A comprehensive approach to detect hybridization sheds light on the evolution of Earth’s largest lizards

Hybridization between species occurs more frequently in vertebrates than traditionally thought, but distinguishing ancient hybridization from other phenomena that generate similar evolutionary patterns remains challenging. Here, we used a comprehensive workflow to discover evidence of ancient hybridization between the Komodo dragon (Varanus komodoensis) from Indonesia and a common ancestor of an Australian group of monitor lizards known colloquially as sand monitors. Our data comprise >300 nuclear loci, mitochondrial genomes, phenotypic data, fossil and contemporary records, and past/present climatic data. We show that the four sand monitor species share more nuclear alleles with V. komodoensis than expected given a bifurcating phylogeny, likely as a result of hybridization between the latter species and a common ancestor of sand monitors. Sand monitors display phenotypes that are intermediate between their closest relatives and V. komodoensis. Biogeographic analyses suggest that V. komodoensis and ancestral sand monitors co-occurred in northern Australia. In agreement with the fossil record, this provides further evidence that the Komodo dragon once inhabited the Australian continent. Our study shows how different sources of evidence can be used to thoroughly characterize evolutionary histories that deviate from a treelike pattern, that hybridization can have long-lasting effects on phenotypes, and that detecting hybridization can improve our understanding of evolutionary and biogeographic patterns.

物种间的杂交事件在脊椎动物类群中的发生频率远高于以往的传统认知，但如何将古杂交事件与其他可产生相似进化模式的现象区分开来，仍是一项亟待解决的难题。本研究依托一套完备的分析流程，旨在发掘印度尼西亚科莫多巨蜥（Varanus komodoensis）与一类俗称为沙巨蜥的澳大利亚巨蜥类群的共同祖先之间存在古杂交的相关证据。本研究的数据涵盖超过300个核基因座、线粒体基因组、表型数据、化石与现生标本记录，以及古今气候数据集。研究结果显示，相较于分叉式系统发育的预期结果，四种沙巨蜥物种与科莫多巨蜥共享的核等位基因数量更多，这一现象大概率源于科莫多巨蜥与沙巨蜥类群共同祖先之间的杂交事件。沙巨蜥的表型特征恰好介于其近亲类群与科莫多巨蜥之间。生物地理学分析表明，科莫多巨蜥与沙巨蜥的祖先曾在澳大利亚北部区域共存。结合化石记录来看，该结果进一步佐证了科莫多巨蜥曾广泛栖息于澳大利亚大陆的推论。本研究证实，可通过多源证据全面刻画偏离树形模式的进化历史；杂交事件可对物种表型产生长期影响；而检测杂交事件有助于深化我们对进化与生物地理学模式的认知。