Polygyny does not explain the superior competitive ability of dominant ant associates in the African ant-plant, Acacia (Vachellia) drepanolobium

1. The Acacia drepanolobium (also known as Vachellia drepanolobium) ant-plant symbiosis is considered a classic case of species coexistence, in which four species of tree-defending ants compete for nesting space in a single host tree species. Coexistence in this system has been explained by trade-offs in the ability of the ant associates to compete with each other for occupied trees versus the ability to colonize unoccupied trees.
2. We seek to understand the proximal reasons for how and why the ant species vary in competitive or colonizing abilities, which are largely unknown.
3. In this study, we use RADseq derived SNPs to identify relatedness of workers in colonies to test the hypothesis that competitively dominant ants reach large colony sizes due to polygyny, i.e., the presence of multiple egg-laying queens in a single colony.
4. We find that variation in polygyny is not associated with competitive ability; in fact, the most dominant species, unexpectedly, showed little evidence of polygyny. We also use these markers to investigate variation in mating behavior among the ant species, and find that different species vary in the number of males fathering the offspring of each queen. Finally, we show that the nature of polygyny varies between the two commonly polygynous species, Crematogaster mimosae and Tetraponera penzigi: in C. mimosae, queens in the same colony are often related, while this is not the case for T. penzigi.
5. These results shed light on factors influencing the evolution of species coexistence in an ant-plant mutualism, as well as demonstrating the effectiveness of RADseq-derived SNPs for parentage analysis.

1. 镰荚金合欢（Acacia drepanolobium，又名Vachellia drepanolobium）的蚁植共生体系被视为物种共存的经典案例，该体系中共有4种护树蚂蚁在单一宿主树种上争夺筑巢空间。学界此前将该系统的物种共存归因于蚁类共生伙伴在占据宿主树木的种间竞争能力与开拓未被占据宿主树木的定植能力之间存在进化权衡。
2. 本研究旨在阐明蚂蚁物种在竞争或定植能力上存在差异的直接原因，而这一问题目前仍鲜为人知。
3. 本研究利用基于限制性酶切位点相关DNA测序（Restriction-site Associated DNA sequencing，RADseq）得到的单核苷酸多态性（Single Nucleotide Polymorphisms，SNPs）标记，通过鉴定蚁群内工蚁的亲缘关系，来验证“具有竞争优势的蚂蚁因多后制（polygyny，即单个蚁群内存在多只产卵蚁后）而拥有更大蚁群规模”这一假说。
4. 研究结果显示，多后制的差异与蚂蚁的竞争能力并无关联；事实上，最具竞争优势的物种几乎未表现出多后制的迹象，这与预期不符。本研究还利用上述标记探究了不同蚂蚁物种的交配行为差异，发现不同物种在为单只蚁后后代提供父本的雄蚁数量上存在显著差异。最后，本研究表明两种常见多后制物种——Crematogaster mimosae与Tetraponera penzigi的多后制本质存在差异：在C. mimosae的同巢蚁后之间通常具有亲缘关系，而T. penzigi则不存在这一情况。
5. 本研究结果不仅阐明了蚁植互利共生（ant-plant mutualism）体系中物种共存演化的影响因素，同时证实了基于RADseq得到的SNPs标记在亲权分析中的有效性。