Genetic evidence for cohort splitting in the merovoltine stoneflyPteronarcys californica(Newport) in Blacksmith Fork, Utah

Abstract

The life cycles of many stream insects results in the simultaneous existence of multiple cohorts within populations. If larval development is fixed in these long-lived species, successive cohorts may be reproductively isolated and genetically distinct. We examined levels of genetic differentiation in a 333 bp region of the mitochondrial cytochrome-b gene among cohorts of the stonefly Pteronarcys californica (Newport) (Plecoptera, Pteronarcyidae) at two sites along Blacksmith Fork, Utah. Body measurements were collected from 224 specimens. The frequency distribution of (head width) X (head þ thorax length) suggests four (Low site) and five year (High site) life cycles comprising an *one year embryonic diapause followed by either three or four years of larval development. Seventy-seven individuals were sequenced, from which we detected five unique mtDNA haplotypes. The overall AMOVA suggested moderate genetic differentiation among cohorts (F ST ¼ 0.11, p ¼ 0.01; F ST ¼ 0.11, p ¼ 0.007). However, pairwise analyses indicated that a single cohort drove the overall F ST value. Thus, we conclude that there is considerable gene flow among most cohorts of P. californica in Blacksmith Fork.

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.…”

Estimating densities of larval Salmonflies (Pteronarcys californica) through multiple pass removal of post-emergent exuvia in Colorado rivers

“…Separating cohorts and year classes of merovoltine species has proven difficult because of varying growth rates [16] and contraction or expansion of abdomens in preserved insect specimens can further confound this task. Our colleagues [39] used head capsule width and combined head and thorax lengths to produce “body size” or “body area” to assign cohorts within a stream.…”

Estimating densities of larval Salmonflies (Pteronarcys californica) through multiple pass removal of post-emergent exuvia in Colorado rivers

“…Length frequency histograms for male and female larvae were separately constructed based on TL to separate annual year classes. Body size measurements can be used to separate cohorts of this merovoltine species due to the long lifespan, fast growth, and large size [16,38,39]. We based our approach on separating annual cohorts on Townsend and Pritchard [16] except we constructed length frequency histograms from total length measurements rather than head-capsule width and wingpad length.…”