Genetic evidence for 'leaky' cohorts in the semivoltine stonefly Peltoperla tarteri (Plecoptera : Peltoperlidae)

1. Genetic techniques are being used increasingly to address questions about dispersal and gene flow of freshwater invertebrates. However, population genetic structure can be affected by factors other than dispersal. 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, successive cohorts may be reproductively isolated and, as a result, genetically distinct. In such cases, significant levels of genetic differentiation between cohorts could confound estimates of dispersal based on population genetic structure. 2. Peltoperla tarteri is a stonefly that can be abundant in Appalachian headwater streams. Although P. tarteri is univoltine at the type locality (Big Paint Hollow, WV, U.S.A.), the study populations in southwestern Virginia, U.S.A., were semivoltine. This semivoltine life cycle results in the simultaneous existence of multiple cohorts with the potential for significant genetic differentiation among them. 3. Levels of genetic differentiation among P. tarteri cohorts were analysed with mitochondrial DNA (mtDNA) sequence data from the non-coding origin of replication or 'control' region from 93 individuals from two successive cohorts (collected in 1998 and 1999). 4. Analysis of molecular variance (AMOVA) indicated no genetic differentiation among cohorts (F-ST = 0.0), and gene flow among cohorts was very high (Nm = infinity). 5. High levels of gene flow among cohorts suggest that larval development of P. tarteri is not fixed. Gene flow among cohorts most likely occurs as a result of a cohort split in which some individuals complete development in one or three years instead of two.