Gone with Gondwana: amphipod diversification in freshwaters preceded the breakup of the supercontinent

Aim The evolutionary origins of modern taxa are best understood as arising from the interplay of vicariance and dispersal. Vicariant events have long been considered responsible for Gondwanan distributions; such species are relics of the eponymic supercontinent on which they were thought to have originated. One such set of taxa are the freshwater members of the amphipod superfamily Hyaloidea, which due to their marine relatives and current distributions serve as an excellent model for testing vicariance and dispersal hypotheses. We investigated the evolutionary and biogeographic histories of the Hyaloidea using a molecular phylogenetic approach. Location North/South America, Australia Taxon Superfamily Hyaloidea Methods Publicly available nucleotide sequences from GenBank and BOLD were collected for 148 members of the order Amphipoda. These data, including sequences from two nuclear genes and one mitochondrial gene were aligned in order to examine evolutionary relationships within the order. Phylogenetic trees were reconstructed using both Maximum likelihood analyses and Bayesian inference, Bayesian trees were time calibrated based on previously estimated divergence dates. Results Maximum likelihood analyses and Bayesian inference, using two nuclear genes and one mitochondrial gene, reveal the freshwater amphipods within the superfamily (Hyalellidae/Chiltoniidae) as a monophyletic group within the superfamily. Members of this lineage are shown to have diverged from their marine ancestors during the Early Cretaceous period, suggesting a singular invasion of freshwater. Main conclusions A single Cretaceous divergence event for these animals rather than multiple Paleogene events is supportive of a vicariant origin following the breakup of Gondwana, instead of a recent marine invasion as has been previously hypothesized. The ancestor to the freshwater Hyaloidea likely invaded what is now Antarctica, allowing for dispersal across southern Gondwana. Fragmentation of the supercontinent would have severed gene flow between taxa, setting hyalellids and chiltoniids on separate evolutionary trajectories. A Cretaceous invasion by the Hyaloidea suggests that freshwater members represent a much older lineage than previously considered, occupying continental freshwaters prior to the gammarids in the Mesozoic and cotemporally with the crangonyctids in the Cenozoic. Our results highlight the utility of amphipods for testing biogeographic hypotheses that infer the origin of freshwater taxa.研究目的：现代类群的演化起源，最佳阐释路径为隔离分化（vicariance）与扩散（dispersal）的共同作用。长期以来，隔离分化事件被认为是冈瓦纳分布格局（Gondwanan distributions）的成因，这类物种被视为起源于其赖以生存的同名超大陆的孑遗类群。端足目（Amphipoda）钩虾超科（Hyaloidea）的淡水类群正是这类典型类群之一；鉴于其海洋近缘类群与现存分布格局，该类群是验证隔离分化与扩散假说的理想模型类群。我们利用分子系统发育学方法，探究了钩虾超科的演化与生物地理历史。 研究区域：南北美洲与澳大利亚。 研究类群：钩虾超科。 研究方法：从GenBank与BOLD数据库获取公开可用的核苷酸序列，共涵盖端足目148个物种，包含2个核基因与1个线粒体基因的序列。通过序列联配分析该目内的演化关系。分别采用最大似然法（Maximum likelihood）与贝叶斯推断法（Bayesian inference）重建系统发育树，其中贝叶斯树基于已报道的分化时间进行时间校准。 研究结果：基于2个核基因与1个线粒体基因的最大似然分析与贝叶斯推断结果显示，钩虾超科内的淡水钩虾（包括Hyallelidae科与Chiltoniidae科）构成单系群（monophyletic group）。该谱系类群与海洋祖先的分化发生于早白垩世（Early Cretaceous），表明淡水生境仅被单次入侵。 主要结论：这些类群的分化仅发生于白垩纪单次事件，而非多次古近纪（Paleogene）事件，这支持了冈瓦纳超大陆（Gondwana）解体后产生的隔离分化起源假说，而非此前提出的近期海洋入侵假说。淡水钩虾超科的祖先极有可能先入侵了现今的南极洲区域，借此实现跨冈瓦纳南部的扩散。超大陆的碎裂阻断了类群间的基因交流，使Hyallelidae与Chiltoniidae科走上独立的演化路径。钩虾超科的白垩纪入侵事件表明，淡水类群的演化历史远较此前认知更为古老：它们在中生代（Mesozoic）便已占据大陆淡水生境，早于钩虾科类群，且在新生代（Cenozoic）与蜾蠃蜚科（Crangonyctidae）类群共存。本研究结果凸显了端足类在验证推断淡水类群起源的生物地理假说中的应用价值。