Centre for Social Evolution, Department of Biology, University of Copenhagen, Universitetsparken 15, DK-2100, Copenhagen, Denmark.
Centre for Integrative Bee Research (CIBER), Department of Entomology, University of California Riverside, Riverside, CA, 92521, USA.
BMC Evol Biol. 2018 Mar 23;18(1):28. doi: 10.1186/s12862-018-1144-y.
Promiscuous mating and sperm competition often induce arms races between the sexes with detrimental outcomes for females. However, ants with multiply-inseminated queens have only a single time-window for sperm competition and queens are predicted to gain control over the outcome of sperm storage quickly. The seminal fluid of Acromyrmex leaf-cutting ants reduces the viability of rival sperm, but how confrontations between unrelated ejaculates affect sperm storage remains unknown.
We investigated the effects of ejaculate admixture on sperm motility in A. echinatior and found that the proportion of motile spermatozoa, sperm swimming speed, and linearity of sperm movement increased when rival ejaculates were mixed in vitro. Major effects induced by the seminal fluid of rival males were of similar magnitude to those generated by queen reproductive tract secretions, whereas own seminal fluid induced lower sperm activation levels.
Our results suggest that ant sperm respond via a self-non-self recognition mechanism to similar or shared molecules expressed in the reproductive secretions of both sexes. Lower sperm motility in the presence of own seminal fluid indicates that enhanced motility is costly and may trade-off with sperm viability during sperm storage, consistent with studies in vertebrates. Our results imply that ant spermatozoa have evolved to adjust their energetic expenditure during insemination depending on the perceived level of sperm competition.
滥交和精子竞争常常导致性别之间的军备竞赛，对女性造成不利后果。然而，具有多次受精的蚁后只有一个单一的精子竞争窗口期，并且蚁后被预测能够迅速控制精子储存的结果。切叶蚁的精液会降低竞争精子的活力，但尚未可知不同精液之间的对抗如何影响精子储存。
我们研究了在 A. echinatior 中混合射精对精子活力的影响，发现当体外混合竞争精液时，运动精子的比例、精子游动速度和运动直线性增加。来自竞争雄性的精液的主要影响与来自蚁后生殖道分泌物的影响相似，而自身精液则诱导较低的精子激活水平。
我们的结果表明，蚂蚁的精子通过自我-非自我识别机制对两性生殖分泌物中表达的相似或共享分子做出反应。在自身精液存在的情况下，精子的运动能力较低，这表明在精子储存过程中，增强的运动能力是有代价的，并且可能与精子活力成反比，这与脊椎动物的研究结果一致。我们的研究结果表明，蚂蚁的精子已经进化为根据感知到的精子竞争程度，在受精过程中调整其能量消耗。
