Data from: Sweet tetra-trophic interactions: multiple evolution of nectar secretion, a defensive extended phenotype in cynipid gallwasps

Many herbivores employ reward-based mutualisms with ants to gain protection from natural enemies. We examine the evolutionary dynamics of a tetra-trophic interaction in which gallwasp herbivores induce their host oaks to produce nectar-secreting galls, which attract ants that provide protection from parasitoids. We show that, consistent with other gall defensive traits, nectar secretion has evolved repeatedly across the oak gallwasp tribe and also within a single genus (Disholcaspis) that includes many nectar-inducing species. Once evolved, nectar secretion is never lost in Disholcaspis, consistent with high defensive value of this trait. We also show that evolution of nectar secretion is correlated with a transition from solitary to aggregated oviposition, resulting in clustered nectar-secreting galls, which produce a resource that ants can more easily monopolize. Such clustering is commonly seen in ant guard mutualisms. We suggest that correlated evolution between maternal oviposition and larval nectar-induction traits has enhanced the effectiveness of this gall defense strategy.诸多植食性生物会通过与蚂蚁构建基于报酬的互利共生关系，以获取天敌防护。本研究针对一类四营养级互作体系的演化动力学展开分析：植食性瘿蜂诱导其寄主栎树产生分泌花蜜的虫瘿，此类虫瘿可吸引蚂蚁，从而为瘿蜂提供抵御寄生蜂的保护。研究结果显示，与其他虫瘿防御性状一致，花蜜分泌性状在栎瘿蜂族内多次独立演化，同时在包含多种花蜜诱导物种的盘胸瘿蜂属（Disholcaspis）内部也发生了演化。该性状一旦演化形成，便不会在盘胸瘿蜂属内丢失，这与该性状具备极高防御价值的推论相符。本研究同时发现，花蜜分泌性状的演化与产卵行为从单独产卵向聚集产卵的转变存在相关性：聚集产卵会形成密集分布的分泌花蜜虫瘿，这类资源更易于被蚂蚁独占。这种虫瘿聚集现象在蚂蚁守卫型互利共生关系中颇为常见。我们认为，母体产卵行为与幼虫花蜜诱导性状之间的协同演化，提升了该虫瘿防御策略的整体有效性。