Adaptation to different host plant ages facilitates insect divergence without a host shift

Abstract

Host shifts and subsequent adaption to novel host plants are important drivers of speciation among phytophagous insects. However, there is considerably less evidence for host plant-mediated speciation in the absence of a host shift. Here, we investigated divergence of two sympatric sister elm leaf beetles, Pyrrhalta maculicollis and P. aenescens, which feed on different age classes of the elm Ulmus pumila L. (seedling versus adult trees). Using a field survey coupled with preference and performance trials, we show that these beetle species are highly divergent in both feeding and oviposition preference and specialize on either seedling or adult stages of their host plant. An experiment using artificial leaf discs painted with leaf surface wax extracts showed that host plant chemistry is a critical element that shapes preference. Specialization appears to be driven by adaptive divergence as there was also evidence of divergent selection; beetles had significantly higher survival and fecundity when reared on their natal host plant age class. Together, the results identify the first probable example of divergence induced by host plant age, thus extending how phytophagous insects might diversify in the absence of host shifts.

Divergence before the host shift? Prezygotic reproductive isolation among three varieties of a specialist fly on a single host plant

“…Sunflowers continue to grow until they flower in August, and S. longipennis oviposit into the upper nodes of the plant (A. Hippee, unpublished), so the allochrony between varieties may reflect partitioning of the stem resource, with the three varieties ovipositing and feeding at different plant growth stages. Such ‘host age‐associated divergence’ in the absence of host shifting has recently been described for two sister species of elm leaf beetle, although beetle species specialized on either seedlings or mature trees and did not overlap on individual plants (Zhang et al ., ). Future work should focus on making additional efforts towards collecting large numbers of S. longipennis var.…”

Intraspecific phytochemical variation shapes community and population structure for specialist caterpillars

“…Intraspecific phytochemical variation is an underappreciated source of geographic variation in species interactions and their evolutionary outcomes (Bolnick et al, 2011;Zhang et al, 2015). At what scales might this variation shape community structure and generate divergent selection and population differentiation?…”

“…The plants located at the highest elevation (> 2400 m) contained higher concentrations of prenylated benzoic acid (Figs S3-S5; Tables S1, S2), which could be a source of divergent selection. Local adaptation of E. encina to high-elevation hosts with high prenylated benzoic acid concentrations could reduce gene flow by causing selection against migrants and low hybrid fitness (Zhang et al, 2015), which could explain the observed genetic differentiation among low-and high-elevation caterpillars. One individual caterpillar was collected at a high-elevation site that was not genetically grouped with the other highest elevation caterpillars (Fig.…”

Phylogenetic test of speciation by host shift in leaf cone moths (Caloptilia) feeding on maples (Acer)

“…Ecological shift within a host plant is also a significant process (Condon and Steck 1997;Cook et al 2002;Joy and Crespi 2007;Althoff 2014;Mishima et al 2014). For example, Zhang et al (2015) demonstrated divergence induced by host plant ages in sympatric sister beetles (Pyrrhalta maculicollis and Pyrrhalta aenescens) feeding on elm. There is clearly a need to sample from a broader geographic area and to collect additional information on microniche divergence among leaf cone moths to fully understand the process underlying their diversification.…”