Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, M5S 3B2, Canada.
Department of Biology, University of Toronto Mississauga, Ontario, L5L 1C6, Canada.
Am J Bot. 2018 Jun;105(6):977-985. doi: 10.1002/ajb2.1098. Epub 2018 Jun 19.
The strength of plant-herbivore interactions varies in space and time, but the factors that explain this variation are poorly understood. Several lines of research suggest that variation in plant reproductive systems and latitude may explain resistance against herbivores, but how these factors jointly affect plant-herbivore interactions has not been investigated in detail. We examined the effects of latitude, sexual system, and plant gender on herbivory in Sagittaria latifolia, an aquatic plant in which populations are typically monoecious (separate female and male flowers) or dioecious (separate female and male plants).
We surveyed 43 populations of S. latifolia between 42 and 48° N in Ontario, Canada. In each population, we recorded the sexual system and obtained estimates of herbivore damage to ramets of known gender (i.e. female, male, or hermaphrodite) by the weevil Listronotus appendiculatus, the principal herbivore of S. latifolia. Herbivore damage was quantified as the percent leaf area removed by adult L. appendiculatus weevils, and the abundance of larvae feeding within flowering stalks, which was correlated with the amount of damage by herbivores to the inflorescence.
Leaf herbivory significantly decreased with increasing latitude but did not vary with sexual system or plant gender. By contrast, larvae were more abundant in dioecious populations and on female plants, corresponding to increased stem damage, providing evidence for sex-biased larval abundance in S. latifolia. These effects of sexual system and gender on larval abundance were strongest at lower latitudes.
Our study found latitudinal variation in leaf herbivory and sex-biased resistance to weevil larvae that feed on the reproductive tissues of S. latifolia, which is predicted to be a necessary condition for herbivory to influence the evolution of dioecy.
植物-食草动物相互作用的强度在空间和时间上都有所不同，但解释这种变化的因素知之甚少。有几条研究线索表明，植物繁殖系统和纬度的变化可能解释了对食草动物的抵抗力，但这些因素如何共同影响植物-食草动物的相互作用还没有被详细研究。我们研究了纬度、性别系统和植物性别对菹草（水生植物，种群通常为雌雄同体（雌雄花分开）或雌雄异株（雌雄植物分开））的食草作用的影响。
我们在加拿大安大略省的 43 个菹草种群中进行了调查，纬度在 42°至 48°N 之间。在每个种群中，我们记录了性别系统，并通过主要的菹草食草动物——长角象鼻虫（Listronotus appendiculatus），记录了已知性别的分株（即雌株、雄株或雌雄同体）的食草损害估计值。食草损害量化为成年长角象鼻虫去除的叶片面积百分比，以及在开花茎内取食的幼虫数量，这与食草动物对花序的损害程度相关。
叶片食草显著随纬度的增加而减少，但不受性别系统或植物性别影响。相比之下，幼虫在雌雄异株种群和雌株中更为丰富，这与茎的损害增加相对应，为菹草中幼虫丰度的性别偏倚提供了证据。这些性别系统和性别对幼虫丰度的影响在较低纬度时最强。
我们的研究发现，菹草的叶片食草性存在纬度变化，以及对以菹草生殖组织为食的象鼻虫幼虫的性别偏倚抗性，这是食草作用影响雌雄异株进化的必要条件。
