Data from: Copulation order, density cues and variance in fertilization success in a cestode

Simultaneous hermaphrodites maximize their fitness by optimizing their investment into male or female functions. Allocation of resources to male function (tissues, traits, and/or behaviours increasing paternity) is predicted to increase as density, and the associated level of sperm competition, increases. We tested whether the simultaneous hermaphroditic cestode Schistocephalus solidus uses cues of potential partner densities in its fish intermediate host to improve its male reproductive success in the final host. We had two worms, one originating from a multiple infection in the fish intermediate host and one from a single infection, sequentially compete to fertilize the eggs of a third worm. The fertilization rates of the two competitors nearly always differed from the 50-50 null expectation, sometimes considerably, implying there was a 'winner' in each experimental competition. However, we did not find a significant effect of density in the fish host (single vs multiple) or mating order on paternity. Additional work will be needed to identify the traits and environmental conditions that explain the high variance in male reproductive success observed in this experiment.

同时雌雄同体生物可通过优化对雄性或雌性生殖功能的资源投入，最大化自身进化适合度。当种群密度及伴随的精子竞争水平升高时，投入雄性功能——即提升父权成功率的组织、性状和/或行为——的资源分配预计会增加。我们以同时雌雄同体的绦虫物种裂头绦虫（Schistocephalus solidus）为研究对象，检验其是否会利用鱼类中间宿主体内的潜在伴侣密度线索，来提升自身在终末宿主中的雄性生殖成功率。实验中我们选取两条蠕虫：一条源自鱼类中间宿主的多重感染个体，另一条源自单一感染个体，让二者依次竞争以完成对第三条蠕虫卵子的受精。两条竞争个体的受精成功率几乎始终偏离50%的零假设预期，有时偏差幅度较大，这表明每一组实验竞争中均存在一个“优胜者”。然而，我们并未发现鱼类宿主的感染密度（单一感染vs多重感染）或交配顺序对父权成功率存在显著影响。未来仍需开展额外研究，以明确能够解释本实验中观测到的雄性生殖成功率高变异性的性状与环境条件。