Cichlids are remarkably social fish. Susan Renn and Hans Hofmann explain that some cichlids are monogamous, while the males from other species surround themselves with a harem. Astatotilapia burtoni, on the other hand,form schools of silvery subordinate males and females that wander through the territories of brightly coloured males. But these social structures are far from rigid, and once a subordinate male becomes big enough, he can overthrow a weaker dominant male and take his place. Having spent years studying A. burtoni's physiology, Hofmann decided to take a genomic approach to understanding the physiology underlying the fish's social interactions when setting up his own lab at Harvard. Teaming up with Renn and Nadia Aubin-Horth,the trio set out to identify key differences in gene expression patterns between the brains of dominant and subordinate A. burtoni males, to begin understanding what sets subordinate and dominant males apart().
But first the team had to design a custom-made cDNA microarray, carrying approximately 4000 of the genes that are expressed in A. burtonibrains. After months of painstakingly isolating genes, sequencing them and searching DNA databases to identify as many of the genes on the microarray chip as possible, the team were ready to test the fish's brains. Setting up nine independent groups of fish, each containing 2–3 males and 2–3 females, the team monitored the fish's behaviour for 5 weeks, clearly establishing which males were dominant and subordinate, before extracting RNA from the fish's brains and comparing the gene expression patterns of dominant males, subordinate males and egg carrying females on the microarray.
According to Renn, the team found that the expression of 87 genes increased in the brains of dominant males, while expression of another 84 genes was increased in the subordinate males' brains. Some of the upregulated genes in the dominant males' brains had already been identified by more classical experiments, such as neuropeptide hormones involved in reproductive dominance and pair bonding in other species. The team also found increased gene expression of tubulin and actin in the dominant males' brains, which could suggest changes in neuronal architecture as the fish ascend the social hierarchy. Also the expression of two neurotransmitter receptors was affected by the male's social status. GABA receptor increased in the dominant males while the kainate receptor increased in the subordinate males. Although the exact role of these receptors in regulating social status is not clear,Hofmann and his collaborators know that the different expression patterns will have profound effects on the electrophysiology of dominant and subordinate male's brains.
After identifying lists of genes whose expression levels change in response to the fish's changing social status, the team used a `systems level'approach, where they looked at differences in the overall gene expression patterns to identify groups, known as modules, that function together to produce specific characteristics. Renn admits that having assumed that the dominant males would simply be souped-up versions of subordinate males, the team were surprised to find that `many genes that are upregulated in females seem to be important for determining social status in males too,' and adds`dominant males are not “super males”.'
Since beginning this work, the trio have gone their separate ways, but all three are keen to find out more about the roles of gene modules in social functions, each from a slightly different perspective. Ultimately they hope to learn more about how gene modules function together to shape cichlid social structures.
