Escaping a nomenclatural limbo caused by hybridization (and the challenges to measure it) in midwife toads (Alytidae: Alytes Wagler, 1829)

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Abstract Advances in molecular techniques have improved our ability to quantify genetic introgression, but they also blur distinctions between hybrids and parental taxa in conservation and taxonomy. Here, we highlight challenges of identifying and defining hybrids within zoological nomenclature. Using high-throughput sequencing data, we show that the holotype of a recently described midwife toad subspecies, Alytes almogavarii inigoi, originates from a wide hybrid zone with its sister taxon, Alytes almogavarii almogavarii. Although the International Code of Zoological Nomenclature permits names based on admixed hybrid individuals (except F1 hybrids), this case raises questions about the applicability of the name to the intended population. To address this, we introduce a quantitative method for distinguishing admixed from parental specimens, using thresholds of foreign ancestry corresponding to known levels of backcrossing. Our findings reveal that the holotype carries 15% foreign ancestry, which is more than a second-generation backcross and thus too much to represent a distinct, non-admixed population. We therefore provide a replacement name by redescribing the Central Pyrenean lineage of Alytes almogavarii from a genetically pure population. This case illustrates the ambiguity surrounding admixed type specimens under current nomenclatural rules and emphasizes the growing need for clearer guidelines as taxonomy enters the genomic era.

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