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

Eloísa B Haga,Marcelo N Rossi

doi:10.1002/ece3.2364

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.

Highlights

Body size is widely considered one of the most important traits of organisms as it directly affects their physiology and their fitness (Brown et al 2004; Fairbairn et al 2007)
Ecology and Evolution published by John Wiley & Sons Ltd
Differences between altitudes were only observed for water content (Mann–Whitney U-test: U = 30; P = 0.018; mean values are presented in Fig. 3) and sexual size dimorphism (SSD) (t = À2.143; df = 22; P = 0.043), with greater values observed at lower altitudes in both cases (Fig. 3)

Summary

Introduction

Body size is widely considered one of the most important traits of organisms as it directly affects their physiology (for example, their metabolic rate) and their fitness (Brown et al 2004; Fairbairn et al 2007). Environmental variables such as temperature, moisture, and the rearing host plant can affect the magnitude of SSD of some bruchines due to local long-term adaptation, and because males and females exhibit differences in body size plasticity (Stillwell and Fox 2007, 2009; Stillwell et al 2007b) This information is very important, because it shows that SSD varies substantially among populations within bruchine species (but considering more than one host plant species); it confirms that these beetles provide adequate model systems for carrying out experimental studies and for investigating geographic variation in body size and SSD.

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Discussion