An experimentally induced host shift in a seed beetle

Abstract

Many insects use a fairly well‐defined set of host plants, but are occasionally observed on an atypical host. The seed beetle Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae: Bruchinae) has rarely been reported to attack lentil, which is distantly related to its usual legume hosts. An initial assay of an Asian beetle population revealed that none of the 100 larvae entering lentil seeds survived to adult emergence. Nevertheless, three attempts at mass selection, in which more than 2 000 adults were added to lentil seeds, eventually yielded self‐sustaining populations. In each case, a severe bottleneck was followed by a rapid increase in survival, which exceeded 65% after only five generations and surpassed 85% in <20 generations. Subsequent large‐scale experiments indicated that survival in lentil is initially <2% and that most larvae die before they have completely entered a seed. The only potential trade‐off associated with rapid adaptation to lentil was a modest increase in the time required to develop in the ancestral host, mung bean. Reciprocal crosses between a lentil‐adapted line (F10) and a mung‐bean line produced offspring with intermediate survival, very long development times, and small size. Although the Asian population has been kept under uniform laboratory conditions for more than 200 generations, it appears to maintain rare alleles that permit the colonization of an extremely poor host. Such standing genetic variation can account for the sporadic appearance of unusual ‘biotypes’ among herbivorous insects.

Specificity of adaptation to a novel host plant by a seed beetle

“…These differences in realized fecundity are not likely due to variation in potential fecundity because females from the mung-bean line did not lay fewer eggs on either the highly acceptable, ancestral host (mung bean) or an inert, artificial host (glass balls). Because greater oviposition on novel hosts was observed in all three lentil lines, it is also not likely a consequence of genetic drift during the population bottlenecks that occurred when the lentil lines were established (Messina et al, 2009a). Instead, lentil-adapted females appear to have evolved a lower level of specificity in their oviposition decisions (Mercader & Scriber, 2007;.…”

“…The absence of cross-adaptation in larval performance also suggests that the mechanism permitting development in lentil is quite specific. Virtually all non-adapted larvae of C. maculatus die soon after they begin consuming the cotyledons in lentil seeds (Messina et al, 2009a), and it seems likely that adapted larvae possess a mechanism for detoxifying specific compounds in lentil seeds (e.g., via altered enzyme activity; Desroches et al, 1997). It would appear that other toxins (or other factors) cause high larval mortality in pea and fava bean.…”

“…We examined cross-adaptation with respect to larval performance by measuring three fitness components on a host: survival from egg hatch to adult emergence, eggto-adult development time, and adult mass at emergence (Messina et al, 2009a;Messina & Jones, 2011). To reduce any non-genetic, parental-host effects, subcultures of the L lines were again reverted to mung bean for a generation.…”

“…Lentil is a member of the legume tribe Fabeae and has only rarely been reported as a host for C. maculatus (Credland, 1987(Credland, , 1990. All lines underwent a severe bottleneck because survival to adult emergence was initially about 1%, and nearly all larvae died as first instars as soon as they began to feed on the cotyledons (Messina et al, 2009a). However, survival in each line rose to >90% after fewer than 30 generations.…”

Sexual conflict and the gender load: correlated evolution between population fitness and sexual dimorphism in seed beetles

“…Simoes et al 2007) and in C. maculatus in even less time (e.g. Messina et al 2009). Further, the black-eyed bean is the main natural host for this species and the rearing conditions used (including nonoverlapping generations and abiotic conditions) mimic natural conditions (Southgate 1979;Messina 1991).…”