Inbreeding and Reproductive Investment in the Ant Formica Exsecta

Abstract

In social animals, inbreeding depression may manifest by compromising care or resources individuals receive from inbred group members. We studied the effects of worker inbreeding on colony productivity and investment in the ant Formica exsecta. The production of biomass decreased with increasing inbreeding, as did biomass produced per worker. Inbred colonies produced fewer gynes (unmated reproductive females), whereas the numbers of males remained unchanged. As a result, sex ratios showed increased male bias, and the fraction of workers increased among the diploid brood. Males raised in inbred colonies were smaller, whereas the weight of gynes remained unchanged. The results probably reflect a trade-off between number and quality of offspring, which is expected if the reproductive success of gynes is more dependent on their weight or condition than it is for males. As males are haploid (with the exception of abnormal diploid males produced in very low frequencies in this population), and therefore cannot be inbred themselves, the effect on their size must be mediated through the workers of the colony. We suggest the effects are caused by the inbred workers being less proficient in feeding the growing larvae. This represents a new kind of social inbreeding depression that may affect sex ratios as well as caste fate in social insects. K E Y W O R D S :Inbreeding, reproduction, resource allocation, sex ratio, social insects, trade-off.

Do Cuticular Hydrocarbons Provide Sufficient Information for Optimal Sex Allocation in the Ant Formica exsecta?

“…Another possibility is that nurse workers can assess the number of queens directly or by colony egg production, and use this as a proxy to adjust sex ratio in polygynous colonies. This is consistent with split-sex ratio theory (Boomsma, 1993), because polygynous colonies will tend to have higher eggproduction and a lower than population-level-average relatedness asymmetry, and also with the idea that specialization in the cheaper sex (males weigh less in F. exsecta; Vitikainen et al, 2011) requires greater initial eggproduction for equal total biomass production. On the other hand, this does not explain the tendency of monogynous/ polyandrous colonies to specialize in male production.…”

“…Fletcher and Michener, 1987), but this lack of relationship may be explained by the uniform levels of genetic variation we found among the different colony types. The breeding patterns and the genetic structure of the population, which indicate non-trivial levels of inbreeding Haag-Liautard et al, 2009;Vitikainen et al, 2011), may have contributed to the depauperate chemical profile through genetic purging, a question which will have to be addressed in future studies. We also found that the within-colony CHC variability does not allow accurate assessment of colony kin structure, and that workers do not seem to use any single compound or set of compound as a proxy for sex ratio adjustments.…”

“…This is indeed the case in F. exsecta, but many more compounds have been found in F. truncorum (Akino, 2006), which goes against this interpretation. Second, given that inbreeding is rife in the population of F. exsecta studied here Haag-Liautard et al, 2009;Vitikainen et al, 2011), genetic variation for cue diversity may have been purged. We found no significant effects of inbreeding on CHC variability, but the measure we used as a covariate in the analyses does not reflect the absolute level of inbreeding, only the relative level.…”

“…The sex of the pupae was determined based on morphology, and the sex ratio was calculated as the proportion of queens of all sexual brood (c.f., Vitikainen et al, 2011).…”

“…We included HL in the analysis, given that two earlier studies have indicated a significant effect of inbreeding on sex ratios (Haag-Liautard et al, 2009;Vitikainen et al, 2011). To test the effects of within-colony CHC variability, relatedness, genetic diversity, and inbreeding on colony sex ratio we used a multiple regression with stepwise backward elimination.…”