Cichlid Social Status Changed Through Gene Expression

Common thinking says that genetic codes are fixed throughout life, but this is not really the case, according to the results of a new study focusing on the roles that genes play in fish dominance.

For African cichlid fish, a colorful male does better in life, typically gaining more breeding opportunities and greater access to food. Of course, much of the perceived dominance of the fish by its community comes from its color and behavior. But what are the mechanisms behind those superficial metrics?

Research by Stanford University biologists suggests that the outcome is regulated at a genetic level, which they’ve proved by tweaking genes in male cichlids. They have found that regulating genes can also happen in an ongoing way through the processes of epigenetics, offering the potential to change fish behavior over a lifetime.

Gene expression can be turned on and off like a light switch via several mechanisms. Through one of those called DNA methylation, methyl molecules are added to genes, preventing them from being expressed. It was this approach that Stanford scientists relied on in their investigation into the dominance behaviors of male African cichlids.

Status differences exist in all social organisms, but researchers say that their results show how social dominance can be regulated through methylation. They note that the process can also be influenced through experiences that occur over an organism’s lifetime, which cause certain genes to express themselves more or less.

For the study into African cichlids, scientists focused on Astatotilapia burtoni, a type of cichlid commonly found in Lake Tanganyika. The males of these species rely on dominance, commonly fighting each other to obtain it, to gain access to food and females.

Sporting bright, rainbow-colored scales, high-ranking males aggressively defend their foraging grounds and lure females into their territory to dine on decaying matter on the lakebed. In contrast, the low-ranking males, which are dull grey in color, comprise 80 percent of the population but cannot reproduce and must swim with females to get access to food.

African cichlid fish. (Credit: L.A. Cicero / Stanford University)

And so researchers wanted to see if it was possible to change the behavior of non-dominant males using different gene expressions. They directly tested the idea in their laboratory.

Several pairs of non-dominant males matched in size were each placed in an aquarium that could support only one territory. In each pair, one male was injected with a methylating agent while the other received a methylation suppressor, and the two fish fought for dominance.

The behavioral change was visible in minutes, scientists say. Within that timeframe, one fish would begin to dominate the other, with those fish injected with the methylating agent found most likely to be the winners. The fish that received methylation suppressing agents were most likely to lose battles for dominance.

Results of the work suggest that epigenetic processes may actually cause, rather than reflect, changes in social status for this species of African cichlid. The next steps for the researchers is to determine which genes are responding to methylation and causing the change in dominance behavior.

Full results of the work are published in the journal PLOS ONE.