotypic changes in infested NZ populations relative to uninfested NZ populations to determine whether rapid adaptive evolution in response to florivory occurred. Irrespective of country of origin or location, webworms significantly reduced plant fitness. Webworms reduced pollination success in small plants but not in larger plants. Although defence chemistry remained unchanged, plants in infested populations were larger after 3–6 years of webworm florivory. As plant size is a strong predictor of realized fitness, evolution of large size as a component of florivore tolerance may occur more rapidly than evolution of enhanced chemical defence.

欧洲野生欧防风（Pastinaca sativa）与其协同演化的食花昆虫（florivore）欧防风织蛾（Depressaria pastinacella）之间的互作关系，在北美定殖已逾150年，由此促成了二者局部化学表型匹配的演化进程。近期织蛾入侵新西兰，首次使当地欧防风面临食花昆虫的选择压力，这为实时评估适应性响应速率提供了绝佳的实验契机。我们分别在美国与新西兰搭建了双向互置共同花园实验，所用种子来自三类种群：(1) 长期与织蛾共存的北美种群；(2) 从未遭受织蛾侵染的新西兰原生种群；(3) 分别受织蛾侵染3年（自2007年起）或6年（自2004年起）的新西兰种群。我们测定了食花昆虫取食对植株实际适合度、繁殖投入以及传粉成功率的影响，并对比了受侵染与未受侵染的新西兰种群间的表型差异，以明确当地欧防风是否会针对食花昆虫产生快速适应性演化。无论种群起源地与种植地点为何，织蛾均显著降低了植株的实际适合度。织蛾会降低小型植株的传粉成功率，但对大型植株无显著影响。尽管植株的防御化学组成未发生改变，但经历3~6年织蛾取食的受侵染种群，其个体尺寸显著更大。由于植株尺寸是实际适合度的强预测因子，因此以增大体型作为食花昆虫耐受策略的演化速率，可能快于增强化学防御的演化速率。