Exploring phenotypic divergence in Prosymna greigerti (Squamata: Prosymnidae)

The African snake Prosymna greigerti occurs across the arid transitional zone between the Sahara and Sahel, where its range broadly overlaps with P. collaris . The taxonomic status of P. collaris , distinguished from P. greigerti solely by the presence of a nuchal collar, has been unstable, alternating between subspecies of P. greigerti and full species status. In 2023, it was re-elevated to full species rank based on phenotype alone, in the absence of genetic data. To assess its genetic distinctiveness, we analysed two mitochondrial gene fragments (16S and ND2) across the distribution of both taxa. Our results reveal a strong spatial structure and a clear isolation-by-distance pattern in mitochondrial variation, but no phylogenetic clustering by phenotype. Collared individuals are nested within multiple clades otherwise composed of non-collared P. greigerti , indicating that the nuchal collar trait does not correspond to a distinct mitochondrial lineage. Therefore mitochondrial evidence does not provide positive support for the recent elevation of P. collaris to species rank. In the absence of nuclear data, this pattern is consistent with a single species showing geographically structured mitochondrial variation and phenotypic polymorphism, but could also reflect mitochondrial introgression or sex-biased dispersal between two lineages. Pending genomic data, we advocate a conservative taxonomic interpretation that recognizes the diagnostic value of morphology while minimizing taxonomic instability – the subspecies rank. Additionally, our results confirm previous reports of mitochondrial discordance involving P. meleagris , consistent with possible mitochondrial introgression, which may have nomenclatural implications for P. greigerti .