Wednesday, May 18, 2022

[Herpetology • 2022] Varanus citrinus Between A Rock and A Dry Place: Phylogenomics, Biogeography, and Systematics of Ridge-tailed Monitors (Varanidae: Varanus acanthurus complex)

 Varanus citrinus
Pavón-Vázquez, Esquerré, Fitch, Maryan, Doughty, Donnellan & Scott Keogh, 2022

photo by Jules E. Farquhar.

• We used single nucleotide polymorphisms mitochondrial sequences, and morphological data to infer the evolutionary history of ridge-tailed monitor lizards.
• We identified nine populations which we propose belong to four species.
• The geographic distribution of the populations and admixture patterns reflect the aridification of Australia and highlight the importance of rocky escarpments as mesic refugia.
• We identified and described a new species from a region that has been recognized as a historical refugium in northern Australia.

Genomic data are a powerful tool for the elucidation of evolutionary patterns at the population level and above. The combined analysis of genomic and morphological data can result in species delimitation hypotheses that reflect evolutionary history better than traditional taxonomy or any individual source of evidence. Here, we used thousands of single nucleotide polymorphisms, mitochondrial sequences, and comprehensive morphological data to characterize the evolutionary history of the ridge-tailed monitors in the Varanus acanthurus complex (V. acanthurus, V. baritji, and V. storri), a group of saxicolous lizards with a wide distribution in Australia, the driest vegetated continent. We found substantial genetic structure in the group and identify nine geographically clustered populations. Based on admixture patterns and species delimitation analyses we propose a taxonomic scheme that differs from current taxonomy. We consider V. acanthurus as monotypic, synonymize V. baritji with V. a. insulanicus (as a redefined V. insulanicus), elevate the subspecies of V. storri to full species (V. storri and V. ocreatus), and describe a new species from a previously identified center of endemism. The relationships among the species remain unresolved, likely as a result of fast speciation. Our study highlights the capability of large datasets to illuminate admixture patterns, biogeographic history, and species limits, even when phylogeny is not completely resolved. Furthermore, our results highlight the impact that the Cenozoic aridification of Australia had on saxicolous taxa and the role of mesic rocky escarpments as refugia. These habitats apparently allowed the persistence of lineages that became sources of colonization for arid environments.

Keywords: Aridification, Australia, morphometrics, phylogeography, refugia, species delimitation

Geographic distribution of species in the Varanus acanthurus complex. Only localities with sequenced or morphologically examined individuals are mapped. Lines indicate state/territory borders.
Photographs by Stephen M. Zozaya.

Varanus citrinus 
probably female from Cape Crawford, NT.
photo by Jules E. Farquhar.

Morphological variation in Varanus citrinus.
 A–B) Cape Crawford, Northern Territory (NT). C) McArthur River Station, NT. D) Cape Crawford, NT. E–F) McArthur, NT.
A–C are probably male, while D–F are probably female.
A, by Stephen M. Zozaya. B by Brendan Schembri. C by Nic Gambold. 
D by Jules E. Farquhar. E and F by Ross McGibbon.

 Varanus citrinus sp. n.

Etymology. The Latin specific epithet is treated as an adjective. Citrinus means “related to lemon trees”, and refers to the bright yellow throats of males of the new species. We propose the common names “Gulf Ridge-tailed Monitor” or “Gulf Ridge-tailed Goanna”.

Carlos J. Pavón-Vázquez, Damien Esquerré, Alison J. Fitch, Brad Maryan, Paul Doughty, Stephen C. Donnellan and J. Scott Keogh. 2022. Between A Rock and A Dry Place: Phylogenomics, Biogeography, and Systematics of Ridge-tailed Monitors (Squamata: Varanidae: Varanus acanthurus complex). Molecular Phylogenetics and Evolution. In Press, 107516. DOI: 10.1016/j.ympev.2022.107516