How Genes Play a Role in Social Networks

If mirrors of twenty people had lived their lives in dozens of different worlds, would the same people be popular in each world?

If you replace “fruit flies” with “people” in this question, you have a fair description of a Rice University study showing that the evolution of social structures and individuals’ attitudes within those structures is based in part on genetics.

Cloned fruit flies starred in the study that the researchers jokingly likened to the “Truman Show,” with video cameras that monitor how the flies behave in a controlled environment.

In the study published online this week in Nature Communications, Rice biologists Eric Weiss and Julia Saltz measured social interactions between individual flies in 98 genetically identical groups. Each group contained 20 clones. The 20 differed genetically from each other, but the same clones were included in each of the 98 groups, which lived in separate enclosures under different environmental conditions.

Wice and Saltz found that the same clones occupy the same social situations in each closed “world,” regardless of the difference in living conditions.

“Social structure varies greatly across the animal world, and the big question we’re interested in is ‘How did this difference evolve?'” said Saltz, an assistant professor of biosciences at Rice. “For evolution to occur, the social structure must be heritable, and we have shown that it is.”

Weiss, Ph.D. A student in Saltz’s lab said that although genetic variation explained part of the variation in the flies’ social network locations, the estimates of heritability that he and Saltz discovered were sufficient to fuel evolutionary change.

“To know whether or not the structure of social groups and the structure of networks will evolve over time, we have to know the genetic basis of how individuals overlap their social networks and we also have to know how natural selection works on social group structure,” Weiss said. These two things simultaneously in this experiment, which have not been done before.”

Weiss said previous research has shown that social network structure can evolve by natural selection, but few genetic components of social network structure have been described.

“We combined these things simultaneously to see how the structure of social groups would evolve and how it might respond to choice,” Weiss said.

The study found that the amount of variance in social status that was explained by genes “was in the range of 2.4% to 16.6%,” Weiss said. Weiss said that given that nearly all organisms exhibit some form of social organization, the researchers’ findings could apply to species as diverse as humans and bacteria.

Saltz said some studies have explored whether human popularity can be explained in part by genetics. But, she added, it’s essentially impossible to design a beta test.

“The best way to do that is to have identical twins, the Truman Show for them,” she said, referring to the 1998 movie in which the main character unintentionally lives in a controlled environment as part of a reality TV show. “You would put one twin in a bubble.” The Truman Show and the other one in the other Truman Show bubble, and then you see if they have the same twin friends.”

Basically, this describes the experience that she and Wice had with flies.

“Any two genotypes in our sample are as closely related genetically as two randomly selected individuals,” Weiss said. “But we can basically make copies of all the flies and basically test identical twins over and over again.”

Because Wice collected data on social interactions by videotaping the flies in each enclosure, conducting the experiment was much like producing 98 Truman Shows at one time, but “with 20 Trumans on each show,” Wice said.

Attitudes within social networks were measured in five ways based on thousands of interactions between flies that were recorded and indexed by computer programs that “watched” tens of hours of video for 98 groups. Although the word “popularity” does not appear in the paper, Saltz said it was appropriate because some of the basic criteria measured were equivalent to “how many friends you have and whether or not your friends are friends with each other.”

Remarkably, Wice and Saltz found that social status within the networks remained the same, even when they changed the environment by altering the quality of the food in the packages. In some foods, foods contained more protein and in others they contained more carbohydrates. In other cases, the flies had fewer calories. In all, there were five types of food, and about 20 groups of flies lived on each species.

“Our findings show that we expect social structure to evolve differently in different food environments,” Weiss said. “This is important, but more research is needed to determine what kind of changes arise from dietary differences.”

Ultimately, Wice and Saltz would like to learn more about the ways in which nutrition, aggressive behavior, and other factors influence the development of social structure.

“What creates the social structure?” Saltz said. “Group structures are by their nature emerging properties of many different individuals, and there must be some basic principles that shape the development of those structures.”

Weiss said that the fact that an individual’s position within a social network depends on the behavior of other individuals complicates the study of social-evolutionary structure, noting that some of the tools used in their analysis did not exist when Weiss began his Ph.D. since five years.

“It’s not independent data, and that violates a lot of statistical tests and assumptions,” Weiss said. “Tools are getting better all the time, and it wouldn’t surprise me if new tools appeared in the next five years that would allow us to learn more about the data we collect today.”

Weiss EW, Saltz JB. The choice on which social networking sites are inheritable depends on the context in Drosophila black belly. Nat Combs. 2021; 12 (1): 3357. doi: 10.1038/s41467-021-23672-1.

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