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Sex ratio of social group — and sex of ‘patient zero’ — affects the spread of infectious disease in flies

The sex ratio of a social group can influence the risk of getting an infectious disease as much as, and sometimes more, than an individual's traits, a Rice University study finds.

If you've ever been assigned a seat next to a coughing person you know your surroundings can affect your risk of infectious disease. Yet many studies that investigate risk of infection tend to focus on the individual and what traits he or she has (or lack) that could provide protection, or boost the chances, of infection.

In truth, an individual's risk of becoming infected is a combination of his or her own traits and social context. A recent study led by a team of researchers from Rice University investigates both.

In the study, the team uses fruit flies and an infectious fungus to understand how fruit fly genotype (an individual trait), the sex ratio of the group, and the sex of the first fruit fly infected (the identity of "patient zero") can influence individual infection risk.

To disentangle the relative importance of individual traits and social context, the researchers created 66 social groups of fruit flies from three genotypes with sex ratios ranging from zero to 100 percent female.

Postdoctoral fellow Nick Keiser, PhD, explains in the Rice press release:

We measured their behaviors as a social group and then we added to each a single infected individual, a male or female patient zero, and allowed them to interact for two hours.


Because they all had paint dots on their backs, we could watch their social behaviors, their aggregation with one another and their mating dynamics. When we separated them to see who became infected and if there were any patterns, we found no genotypic differences in infection risk.

We were surprised. We thought there would be a clear link.

Instead, the team found that the social context of each group had the most influence on infection rates. Two more surprise findings were that the male fruit flies were at greater risk of infection if they were in a female-dominated group; also when "patient zero" was female, the males in the community suffered four times greater risk of infection than the females did.

These findings suggest, "it's not just your traits, but the traits of the social group that you find yourself in, or maybe that you choose, that influences your infection risk," Keiser said. "No individual [or fruit fly] is an island."

Photo by 班森

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