Phylogenetic signal in module composition and species connectivity in compartmentalized host-parasite networks

Across different taxa, networks of mutualistic or antagonistic interactions show consistent architecture. Most networks are modular, with modules being distinct species subsets connected mainly with each other and having few connections to other modules. We investigate the phylogenetic relatedness of species within modules and whether a phylogenetic signal is detectable in the within- and among module connectivity of species using 27 mammal-flea networks from the Palaearctic. In the 24 networks that were modular, closely-related hosts co-occurred in the same module more often than expected by chance; in contrast, this was rarely the case for parasites. The within- and among-module connectivity of the same host or parasite species varied geographically. However, among-module but not within-module connectivity of host and parasites was somewhat phylogenetically constrained. These findings suggest that the establishment of host-parasite networks results from the interplay between phylogenetic influences acting mostly on hosts and local factors acting on parasites, to create an asymmetrically constrained pattern of geographic variation in modular structure. Modularity in host-parasite networks seems to result from the shared evolutionary history of hosts and by trait convergence among unrelated parasites. This suggests profound differences between hosts and parasites in the establishment and functioning of bipartite antagonistic networks.

不同分类群的互利或拮抗互作网络均呈现出一致的架构模式。大多数网络呈模块化结构，其中模块（module）为彼此间连接紧密、仅与其他模块存在少量连接的独立物种子集。本研究基于古北界（Palaearctic）的27组哺乳动物-跳蚤网络，探究了模块内物种的系统发育亲缘关系（phylogenetic relatedness），以及物种的模块内与模块间连接度是否存在可检测的系统发育信号（phylogenetic signal）。在24个呈模块化结构的网络中，亲缘关系相近的宿主物种同处一个模块的概率显著高于随机预期；与之相反，寄生物物种几乎未表现出这一规律。同一宿主或寄生物物种的模块内与模块间连接度存在地理变异。不过，宿主与寄生物的模块间连接度存在一定程度的系统发育约束，而模块内连接度则无此现象。上述研究结果表明，宿主-寄生物网络的形成源于主要作用于宿主的系统发育影响，与主要作用于寄生物的局部环境因子之间的相互作用，最终形成了模块化结构地理变异的非对称约束模式。宿主-寄生物网络的模块化结构，似乎源于宿主共有的进化历史，以及不同类群寄生物之间的性状趋同演化。这一发现揭示了宿主与寄生物在二部拮抗网络的构建与运行机制上存在的深刻差异。