Evolutionary relationships, cospeciation, and host switching in avian malaria parasites

We used phylogenetic analyses of cytochrome b sequences of malaria parasites and their avian hosts to assess the coevolutionary relationships between host and parasite lineages. Many lineages of avian malaria parasites have broad host distributions, which tend to obscure cospeciation events. The hosts of a single parasite or of closely related parasites were nonetheless most frequently recovered from members of the same host taxonomic family, more so than expected by chance. However, global assessments of the relationship between parasite and host phylogenetic trees, using Component and ParaFit, failed to detect significant cospeciation. The event-based approach employed by TreeFitter revealed significant cospeciation and duplication with certain cost assignments for these events, but host switching was consistently more prominent in matching the parasite tree to the host tree. The absence of a global cospeciation signal despite conservative host distribution most likely reflects relatively frequent acquisition of new hosts by individual parasite lineages. Understanding these processes will require a more refined species concept for malaria parasites and more extensive sampling of parasite distributions across hosts. If parasites can disperse between allopatric host populations through alternative hosts, cospeciation may not have a strong influence on the architecture of host–parasite relationships. Rather, parasite speciation may happen more often in conjunction with the acquisition of new hosts followed by divergent selection between host lineages in sympatry. Detailed studies of the phylogeographic distributions of hosts and parasites are needed to characterize these events.

本研究通过对疟原虫及其鸟类宿主的细胞色素b（cytochrome b）序列开展系统发育分析，以解析宿主与疟原虫谱系间的共进化关联。诸多鸟类疟原虫谱系拥有宽泛的宿主分布范围，这往往会掩盖共成种（cospeciation）事件的信号。但即便如此，单一疟原虫或近缘疟原虫的宿主，大多仍隶属于同一宿主分类科，该结果的随机发生概率远低于预期。然而，借助Component与ParaFit工具对疟原虫与宿主系统发育树的全局关联进行评估时，并未检测到显著的共成种信号。TreeFitter所采用的基于事件的分析方法，在为各类事件设定特定成本赋值的前提下，揭示出了显著的共成种与复制事件，但在匹配疟原虫系统发育树与宿主系统发育树的过程中，宿主转换（host switching）始终占据更为主导的地位。尽管宿主分布具有保守性，但全局共成种信号的缺失，大概率反映出单个疟原虫谱系相对频繁地获得了新宿主。要厘清这些演化过程，需要为疟原虫建立更为精准的物种界定标准，并对疟原虫在各类宿主中的分布开展更广泛的采样工作。若疟原虫能够通过替代宿主在异域（allopatric）宿主种群间扩散，那么共成种作用对宿主-疟原虫关系的架构可能不会产生强烈影响。相较而言，疟原虫的成种作用或许更常伴随新宿主的获得，随后在同域（sympatry）分布的宿主谱系间发生趋异选择。要系统表征这些演化事件，仍需对宿主与疟原虫的系统地理学分布开展细致深入的研究。