Ant workers selfishly bias sex ratios by manipulating female development

Abstract

Kin selection theory predicts that social insects should perform selfish manipulations as a function of colony genetic structure. We describe a novel mechanism by which this occurs. First, we use microsatellite analyses to show that, in a population of the ant Leptothorax acervorum, workers' relatedness asymmetry (ratio of relatedness to females and relatedness to males) is significantly higher in monogynous (singlequeen) colonies than in polygynous (multiple-queen) colonies. Workers rear mainly queens in monogynous colonies and males in polygynous colonies. Therefore, split sex ratios in this population are correlated with workers' relatedness asymmetry. Together with significant female bias in the population numerical and investment sex ratios, this finding strongly supports kin-selection theory. Second, by determining the primary sex ratio using microsatellite markers to sex eggs, we show that the ratio of male to female eggs is the same in both monogynous and polygynous colonies and equals the overall ratio of haploids (males) to diploids (queens and workers) among adults. In contrast to workers of species with selective destruction of male brood, L. acervorum workers therefore rear eggs randomly with respect to sex and must achieve their favoured sex ratios by selectively biasing the final caste (queen or worker) of developing females.

Modelling information exchange in worker–queen conflict over sex allocation

“…So far little is known about which party tends to be in control of y (Reuter & Keller 2001), but in the ant Lepthotorax acervorum it appears that workers can selectively bias the final caste of developing females (Hammond et al 2001). Just to compare, we have examined the case in which the queen has control of both x and y and the workers can only determine z.…”

Effects of Brood Manipulation Costs on Optimal Sex Allocation in Social Hymenoptera

“…If, for instance, queens vary in mating frequency, workers in colonies with aboveaverage relatedness asymmetry (i.e., below-average mating frequency) should preferentially produce females, whereas workers in other colonies should specialize in the pro-duction of males. Evidence for this hypothesis has been found in a number of species (e.g., Müller 1991;Queller et al 1993;Sundström 1994;Sundström et al 1996;Walin and Seppä 2001;Hammond et al 2002). In others, however, sex ratio specialization occurred but was not associated with differences in relatedness asymmetry (e.g., Helms 1999;Brown and Keller 2000;Fjerdingstad et al 2002).…”

“…Workers can therefore influence sex allocation by influencing the process of caste determination and increasing the proportion of females reared as sexuals. This mechanism of sex ratio biasing has been found to be used by workers of the ant Leptothorax acervorum (Hammond et al 2002). Sex ratio manipulation through the biasing of female caste determination should also be associated with costs because raising more females as gynes reduces investment in new workers, thereby compromising colony growth and overall productivity (Reuter and Keller 2001).…”

Sex ratio and dynamic behavior in populations of the exotic blowfly Chrysomya albiceps (Diptera, Calliphoridae)

“…In some species, sex is determined by the joint effects of temperature and sex-determining genes (Conover and Demond, 1991). Sex allocation has been investigated more intensely in social insects as a function of factors such as colony size, genetic structure, food availability, season and parental investment (Bourke, 2001;Passera et al, 2001;Hammond et al, 2002).…”