Table_5_Distance Dependent Contribution of Ants to Pollination but Not Defense in a Dioecious, Ambophilous Gymnosperm.DOCX

Dioecious plants are obligate outcrossers with separate male and female individuals, which can result in decreased seed set with increasing distance between the sexes. Wind pollination is a common correlate of dioecy, yet combined wind and insect pollination (ambophily) could be advantageous in compensating for decreased pollen flow to isolated females. Dioecious, ambophilous gymnosperms Ephedra (Gnetales) secrete pollination drops (PDs) in female cones that capture airborne pollen and attract ants that feed on them. Plant sugary secretions commonly reward ants in exchange for indirect plant defense against herbivores, and more rarely for pollination. We conducted field experiments to investigate whether ants are pollinators and/or plant defenders of South American Ephedra triandra, and whether their contribution to seed set and seed cone protection varies with distance between female and male plants. We quantified pollen flow in the wind and assessed the effectiveness of ants as pollinators by investigating their relative contribution to seed set, and their visitation rate in female plants at increasing distance from the nearest male. Ants accounted for most insect visits to female cones of E. triandra, where they consumed PDs, and pollen load was larger on bigger ants without reduction in pollen viability. While wind pollination was the main contributor to seed set overall, the relative contribution of ants was distance dependent. Ant contribution to seed set was not significant at shorter distances, yet at the farthest distance from the nearest male (23 m), where 20 times less pollen reached females, ants enhanced seed set by 30% compared to plants depending solely on wind pollination. We found no evidence that ants contribute to plant defense by preventing seed cone damage. Our results suggest that, despite their short-range movements, ants can offset pollen limitation in isolated females of wind-pollinated plants with separate sexes. We propose that ants enhance plant reproductive success via targeted delivery of airborne pollen, through frequent contact with ovule tips while consuming PDs. Our study constitutes the first experimental quantification of distance-dependent contribution of ants to pollination and provides a working hypothesis for ambophily in other dioecious plants lacking pollinator reward in male plants.

雌雄异株的植物为严格异交植物，具有分开的雌雄个体，这可能导致随着雌雄个体之间距离的增加而种子产量下降。风媒授粉是雌雄异株植物的一个常见伴随特征，但结合风媒和昆虫授粉（两媒性）可能在补偿向孤立雌株减少的传粉量方面具有优势。雌雄异株的两媒性裸子植物麻黄（Gnetales）在雌球果中分泌传粉滴（PDs），这些传粉滴能捕捉空中花粉并吸引以之食的蚂蚁。植物的糖质分泌物通常作为奖励吸引蚂蚁，以换取植物对食草动物的间接防御，以及更少见的授粉服务。我们进行了田间实验，以研究蚂蚁是否是南美麻黄（Ephedra triandra）的授粉者及/或植物防御者，以及它们对种子产量和种子球果保护的贡献是否随雌雄植物间距离的变化而变化。我们量化了风中的花粉流动，并通过调查它们对种子产量的相对贡献和它们在距离最近雄株越来越远的女株上的访问率，来评估蚂蚁作为授粉者的有效性。蚂蚁是E. triandra雌球果上昆虫访问的主要来源，在那里它们消费PDs，较大的蚂蚁携带的花粉量更大，而花粉的存活率并未降低。尽管风媒授粉是总体种子产量的主要贡献者，但蚂蚁的相对贡献与距离相关。在较短的距离内，蚂蚁对种子产量的贡献并不显著，但在距离最近雄株最远的距离（23米）处，那里到达雌株的花粉减少了20倍，蚂蚁将种子产量提高了30%，相比于仅依赖风媒授粉的植物。我们没有发现蚂蚁通过防止种子球果受损来为植物防御做出贡献的证据。我们的结果表明，尽管蚂蚁的短距离移动，但它们可以通过在消费PDs时与胚珠尖端频繁接触，从而将空中花粉有针对性地传递给雌株，以补偿风媒授粉植物雌雄异株中孤立雌株的花粉限制。我们提出，蚂蚁通过目标性地传递空中花粉，提高植物繁殖成功率，这种机制在其他缺乏授粉奖励的雌雄异株植物中可能存在。我们的研究构成了对蚂蚁授粉距离依赖性贡献的第一个实验量化，并为其他雌雄异株植物中的两媒性提供了一个工作假说。