In the Wake of Invasion: Tracing the Historical Biogeography of the South American Cricetid Radiation (Rodentia, Sigmodontinae)

The Great American Biotic Interchange (GABI) was greatly influenced by the completion of the Isthmus of Panama and impacted the composition of modern faunal assemblages in the Americas. However, the contribution of preceding events has been comparatively less explored, even though early immigrants in the fossil records are evidence for waif dispersals. The cricetid rodents of the subfamily Sigmodontinae are a classic example of a species-rich South American radiation resulting from an early episode of North American invasion. Here, we provide a temporal and spatial framework to address key aspects of the historical biogeography and diversification of this diverse mammal group by using mitochondrial and nuclear DNA datasets coupled with methods of divergence time estimation, ancestral area reconstruction and comparative phylogenetics. Relaxed-clock time estimates indicate that divergence of the Sigmodontinae began in the middle–late Miocene (ca. 12–9 Ma). Dispersal-vicariance analyses point to the arrival of a single lineage of northern invaders with a widespread ancestral distribution and imply that the initial differentiation between Central and South America gave rise to the most basal groups within the subfamily. These two major clades diversified in the late Miocene followed by the radiation of main tribes until the early Pliocene. Within the Oryzomyalia, tribes diverged initially in eastern South America whereas multiple dispersals into the Andes promoted further diversification of the majority of modern genera. A comparatively uniform background tempo of diversification explains the species richness of sigmodontines across most nodes, except for two akodontine genera with recent increases in diversification rates. The bridging of the Central American seaway and episodes of low sea levels likely facilitated the invasion of South America long before the onset of the post-Isthmian phase of the GABI.

美洲生物大交流（Great American Biotic Interchange, GABI）的进程极大地受巴拿马地峡（Isthmus of Panama）形成完成的影响，同时深刻塑造了美洲现代动物群组合的组成格局。然而，相较于该核心事件本身，此前发生的生物交流事件的贡献却鲜少被探索，尽管化石记录中的早期移民类群正是漂泊扩散（waif dispersals）的直接证据。仓鼠科棉鼠亚科（Sigmodontinae）啮齿类便是一个经典研究案例：它们由早期侵入南美洲的北美类群演化而来，最终形成了物种极为丰富的南美辐射类群。本研究整合线粒体与核DNA数据集，结合分化时间估算、祖先区域重建以及比较系统发育学等分析方法，为这一多样哺乳动物类群的历史生物地理学与演化分化关键问题提供了系统性时空分析框架。宽松分子钟（relaxed-clock）时间估算结果显示，棉鼠亚科的分化起始于中新世（Miocene）中晚期（约1200万至900万年前，Ma）。扩散-分替分析（dispersal-vicariance analyses）表明，单一的北方入侵者演化支（clades）拥有广泛的祖先分布范围，且中美洲与南美洲的初始地理分异催生了该亚科内的最基部类群（basal groups）。这两大主要演化支在中新世晚期发生早期分化，随后各主要分类族类群直至上新世（Pliocene）早期均发生大规模辐射演化。在稻鼠总族（Oryzomyalia）中，各族类群最初在南美东部发生分化，而多次向安第斯山脉（Andes）的跨区域扩散事件则推动了多数现代属类的进一步分化与物种积累。除两个近年演化速率显著提升的棘鼠族（akodontine）属类之外，整体相对均匀的演化分化速率背景可以解释棉鼠亚科绝大多数演化节点处的物种丰富度格局。巴拿马海道的闭合以及多轮低海平面事件，极有可能在美洲生物大交流的地峡后阶段开启之前，就已经为北美类群侵入南美洲提供了便利的扩散通道。