Phylogenetic position of the poorly known montane cascade frog Amolops monticola (Ranidae) and description of a new closely related species from Northeast India

The phylogenetic position of Amolops monticola, a cascade frog species known for over 150 years, remains unknown. Yet over the years new taxa have been frequently described in the ambiguously recognised A. monticola species group, based on morphology and presumed phylogenetic affinities. Here we report fresh collections of A. monticola from the Indian Himalayas and clarify its identity with new molecular and morphological evidence. Furthermore, our surveys in the Northeast Indian State of Arunachal Pradesh have led to the discovery of another new species in the A. monticola group from Siang River basin encompassing the Adi hills (historically known as Abor hills). The new species, formally described as Amolops adicola sp. nov., is morphologically distinguished from its congeners by a suite of characters that include adult size, body colouration and markings, skin texture, snout shape, foot webbing and digit tip morphology, in addition to significant morphometric distinctness observed in principal component analysis. Phylogenetically, the new species is recovered as a well-supported sister taxon of A. monticola, based on Bayesian and maximum likelihood inferences as well as Bayesian Poisson Tree Processes (bPTP) species delimitation analysis using 2001 bp of mitochondrial DNA (16S rRNA, cytochrome c oxidase subunit I and NADH dehydrogenase 2). It differs from other known members of the group by considerable genetic distances in the studied loci: ≥ 4.3% at 16S, ≥ 6.1% at COI and ≥ 7.0% at ND2. The study also shows for the first time the phylogenetic affinities of another Indian member of the A. monticola group, A. kohimaensis. Altogether, the new insights resolve the long-existing enigma surrounding A. monticola and facilitate a better understanding of systematic relationships within the large A. monticola species group. Our findings also emphasise the need for further dedicated studies on the cascade frogs of Northeast India.山湍蛙（Amolops monticola）是一种已被学界认知超150年的湍蛙（cascade frog）物种，其系统发育位置至今仍未明确。多年来，学界常基于形态学特征及推测的系统发育亲缘关系，在界定模糊的山湍蛙种组（A. monticola species group）中发表新的分类单元。本研究报道了采自印度喜马拉雅山区的山湍蛙新标本，并结合全新的分子与形态学证据厘清了其分类界定。此外，我们在印度东北部阿鲁纳恰尔邦（Arunachal Pradesh）开展的调查中，于涵盖阿迪丘陵（历史上称阿博尔丘陵）的西昂河流域，发现了山湍蛙种组的另一新物种。该新种被正式命名为Amolops adicola sp. nov.，与同属近缘物种相比，其形态差异体现在成虫体型、体色与斑纹、皮肤纹理、吻部形态、趾间蹼结构以及指（趾）端形态等一系列特征，同时主成分分析结果显示其在形态测量学上也存在显著分化。系统发育分析方面，基于线粒体DNA 2001 bp片段（包含16S rRNA、细胞色素c氧化酶亚基I及NADH脱氢酶2），通过贝叶斯推断、最大似然法以及贝叶斯泊松树过程（Bayesian Poisson Tree Processes, bPTP）物种界定分析，结果显示该新种与山湍蛙构成支持度极高的姐妹群分类单元。在所检测的基因位点中，该新种与该种组其他已知物种间存在显著的遗传距离：16S基因≥4.3%、COI基因≥6.1%、ND2基因≥7.0%。本研究还首次明确了山湍蛙种组另一印度分布物种A. kohimaensis的系统发育亲缘关系。综上，本研究的新发现解决了长期以来围绕山湍蛙的分类谜团，同时有助于学界更深入理解庞大的山湍蛙种组内的系统分类关系。本研究结果同时强调，亟需针对印度东北部的湍蛙类开展更具针对性的专项研究。