Drainage History, Evolution, and Conservation of Tonguetied Minnow (Exoglossum laurae), a Rare and Imperiled Teays River Endemic
Legacies of ancient riverine systems are often manifest in patterns of genetic diversity within aquatic species. The ancient Teays River, a principal drainage of the eastern United States, engaged in several ephemeral connections with neighboring palaeodrainages prior to and during the Pleistocene, when cyclical glacial advance and retreat reconfigured the region's fluvial systems. This study assayed DNA-sequence diversity at one mitochondrial (mtDNA) and three single-copy nuclear DNA (scnDNA) loci from the Tonguetied Minnow (Exoglossum laurae), a species distributed as four disjunct populations, one each within the Upper Great Miami, Upper Allegheny, Upper Genesee, and New rivers. Mitochondrial DNA variation revealed that the New River harbors the highest diversity (h = 0.73) and that the Tonguetied Minnow is composed of two ancient lineages, a Teays River lineage and a Pittsburgh River lineage. Analyses of the scnDNA loci revealed sharing of alleles among populations of E. laurae and between the Tonguetied Minnow and its only congener, the Cutlip Minnow (E. maxillingua), sampled from the Roanoke and Potomac rivers. The probability of interspecific hybridization in the New and Upper Genesee rivers was estimated as 0.16 and 0.34, respectively, but it is likely that some degree of incomplete lineage sorting contributed to these estimates. Probabilities of interspecific hybridization for Cutlip Minnow were 0.62 and 0.65, for the Roanoke and Potomac rivers, respectively, and might reflect ancient hybridization resulting from stream capture events involving these drainages by the Teays River. Management strategies should focus on maintaining the security of the Pittsburgh River lineage in the Upper Great Miami and Upper Allegheny River drainages. Finally, insights into the Tonguetied Minnow's rather convoluted taxonomic history are few, but genetic variation is inconsistent with subspecies status for Tonguetied Minnow in the Upper Great Miami River drainage.
(A) Principal pre-Pleistocene drainages of eastern North America, including the ancient Teays River and Pittsburgh River (georeferenced and digitized in ArcGIS v. 10 from the map of Hocutt et al., 1978). Arrows indicate routes of transfer of Tonguetied Minnow among palaeodrainages. (B) Tonguetied Minnow drainages and extent of native ranges of Tonguetied Minnow and Cutlip Minnow. Note that the native range for Tonguetied Minnow is based on all historical and contemporary collection records and thus likely overestimates the current range of the species. State boundaries represented by dashed light gray lines and included in both panels for reference.
Haplotype and allele networks for Tonguetied Minnow: (A) nd2, (B) β-act, (C) mlc3, (D) myh6. Hashmarks indicate the number of mutations.
Bars represent the probability of interspecific hybridization between Tonguetied Minnow and Cutlip Minnow (PT×C; left axis). Open bars indicate drainages with only Tonguetied Minnow, light gray bars indicate drainages with both Tonguetied and Cutlip minnows, and dark gray bars indicate drainages with only Cutlip Minnow. The black line indicates the proportion of Cutlip Minnow nd2 haplotypes discovered within each drainage (right axis).
Photographic image of Tonguetied Minnow jaws from individuals collected from the Upper Genesee River: (A) OSUM 113514, (B) OSUM 113520, (C) OSUM 113523. Two Cutlip Minnow (E. maxillingua) are included for comparison in each panel and were obtained from a Potomac River sample collected in 1976 (OSUM 34899).
Contributor Notes
Associate Editor: M. T. Craig.