Final Activity Report Summary - INVICTA2 (Caste-specific gene expression in the fire ant, Solenopsis invicta) Colonies of the single-queen-per-colony social form of the invasive fire ant, solenopsis invicta, are typically specialised in the production of a single sex, i.e. virgin queens versus males, during the reproductive season. Queens determine the sex of their eggs by fertilising or not fertilising them. The first are destined to become female, i.e. workers or virgin queens, while the latter become males. It has previously been shown that a colonys final sex allocation ratio, i.e. the energy invested in virgin queens versus males, is mainly determined by the proportion of female eggs laid by the mother queen. It has recently become clear that queens have a strong influence over the caste fate of her daughters, i.e. over whether her female eggs will develop into virgin queens or small, medium or large size workers. Our main aim was to identify genes whose expression pattern, either up or down regulated, in queens could explain the observed variation between queens in primary sex ratio. Using powerful microarray technology we compared the expression of more than 22 000 genes between 30 solenopsis invicta mother queens with widely varying primary sex ratios. We identified a previously unknown neurotransmitter receptor, whose expression was significantly, highly (with R2 equal to 0.81) and positively correlated with the proportion of female eggs laid by a mother queen. This result, which we were pursuing further by the time of the project completion, was potentially of great importance for the study of adaptive between and within species variation in sex ratios in any ant species. It suggested that it might be possible to experimentally change the sex ratio produced by captive ant colonies by adding synthetic neurotransmitters to their food.