Complex selection on life-history traits and the maintenance of variation in exaggerated rostrum length in acorn weevils

Abstract

Trophic interactions can trigger the development of exaggerated specialized characters and promote morphological diversification. For example, acorn weevils (genus Curculio) present strikingly long rostrums, which are used by females to perforate oviposition holes through the seed coat. Species exhibiting longer rostrums are known to exploit larger acorns, and therefore rostrum length is thought to be subject to selection to match the preferred acorn type. However, rostrum length is strongly correlated with body size, and morphological divergence could result from either selection on rostrum length for optimal food exploitation or from other pressures acting on body size. We collected infested acorns at oak forests where the large Curculio elephas and the small-bodied Curculio glandium co-occur. There were no interspecific differences in adult female body size to rostrum length allometric relationships, and rostrum length is equally correlated with body size in either species. MtDNA-based species identification showed that C. glandium larvae were present within acorns of all sizes, whereas C. elephas larvae were restricted to acorns above a minimum size, irrespective of oak species. Hence, exploitation of large acorns can hardly have triggered rostrum enlargement, as the small sized C. glandium adults (with short rostrums) could perforate and oviposit in both small and large acorns. Rather, increased rostrum length is probably a by-product of the larger body sizes of individuals emerging from bigger acorns, which allow increased larval size and enhance larval survival likelihood. Summarizing, when exaggerated feeding traits co-vary with other body features, interspecific morphological variability may result from contrasting selective pressures acting on these correlated characters.

Zero‐sum landscape effects on acorn predation associated with shifts in granivore insect community in new holm oak (Quercus ilex) forests

“…For this reason, we made the identification by mitochondrial DNA sequencing and comparison with reference sequences from previously identified individuals (see Bonal, Espelta, & Vogler, 2011). and C. fagiglandana larvae can be easily distinguished according to morphology.…”

Tropical insect diversity: evidence of greater host specialization in seed‐feeding weevils

“…These results are remarkable considering that the morphological traits were not retrieved from the bibliography (e.g., Hughes and Vogler 2004b) but directly measured and that only larvae grown ad libitum were included, thus minimizing the potential effect of intra-specific phenotypic plasticity. Assuming that seed size is an important driver of morphological variation in weevils and that larval size is a good a proxy of fitness, premises that are well supported by current evidence (Desouhant et al 2000, Hughes and Vogler 2004b, Bonal et al 2011, the existence of latitudinal differences in the relationship between seed and body size points to differences in selection pressures related to biotic interactions. That is, selecting for host specialization might result in a tighter acorn-body size matching to increase the efficiency of resource exploitation Vogler 2004b, Bonal et al 2011).…”

“…This is because larval performance should not differ among different oak hosts in the more generalist weevil species. To test this prediction, we assessed the relationship between the width of larval head (i.e., a proxy of larval body size and adult fitness, Bonal et al 2011) and the volume of the acorns infested by each weevil species. After rearing the larvae, the width and length of all acorns were measured with a digital caliper and their volume derived assuming a geometric equivalence with a prolate spheroid.…”

The ecology of predispersal insect herbivory on tree reproductive structures in natural forest ecosystems

“…Long‐term association with a particular host may result in the loss of genetic variation for the ability to use alternative hosts, which may irreversibly constrain specialists to a restricted range of host plant species (Auger‐Rozenberg et al ., ). For instance in Europe, the chestnut weevil ( C. elephas ) is able to feed and develop on both Quercus and Castanea host genera, while the acorn weevil ( C. glandium ) does not belong to the same genetic clade as C. elephas and exploits only Quercus host species (Hughes & Vogler, ; Bonal et al ., ).…”