Fish and chips: functional genomics of social plasticity in an African cichlid fish

Abstract

Behavior and physiology are regulated by both environment and social context. A central goal in the study of the social control of behavior is to determine the underlying physiological, cellular and molecular mechanisms in the brain. The African cichlid fish Astatotilapia burtoni has long been used as a model system to study how social interactions regulate neural and behavioral plasticity. In this species, males are either socially dominant and reproductively active or subordinate and reproductively suppressed. This phenotypic difference is reversible. Using an integrative approach that combines quantitative behavioral measurements, functional genomics and bioinformatic analyses, we examine neural gene expression in dominant and subordinate males as well as in brooding females. We confirm the role of numerous candidate genes that are part of neuroendocrine pathways and show that specific co-regulated gene sets (modules), as well as specific functional gene ontology categories, are significantly associated with either dominance or reproductive state. Finally, even though the dominant and subordinate phenotypes are robustly defined, we find a surprisingly high degree of individual variation in the transcript levels of the very genes that are differentially regulated between these phenotypes. The results of the present study demonstrate the molecular complexity in the brain underlying social behavior, identify novel targets for future studies, validate many candidate genes and exploit individual variation in order to gain biological insights.

Figure 1

Schematic representation of A) phenotypic and B) physiological characteristics associated with Dominant and Subordinate male phenotypes in A. burtoni. The graphs are based on the following studies: gonad size: Hofmann and Fernald 2000; growth: Hofmann et al., 1999a; GnRH1: White et al., 2002; testosterone: Francis et al., 1993; somatostatin: Hofmann and Fernald 2000; growth hormone: Hofmann et al 1999b; cortisol: Fox et al., 1997.

Figure 2

Social behaviour varied significantly and predictably between the dominant and subordinate males. Y-axis represents (A, C) the percent time or (B, D, E, F) the average number of observed events per 10 minute focal observation. Identified squares, dominants, and circles, subordinates are consistent with figure 5.

Figure 3

Analysis, at all Gene Ontology (GO) levels, revealed 22 categories that were statistically over- (red) or under- (blue) represented. The three separate GO vocabularies (molecular function, biological process and cellular location) provide overlapping information, as many of the genes are annotated according to each. P-values represent uncorrected results for the hypergeometric test. GO number and name are according to 200605 releases.

Figure 4

Venn diagram depicting the relationship of sexually regulated and socially regulated genes. These relationships subdivide the gene classes to indicate modules of gene expression that potentially underlie reproduction (orange), submissive behaviour (lavender), and super-male dominance (green) and opposing super-male dominance (teal). Numbers indicate total unique sequences, and unsequenced array features. Gene names given represent best hit blast annotation for available sequences. The Venn diagram indicates regulation at a BPP of ≥0.99, (the specific BPP, down to 0.80, for regulation is indicated in supplementary tables 1 and 2)

Figure 5

Hierarchical clustering of (left) phenotypes and (right) individuals (based on expression profiles for the genes regulated at level of phenotype). The genes have been ordered according to K-means clustering. The heatmaps (red – up-regulated, green down-regulated) show estimated gene expression levels. Heat values are relative only within, not across genes. The numbers identify individual males consistent with figure 2. Confidence values at the nodes were obtained by bootstrap analysis (1000 permutations with resampling).