A study tracked the prevalence of symbiotic bacteria in pea aphids in order to glean insights into how evolutionary forces — and temperature change — affect biodiversity across generations.

Aphids host a symbiotic bacteria, or endosymbiont, called Hamiltonella defensa, in their blood and internal tissues that safeguards pea aphids from parasitoid egg-laying wasps. Females can pass on Hamiltonella defensa to their offspring, yet not all pea aphids inherit the bacteria.

“Sometimes it just takes a handful of generations for evolution to unfold,” says Professor Jacob A. Russell, senior author of a study that appeared in Molecular Ecology in 2021. “We were motivated to try to understand why this one endosymbiont is maintained in these populations, why is it never lost and why it doesn’t just go to 100% frequency.”

The team — initially led by former student Andrew H. Smith (PhD environmental science ’16) — subjected aphid samples collected from alfalfa fields over the course of six months to routine molecular screening.

Although growth in the parasite population modestly increased Hamiltonella prevalence, the team discovered that temperature drove the most notable shifts in the bacteria’s presence and in how it functioned.

Hamiltonella clearly helped the aphids if the temperature was warm. However, in cooler temperatures, it was estimated that the bacteria harmed them.

This work is among the first to show that maternally transmitted microbes — which are found in most insect species — respond to the pendulum-like nature of changing environments across the seasons. Given the longer-term trends in global climate, it is possible, too, that endosymbionts may govern the future success of insect species in our rapidly warming world.