Gowler Camden D, Leon Kristoffer E, Hunter Mark D, de Roode Jacobus C
Biology Department, Emory University, 1510 Clifton Road, Atlanta, GA, 30322, USA,
J Chem Ecol. 2015 Jun;41(6):520-3. doi: 10.1007/s10886-015-0586-6. Epub 2015 May 9.
In tri-trophic systems, herbivores may benefit from their host plants in fighting parasitic infections. Plants can provide parasite resistance in two contrasting ways: either directly, by interfering with the parasite, or indirectly, by increasing herbivore immunity or health. In monarch butterflies, the larval diet of milkweed strongly influences the fitness of a common protozoan parasite. Toxic secondary plant chemicals known as cardenolides correlate strongly with parasite resistance of the host, with greater cardenolide concentrations in the larval diet leading to lower parasite growth. However, milkweed cardenolides may covary with other indices of plant quality including nutrients, and a direct experimental link between cardenolides and parasite performance has not been established. To determine if the anti-parasitic activity of milkweeds is indeed due to secondary chemicals, as opposed to nutrition, we supplemented the diet of infected and uninfected monarch larvae with milkweed latex, which contains cardenolides but no nutrients. Across three experiments, increased dietary cardenolide concentrations reduced parasite growth in infected monarchs, which consequently had longer lifespans. However, uninfected monarchs showed no differences in lifespan across treatments, confirming that cardenolide-containing latex does not increase general health. Our results suggest that cardenolides are a driving force behind plant-derived resistance in this system.
在三级营养系统中，食草动物在对抗寄生虫感染时可能会从其寄主植物中受益。植物可以通过两种截然不同的方式提供抗寄生虫能力：直接方式是通过干扰寄生虫，间接方式是通过提高食草动物的免疫力或健康状况。在帝王蝶中，幼虫食用马利筋的食物强烈影响一种常见原生动物寄生虫的适应性。被称为强心甾的有毒植物次生化学物质与寄主的抗寄生虫能力密切相关，幼虫食物中强心甾浓度越高，寄生虫生长越低。然而，马利筋强心甾可能与包括营养成分在内的其他植物质量指标共同变化，强心甾与寄生虫表现之间的直接实验联系尚未建立。为了确定马利筋的抗寄生虫活性是否确实归因于次生化学物质而非营养成分，我们用含有强心甾但不含营养成分的马利筋乳胶补充受感染和未受感染的帝王蝶幼虫的食物。在三个实验中，增加食物中强心甾浓度可降低受感染帝王蝶体内寄生虫的生长，因此它们的寿命更长。然而，未受感染的帝王蝶在不同处理下的寿命没有差异，这证实了含强心甾的乳胶不会提高总体健康水平。我们的结果表明，强心甾是该系统中植物衍生抗性的驱动力。
