A strain of Aedes aegypti (L.) was collected in Rio Piedras, Puerto Rico, in 1987 and reared through to the F2 generation. The strain was shown to be heterozygous for resistance to a range of pyrethroids, but there was no evidence of organophosphate and carbamate resistance. Biochemical studies indicated that the pyrethroid resistance was not metabolically based. Electrophysiological studies at the neuromuscular junction of resistant insects indicated that a kdr (nerve insensitivity) type mechanism was operating in this strain. It is not known whether this mechanism was selected by earlier DDT exposure or by the recent ultra-low-volume applications of pyrethroids.

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Intra-species cognitive variation is commonly observed, but explanations for why individuals within a species differ in cognition are still understudied and not yet clear. Cognitive processes are likely influenced by genetic differences, with genes in the monoaminergic systems predicted to be important. To explore the potential role of these genes in association with individual variation in cognition, we exposed red junglefowl (Gallus gallus) chicks to behavioural assays measuring variation in learning (discriminative learning, reversal learning, and cognitive flexibility) and optimism (measured in a cognitive judgement bias test). Following this, we analysed prefrontal cortex gene expression of several dopaminergic and serotonergic genes in these chicks. Of our explored genes, serotonin receptor genes 5HT2A and 5HT2B, and dopaminergic receptor gene DRD1 were associated with measured behaviour. Chicks that had higher 5HT2A were less flexible in the reversal learning task, and chicks with higher 5HT2B also tended to be less cognitively flexible. Additionally, chicks with higher DRD1 were more optimistic, whilst chicks with higher 5HT2A tended to be less optimistic. These results suggest that the serotonergic and dopaminergic systems are linked to observed cognitive variation, and, thus, individual differences in cognition can be partially explained by variation in brain gene expression.