Reticulation between incipient lineages is a common feature of diversification. We examine these phenomena in the Pisgah clade of Desmognathus salamanders from the southern Appalachian Mountains of the eastern United States. The group contains four to seven species exhibiting two discrete phenotypes, aquatic “shovel-nosed” and semi-aquatic “black-bellied” forms. These ecomorphologies are ancient and have apparently been transmitted repeatedly between lineages through introgression. Geographically proximate populations of both phenotypes exhibit admixture, and at least two black-bellied lineages have been produced via reticulations between shovel-nosed parentals, suggesting potential hybrid speciation dynamics. However, computational constraints currently limit our ability to reconstruct network radiations from gene-tree data. Available methods are limited to level-1 networks wherein reticulations do not share edges, and higher-level networks may be non-identifiable in many cases. We present a heuristic approach to recover information from higher-level networks across a range of potentially identifiable empirical scenarios, supported by theory and simulation. When extrinsic information indicates the location and direction of reticulations, our method can successfully estimate a reduced possible set of non-level-1 networks. Phylogenomic data support a single backbone topology with up to five overlapping hybrid edges in the Pisgah clade. These results suggest an unusual mechanism of ecomorphological hybrid speciation, wherein a binary threshold trait causes some hybrid populations to shift between microhabitat niches, promoting ecological divergence between sympatric hybrids and parentals. This contrasts with other well-known systems in which hybrids exhibit intermediate, novel, or transgressive phenotypes. The genetic basis of these phenotypes is unclear and further data are needed to clarify the evolutionary basis of morphological changes with ecological consequences.

中文翻译：

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通过估计高级系统发育网络发现多型蝾螈（Plethodontidae：Desmognathus）分支中的复杂杂交


早期谱系之间的网状结构是多元化的一个共同特征。我们研究了来自美国东部阿巴拉契亚山脉南部的 Desmognathus salamanders 的 Pisgah 分支中的这些现象。该组包含四到七个物种，表现出两种离散的表型，水生的“铲鼻”和半水生的“黑腹”形式。这些生态形态是古老的，显然是通过渗入在谱系之间反复传递的。两种表型的地理邻近种群都表现出混合，并且至少通过铲鼻亲本之间的网状结构产生了两个黑腹谱系，这表明潜在的杂交物种形成动力学。然而，计算限制目前限制了我们从基因树数据中重建网络辐射的能力。可用方法仅限于 1 级网络，其中网状不共享边缘，并且在许多情况下，更高级别的网络可能无法识别。我们提出了一种启发式方法，可以在理论和模拟的支持下，在一系列可能可识别的经验场景中从更高级别的网络中恢复信息。当外在信息表明网状网络的位置和方向时，我们的方法可以成功地估计一组可能的非 1 级网络。系统发育数据支持 Pisgah 分支中最多具有 5 个重叠混合边缘的单个主干拓扑。这些结果表明了生态形态杂交物种形成的一种不寻常机制，其中二元阈值性状导致一些杂交种群在微生境生态位之间转移，从而促进同域杂交种和亲本之间的生态分化。 这与其他众所周知的系统形成鲜明对比，在这些系统中，杂交种表现出中间、新或越界表型。这些表型的遗传基础尚不清楚，需要进一步的数据来阐明具有生态后果的形态变化的进化基础。