Combining animal personalities with transcriptomics resolves individual variation within a wild-type zebrafish population and identifies underpinning molecular differences in brain function S. REY,* 1 S. BOLTANA,* † 1 R. VARGAS,* N. ROHER* and S. MACKENZIE* † *Institut de Biotecnologia i Biomedicina, Universitat Auto `noma de Barcelona, Barcelona, 08193 Bellaterra, Spain, †Institute of Aquaculture, University of Stirling, Stirling, Stirlingshire FK9 4LA, UK Abstract Resolving phenotype variation within a population in response to environmental perturbation is central to understanding biological adaptation. Relating meaningful adaptive changes at the level of the transcriptome requires the identification of pro- cesses that have a functional significance for the individual. This remains a major objective towards understanding the complex interactions between environmental demand and an individual’s capacity to respond to such demands. The interpretation of such interactions and the significance of biological variation between individuals from the same or different populations remain a difficult and under-addressed ques- tion. Here, we provide evidence that variation in gene expression between individuals in a zebrafish population can be partially resolved by a priori screening for animal personality and accounts for >9% of observed variation in the brain transcriptome. Proactive and reactive individuals within a wild-type population exhibit consistent behavioural responses over time and context that relates to underlying differences in regulated gene networks and predicted protein–protein interactions. These differences can be mapped to distinct regions of the brain and provide a foundation towards understanding the coordination of underpinning adaptive molecular events within populations. Keywords: proactive, reactive, behaviour, gene expression, variation Received 18 July 2013; accepted 23 September 2013 Introduction Understanding phenotypical variation and physiological regulation during acclimation to external stimuli varia- tion is a central issue in biology and underpins evolu- tionary studies. In most studies across the biological sciences, individuals within groups are considered as simple replicates, and interindividual variation is actively minimized, for example, using specific genetic backgrounds. Such practice is commonplace in biology and reflects the difficulties in understanding variation and the prevalence of an approach often centred upon differences between mean values of measured variables for a population. However, it is common to observe sig- nificant levels of variation, standard deviation, in many different types of data that more probably obscure the underlying individual differences that may have poten- tial biological significance. Such an approach therefore cannot be used to understand the underlying interac- tions between individuals and their environment. Over the past few years, an increasing interest in the diver- sity of behavioural phenotypes and their consistency in and between individuals within a population has emerged (Sih et al. 2004; Wolf et al. 2007). This has been accompanied by a growing nomenclature including individual coping style, personality and behavioural Correspondence: S. MacKenzie, Fax: + 44 (0) 1786 472133; E-mail: Simon.Mackenzie@stir.ac.uk 1 These two authors contributed equally to the manuscript. © 2013 John Wiley & Sons Ltd Molecular Ecology (2013) 22, 6100–6115 doi: 10.1111/mec.12556