The snails the dinosaurs saw: are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era?

摘要

Pleurocerid snails are a common element of the benthos in rivers and streams throughout the Appalachian highlands from Virginia to Georgia on both sides of the continental divide. Yet their dispersal capabilities are so limited today that significant gene frequency differences have been demonstrated over a scale of meters. We obtained mitochondrial cytochrome oxidase C Subunit I (COI) sequence data from 3 individual snails sampled from each of 13 populations of pleurocerids representing 3 species-Leptoxis carinata (4 populations), Goniobasis ("Elimia") catenaria (4 populations), and Goniobasis ("Elimia"') Proxima (5 populations). To these data we added previously published COI sequences from 3 other G. proxima populations. Levels of intrapopulation sequence divergence were strikingly high, ranging up to 21.91% within populations and 22.6% between populations within species. A neighbor-joining analysis revealed 3 loose clusters corresponding to the species, but featured numerous extreme outliers. Wilcoxon rank-sum tests returned no evidence that the Continental Divide (as it presently stands) makes any contribution to mean levels of interpopulation sequence divergence, nor that simple geographic distance (regardless of modern drainage) has an effect. We suggest that populations of pleurocerid snails inhabiting the Older Appalachians might date to such an age that all geographic signal in the divergence of our test gene has been lost. We review additional lines of evidence from other genetic studies and from ecology, life history, continental biogeography, and the fossil record that suggest that our pleurocerid populations might be living fossils from the Paleozoic uplift of the Appalachians.