The effect of seed traits on geographic variation in body size and sexual size dimorphism of the seed‐feeding beetle Acanthoscelides macrophthalmus

Abstract

Explaining large‐scale patterns of variation in body size has been considered a central question in ecology and evolutionary biology because several life‐history traits are directly linked to body size. For ectothermic organisms, little is known about what processes influence geographic variation in body size. Changes in body size and sexual size dimorphism (SSD) have been associated with environmental variables, particularly for Bruchinae insects, which feed exclusively on seeds during the larval stage. However, the effect of important seed traits on body size variation has rarely been investigated, and whether SSD varies substantially among populations within bruchine species is poorly known. Using the seed‐feeding beetle Acanthoscelides macrophthalmus infesting its host plant Leucaena leucocephala, we investigated whether specific seed traits (hardness, size, water content, carbon/nitrogen ratio, and phenolic content) were determinant in generating geographic variation in body size and SSD of A. macrophthalmus. We also examined the relationships between body size and SSD with latitude and altitude. The body size of both sexes combined was not related to latitude, altitude, and any of the physical and chemical seed traits. However, the female body size tended to vary more in size than the males, generating significant variation in SSD in relation to latitude and altitude. The females were the larger sex at higher latitudes and at lower altitudes, precisely where seed water content was greater. Therefore, our results suggest that water content was the most important seed trait, most severely affecting the females, promoting geographic variation in SSD of A. macrophthalmus.

The interaction effect between intraspecific competition and seed quality on the life‐history traits of the seed‐feeding beetle Acanthoscelides macrophthalmus

“…EL was estimated by calculating the mean value from the longest portion of both elytra. EW was estimated by the mean value obtained from the widest portion of both elytra, and PW corresponded to the widest pronotum portion (Colgoni & Vamosi, 2006;Stillwell et al, 2007b;Haga & Rossi, 2016).…”

“…Although most SSD studies considered an interspecific approach (e.g., Colwell, ; Monnet & Cherry, ), other studies have shown that SSD may vary substantially within a species (Teder & Tammaru, ; Fairbairn, ) because, for example, females and males respond differently to resource quality (Stockhoff, ; Teder & Tammaru, ). For bruchines, some findings suggest that females exhibit greater phenotypic plasticity relative to body size than males as females are more vulnerable when growing on poor hosts and when resources are scarce (Moreau et al, ; Stillwell et al, ; Haga & Rossi, ). Therefore, it is relevant to determine how bruchines of both sexes respond to certain ecological processes, such as competition and variation in resource quality, affecting the degree of SSD (Fairbairn, ).…”

“…Leucaena leucocephala produces about 20 seeds per fruit (Raghu et al, ), with fruit phenology characterized by 2–4 fructification cycles per year (Smith, ; Raghu et al, ; Tuda et al, ). Although L. leucocephala produces many fruits, the number varies considerably among plant populations, as does the size of the seeds (Haga & Rossi, ). Leucaena leucocephala individuals usually inhabit disturbed areas and this plant is widely distributed around the globe, occurring in about 120 countries, where it is considered an important invasive plant (Lowe et al, ).…”

“…In addition, the evolution of body size in bruchines can be mediated by the type of competition (scramble or contest) that takes place inside the seeds, which is directly related to the size of the seeds of the host plants (Messina & Karren, 2003). Modifications in specific life-history traits in bruchine beetles, such as body size (Menezes et al, 2010) and sexual size dimorphism (SSD) (Haga & Rossi, 2016), have also been related to changes in resource quality other than seed size. It is also interesting that host shifts may cause females to exhibit plasticity in egg size (Fox & Savalli, 2000), which can occur due to variation in seed quality.…”

“…The current understanding of how variation in seed traits among plant populations, such as hardness, water content, and chemical compounds (i.e., resource quality), affects the life history of bruchines is limited (Menezes et al, 2010;Haga & Rossi, 2016). Our knowledge of how intraspecific competition and resource quality interact and change life-history traits is even scarcer, although some studies have shown, using fruit flies, flour beetles, and bruchines as model systems, how inter-and intraspecific competition affect the fitness of organisms through changes in life-history traits (Kishi, 2015).…”