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

Abstract

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 (FST=0.0), and gene flow among cohorts was very high (Nm=∞). 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.

Developmental plasticity as a cohesive evolutionary process between sympatric alternate-year insect cohorts

“…Importance of cohort splitting from genetic and demographic standpoints Gene flow between cohorts is consistent with preliminary work on C. mercuriale (Watts et al, 2005) and some other insects (Schultheis et al, 2002(Schultheis et al, , 2008; however, we are able to demonstrate that cohort splitting is geographically widespread and not restricted to a few, perhaps anomalous, populations. Crucially, this demonstrates the wider adaptive consequences of developmental plasticity-that a significant proportion of individuals who alter their developmental rate subsequently reproduce.…”

“…For example, genetic differences between cohorts that may be indicative of reproductive isolation have been uncovered in some studies (Battisti et al, 2000;Santos et al, 2007), whereas the presence of genetic divergence at one location but not another suggests that there can be spatial component to the efficacy of temporal isolation (Coates et al, 2004). Conversely, weak or nonsignificant genetic differentiation among cohorts has been interpreted as evidence for gene flow between alternate cohorts (that is, developmental plasticity; Schultheis et al, 2002Schultheis et al, , 2008, but equally may arise because of insufficient time for genetic divergence (Marcon et al, 1999) or inadequate genetic marker resolution (Kankare et al, 2002). Beyond demonstrating a range of possible outcomes, it is hard to generalise about the efficacy of allochronic divergence as an isolating mechanism because, to our knowledge, no study has explicitly compared levels of genetic divergence between sympatric cohorts at more than just one or two study locations.…”

Female reproductive patterns in the millipede Polydesmus angustus (Diplopoda: Polydesmidae) and their significance for cohort-splitting

“…Mendelian and polygenic models would also require that biennial males reproduce late in the season, with a reduced level of intermating between annual and biennial millipedes. This is most unlikely for species in which all adults are reproductively active for weeks or months (Wise, 1987;Schultheis et al, 2002).…”

Reconciling theory and practise: The role of stream ecology

“…Molecular tools provide us with important insights into metapopulation processes in streams (e.g. Schultheis et al, 2002, Hughes et al, 2004, and may allow us to surmount these difficulties. Stream food web ecology has provided a framework allowing us to integrate population ecology within an energy flow context, in a similar way as has been attempted in marine systems (Walters et al, 1999).…”