Fish N' Chips; Functional Genomics of Social Plasticity in African Cichlid Fish.
Different cichlid species show a diversity of social structures. Suzy Renn explains that some cichlid species are monogamous, while males from other species mate with many females. Astatotilapia burtoni, the species used in her recent study, develop a dominance structure, with a brightly coloured dominant male maintaining a territory while silvery females and subordinate males travel together in a school. But these social structures are far from rigid, and once a subordinate male becomes larger, he can over throw a dominant male. Cichlids produce a rich repertoire of natural social behaviours in a lab tank where they are easily observed, thus making them a powerful model system to study physiological changes accompanying changing social status. As postdoctoral fellows, Renn and Nadia Aubin-Horth teamed up with Hans Hoffman who had initiated a genomic approach to the fish’s social interactions. The trio set out to identify key differences in gene expression patterns between the brains of dominant and subordinate A. burtoni males to lay the foundation for research in their own labs, Renn at Reed College, Aubin-Horth at University of Montreal, and Hofmann now at UT Austin.
First, the team built the custom cDNA microarray carrying approximately 4000 genes. After isolating the genes, sequencing them and searching DNA databases to identify as many as possible, the team was ready to test the fish’s brains. Setting up 9 tanks with 2-3 male and 2-3 female fish, the team monitored behaviour for 5 weeks, to clearly establish social structure, before extracting RNA from the brains to compare the gene expression patterns using the microarray.
According to Renn, the team found the expression of 87 genes was increased in the brains of dominant males, while 84 genes were upregulated in the subordinate males’ brains. These "gene lists" include some changes already known through classical experiments, including increased expression in dominants for two neuropeptides, gonadotropin releasing hormone and arginine vasotocin, that are also involved in reproduction, dominance and pair bonding in other species. The gene lists also revealed new findings such tubulin and actin expression increased in the dominant males’ brains, which could indicate changes in the structure of the brain not previously known. Surprisingly, two neurotransmitter systems, involving GABA and glutamate signalling, were oppositely regulated by social status. The team admit that the detailed functional consequences of these findings are not always clear, but know that the different expression patterns can have profound effects on the electrophysiology of the brains.
Contrary to naive expectation, the team found that dominant males are not simply "super males", but instead, many of the genes upregulated in females are also important for determining social status in males. This "systems level approach" complemented the gene-list and allowed the researchers to discover that co-regulated gene sets are associated with social status in a “modular” fashion. In combination with current cichlid genome projects, these results pave the way for future genomic studies to uncover how molecular modules are associated with behavioural or physiological measures of social status in different contexts and different species.
text by Kathryn Philips, Suzy Renn and Melati Kaye