Scott, Glenn & Rissler, 2018
• We use 3RADseq to resolve Musk Turtle (Sternotherus) systematics.
• We discover cryptic species-level diversity in Sternotherus.
• Multiple species-tree methods infer conflicting relationships for Sternotherus species.
• Robust demographic modeling provides resolution to Sternotherus phylogeny.
• We provide a revised taxonomy for Sternotherus.
Accurate and consistent delimitation of species and their relationships provides a necessary framework for comparative studies, understanding evolutionary relationships, and informing conservation management. Despite the ever-increasing availability of genomic data, evolutionary dynamics can still render some relationships exceedingly difficult to resolve, including underlying speciation events that are rapid, recent, or confounded by post-speciation introgression. Here we present an empirical study of musk turtles (Sternotherus), which illustrates approaches to resolve difficult nodes in the Tree of Life that robust species-tree methods fail to resolve. We sequence 4430 RAD-loci from 205 individuals. Independent coalescent-based analyses, corroborated with morphology and geography, strongly support the recognition of cryptic species within Sternotherus, but with conflicting or weak support for some intraspecific relationships. To resolve species-tree conflict, we use a likelihood-based approach to test support for alternative demographic models behind alternative speciation scenarios and argue that demographic model testing has an important role for resolving systematic relationships in recent, rapid radiations. Species-tree and demographic modeling strongly support the elevation of two nominal subspecies in Sternotherus to species and the recognition of a previously cryptic species (Sternotherus intermedius sp. nov.) described within. The evolutionary and taxonomic history of Sternotherus is discussed in the context of these new species and novel and well-supported systematic hypotheses.
Keywords: Sternotherus, Species delimitation, Demographic models, Cryptic species, Species tree conflict
A revised and consistent taxonomy for Sternotherus
We find strong support for the recognition of S. odoratus (Latreille in Sonnini and Latreille, 1802:122) and S. carinatus (Gray, 1855:211) as they have been previously defined; therefore, the taxonomic status of these species will not be discussed further.
A.1. The Sternotherus minor species group
A.2. Sternotherus depressusTinkle and Webb, 1955
A.3. Sternotherus intermedius Scott et al., new species
Etymology. Medieval Latin intermediātus, past participle of intermediāre. This species is named for its long recognition as being a hypothetical “intermediate” form between S. peltifer and S. minor (e.g. Ernst et al., 1988), as turtles now attributed to S. intermedius have historically been recognized as hybrids between the two aforementioned species due to having a superficially intermediate morphology. The name is a noun in apposition.
Distribution – S. intermedius is endemic to only the Choctawhatchee and Escambia River basins and associated waters from the Apalachicola Bay in southern Alabama and the Florida panhandle (including the Choctawhatchee, Conecah, Yellow, Pea, Blackwater, and Escambia rivers drainages). This distribution is bordered to the north and west by the greater Mobile River Basin (including the Alabama, Coosa, and Tallapoosa River drainages), where it is replaced by S. pelitifer, and to the east by the greater Apalachicola River Basin (including the Chattahoochee and Flint River drainages), where it is replaced by S. minor.
A.4. Sternotheris peltifer Smith and Glass, 1947
A.5. Sternotherus minor Agassiz, 1857
Peter A.Scott, Travis C.Glenn and Leslie J.Rissler. 2018. Resolving Taxonomic Turbulence and Uncovering Cryptic Diversity in the Musk Turtles (Sternotherus) Using Robust Demographic Modeling. Molecular Phylogenetics and Evolution. 120; 1-15. DOI: 10.1016/j.ympev.2017.11.008
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