Gene Flow and Dispersal Among Populations of the Stonefly Peltoperla tarteri (Plecoptera: Peltoperlidae) in the Southern Appalachians
Despite a number of recent studies focused on the issue, patterns of stream insect dispersal in temperate streams of North America remain poorly understood. Movement of benthic invertebrates is notoriously difficult to measure directly using traditional means; however, genetic markers provide an ideal method for estimating stream insect dispersal. In this study, the control region of mitochondrial DNA was used to study gene flow and dispersal among populations of the stonefly Peltoperla tarteri in the Southern Appalachians. The control region of P. tarteri is approximately 1270 base pairs (bp) in length, 81% AT-rich, and contains variable numbers of a 74 bp tandem repeat containing the sequence motif "5'-GGGGGC-3".
Many stream insects have long life cycles that result in the simultaneous existence of multiple cohorts throughout the larval development period. If larval development is fixed, genetic isolation among cohorts may confound genetic estimates of dispersal. Although larval head width data indicated that P. tarteri is semivoltine in southwestern Virginia, low levels of genetic differentiation among cohorts suggested that larval development of P. tarteri is not fixed and that some individuals complete development in one or three years rather than two. This 'cohort splitting' would result in individuals from distinct cohorts maturing at the same time and mating with one another. Such developmental plasticity may allow some individuals to avoid adverse environmental conditions.
The extent and likely mechanism of dispersal was determined by comparing levels of population genetic differentiation within drainages to that among drainages. While there was no consistent relationship between genetic and geographic distance, genetic differentiation was generally high within and among drainages. Nested clade analysis indicates that historical fragmentation and range expansion coupled with contemporary gene flow explain the present day pattern of genetic variation in P. tarteri. In order for these historical patterns to have such a strong influence on present day genetic structure, both larval and adult dispersal must be restricted. However, the genetic data suggest that larval dispersal is more frequent than adult dispersal.