Safe sex: ant defense does not interfere with pollination in passion flowers

ABSTRACT Ant defense against floral enemies incurs a two-fold ant-pollinator conflict, both via pollinator deterrence and nectar or pollen collection by non-pollinating protective ants. Some ant-plants have physical barriers whereas others produce ant-repellent chemicals to avoid ant visitation to flowers and subsequent pollination interference. Passiflora coccinea is a hummingbird-pollinated myrmecophilous plant in which floral enemy repellence occurs without limiting ant access to open flowers. To test the hypothesis that ant activity is restricted within flowers to prevent contact with anthers, we compared ant defense response between reproductive (anthers and stigmas) and non-reproductive (bracts, corona and perianth) floral structures by combining an observational survey with an experimental approach. A few insect species were found to visit flowers without providing pollination service, mostly pollen-collecting bees and nectar-thieving butterflies landing on petals. Ants always attacked floral visitors that landed on non-reproductive structures, but they never attacked insects visiting reproductive structures as ants never accessed anthers. Our results suggest that the differential ant defense response is an adaptative process to prevent ant-pollinator conflict. The eventual mechanism that regulates this process could be closely linked to the corona of filaments that protects nectar chambers, simultaneously restricting ant access to nectar and pollen.

摘要：蚂蚁对花部天敌的防御行为会引发双重的蚂蚁-传粉者冲突：一方面会威慑传粉者，另一方面会导致不提供传粉服务的保护性蚂蚁采集花蜜或花粉。部分蚁栖植物（myrmecophilous plant）会借助物理屏障抵御蚂蚁，另一些则会分泌驱蚁化学物质，以避免蚂蚁造访花朵及后续的传粉干扰。红花西番莲（Passiflora coccinea）是一种由蜂鸟传粉的蚁栖植物，其针对花部天敌的防御行为不会限制蚂蚁造访开放的花朵。为验证"蚂蚁活动被限制在花部特定区域以避免接触花药（anthers）"这一假说，我们结合野外观察与实验手段，对比了蚂蚁对花部生殖结构（花药与柱头（stigmas））和非生殖结构（苞片（bracts）、副花冠（corona）与花被（perianth））的防御反应。研究发现，有少量昆虫造访花朵却不提供传粉服务，其中以采集花粉的蜂类以及停留在花瓣上的盗蜜蝶类为主。蚂蚁总会攻击落在非生殖结构上的花部访客，但从未攻击造访生殖结构的昆虫——这是因为蚂蚁从未接触过花药。本研究结果表明，蚂蚁的差异化防御反应是一种适应性策略，用于规避蚂蚁-传粉者冲突。调控这一过程的核心机制可能与保护花蜜室的丝状副花冠密切相关，该结构可同时限制蚂蚁获取花蜜与花粉。