Geography is more important than host plant use for the population genetic structure of a generalist insect herbivore

Abstract

Population divergence can occur due to mechanisms associated with geographic isolation and/or due to selection associated with different ecological niches. Much of the evidence for selection‐driven speciation has come from studies of specialist insect herbivores that use different host plant species; however, the influence of host plant use on population divergence of generalist herbivores remains poorly understood. We tested how diet breadth, host plant species and geographic distance influence population divergence of the fall webworm (Hyphantria cunea; FW). FW is a broadly distributed, extreme generalist herbivore consisting of two morphotypes that have been argued to represent two different species: black‐headed and red‐headed. We characterized the differentiation of FW populations at two geographic scales. We first analysed the influence of host plant and geographic distance on genetic divergence across a broad continental scale for both colour types. We further analysed the influence of host plant, diet breadth and geographic distance on divergence at a finer geographic scale focusing on red‐headed FW in Colorado. We found clear genetic and morphological distinction between red‐ and black‐headed FW, and Colorado FW formed a genetic cluster distinct from other locations. Although both geographic distance and host plant use were correlated with genetic distance, geographic distance accounted for up to 3× more variation in genetic distance than did host plant use. As a rare study investigating the genetic structure of a widespread generalist herbivore over a broad geographic range (up to 3,000 km), our study supports a strong role for geographic isolation in divergence in this system.

Geographic variation in performance of a widespread generalist insect herbivore

“…The adults emerge from early May to late July, females lay eggs in a single large clutch on a host plant leaf and sibling larvae feed gregariously, forming a web that encompasses the host plant branches, usually including hundreds of sibling larvae. There are two types of FW that are morphologically, genetically and behaviourally distinct and probably comprise different species (Oliver, ; Vidal et al ., ), the black‐ and the red‐headed larvae; here, we focus on the red‐headed type.…”

“…Variable performance together with geographic isolation could influence the genetic divergence of generalist herbivores (e.g. Vidal et al ., ). Knowing how much the performance of a generalist herbivore varies across its geographic range will aid in understanding their diet breadth evolution and ecology.…”

“…On a relatively larger scale, a recent study showed that an extreme generalist ( sensu Normark & Johnson, ), fall webworm ( Hyphantria cunea ), varied in performance on the same plant species across a 700‐km range (within the state of Colorado; Vidal & Murphy, ). This last study indicates that the performance of generalist herbivores can vary even when there is considerable gene flow among populations (Vidal et al ., ). Experiments on an even larger spatial scale (e.g.…”

“…This could be because the plant community composition and traits are more distinct and/or because gene flow among FW populations decreases with geographic distance (Wright, ; e.g. Vidal et al ., ). Besides testing for the possible effects of geographic distance of plant population, we also tested for evidence of local adaptation of FW to the array of host plants used in each location.…”

Genetic Divergence of Two Sitobion avenae Biotypes on Barley and Wheat in China

“…Thus, geographic factors appeared to be more important than host plant use in the divergence of the two S. avenae biotypes. Some studies have proposed several mechanisms that may promote the maintenance of genetic structure of insect geographic populations and evolution of insect biotypes, such as differential composition of secondary endosymbionts, phenotypic plasticity, habitat isolation, and habitat persistence [2,21,24,30,31,73]. Future studies are needed to determine if these mechanisms can explain the development of biotypes in S. avenae.…”