A comparative approach to testing hypotheses for the evolution of sex‐biased dispersal in bean beetles

Abstract

Understanding the selective forces that shape dispersal strategies is a fundamental goal of evolutionary ecology and is increasingly important in changing, human‐altered environments. Sex‐biased dispersal (SBD) is common in dioecious taxa, and understanding variation in the direction and magnitude of SBD across taxa has been a persistent challenge. We took a comparative, laboratory‐based approach using 16 groups (species or strains) of bean beetles (genera Acanthoscelides, Callosobruchus, and Zabrotes, including 10 strains of one species) to test two predictions that emerge from dominant hypotheses for the evolution of SBD: (1) groups that suffer greater costs of inbreeding should exhibit greater SBD in favor of either sex (inbreeding avoidance hypothesis) and (2) groups with stronger local mate competition should exhibit greater male bias in dispersal (kin competition avoidance hypothesis). We used laboratory experiments to quantify SBD in crawling dispersal, the fitness effects of inbreeding, and the degree of polygyny (number of female mates per male), a proxy for local mate competition. While we found that both polygyny and male‐biased dispersal were common across bean beetle groups, consistent with the kin competition avoidance hypothesis, quantitative relationships between trait values did not support the predictions. Across groups, there was no significant association between SBD and effects of inbreeding nor SBD and degree of polygyny, using either raw values or phylogenetically independent contrasts. We discuss possible limitations of our experimental approach for detecting the predicted relationships, as well as reasons why single‐factor hypotheses may be too simplistic to explain the evolution of SBD.

Insects and incest: Sib‐mating tolerance in natural populations of a parasitoid wasp

“…When one sex disperses, the risk of sib‐mating is reduced and so selection on sib‐avoidance will not be strong. Male‐biased dispersal patterns have been reported in nonsocial insects by Downey et al () and might be explained in V. canescens by local resource competition. Males and females of V. canescens are known to be good fliers (Amat et al, ; Desouhant, Driessen, Amat, & Bernstein, ; Metzger, Fischbein, et al, ), meaning that dispersal should contribute strongly to decreasing the probability of sib encounters.…”

Genetic mixture of multiple source populations accelerates invasive range expansion

“…In prior work, we found that most of these populations exhibited very little inbreeding depression following full‐sib mating (Downey et al . ), further suggesting a history of inbreeding that may have been sufficient to purge deleterious alleles. The low genetic diversity expected for inbred laboratory lines may have increased the chances of finding support for a heterosis effect, as we did.…”

“…; Downey et al . ) and differ in phenotypic traits (Dowling, Abiega & Arnqvist ; Arnqvist & Tuda ) and dispersal behaviour (Downey et al . ).…”

“…Our experiment relied on ten source populations of C. maculatus that were obtained from localities distributed world-wide, primarily Africa and Asia but including populations from North and South America (Table 1). These populations exhibit significant genetic structure at neutral mitochondrial and nuclear loci (Tuda et al 2014;Downey et al 2015) and differ in phenotypic traits (Dowling, Abiega & Arnqvist 2007;Arnqvist & Tuda 2010) and dispersal behaviour (Downey et al 2015). All populations readily inter-breed (M.H.…”

Admixture is a driver rather than a passenger in experimental invasions

“…This is an intriguing result that warrants further exploration. Dispersal in the focal species has been shown to increase as a means of reducing competition among kin (Downey et al 2015). Thus, it may be that the propensity to move was reduced as relatives (members of the same source population) were at the lowest frequency in the population from six sources.…”