Heat Changes Wolbachia and Spider Generations

Heat can leave its imprint long after the temperature drops. For one small spider, a short encounter with summer warmth produces an unusual outcome.

The spider exposed to the heat is unaffected. Instead, the effects are passed on to its children and grandchildren.

New research led by the Hebrew University of Jerusalem (HUJI) examined how a single warm period reshaped the community of bacteria living inside a spider.

That microbial reshuffle travelled through successive generations, altering the number of descendants born male rather than female.

Bacteria inside the dwarf spider

This study focused on the dwarf spider Mermessus fradeorum, originally collected from alfalfa fields by the University of Kentucky.

Although it appears unremarkable, the spider contains a densely populated microbial ecosystem.

As many as five bacterial types inhabit its body, and each is transmitted directly from mothers to their offspring. Rather than being unwanted intruders, these bacteria are retained by the spider.

They act more like permanent residents pursuing their own interests. One strain has an especially unusual influence on the spider’s sex.

When males become females

The bacterium responsible is a strain of Wolbachia, which is well known for interfering with the reproduction of insects and related animals. In this spider, it performs a remarkable transformation.

Wolbachia causes genetically male spiders to develop into fertile females. This directly benefits the bacterium because only daughters can pass it to the following generation.

A female-skewed brood therefore creates more future carriers. In essence, feminisation is a strategy that helps the microbe spread.

However, something limits this process. In wild populations, feminising Wolbachia does not reach fixation; instead, it remains at intermediate frequencies, suggesting that an unseen constraint is at work.

Heat affects spider generations

To identify that constraint, researchers exposed young spiders to approximately 27–28°C for one generation. These temperatures resemble those recorded during a hot Kentucky summer afternoon at ground level.

Their relatives remained at 20°C for the entire experiment. The spiders exposed to heat appeared entirely normal.

They developed into females as expected, precisely as the infection dictates. The unexpected effect emerged only after those females reproduced under cooler conditions.

The following generations had never experienced heat themselves. Nevertheless, their broods started producing males again, disrupting the almost entirely female sex ratio normally imposed by Wolbachia.

The effect was strongest among the grandchildren, before easing somewhat in later generations. In practical terms, a warm spell affecting one generation extended its influence to spiders raised only in cool containers.

A rival bacterium takes over

Bacteria were counted individually to explain this delayed response, with part of the work undertaken at the University of Illinois. Heat did not immediately eliminate the feminising Wolbachia.

Instead, it altered the balance between the microbes sharing the spider’s body. Immediately after the warm period, Wolbachia briefly increased in abundance.

However, its ability to pass reliably into the next generation subsequently declined. Another resident bacterium, called Tisiphia, vanished from the lineage altogether.

Competing bacteria disrupt feminisation

As Wolbachia weakened, a competitor gained ground. A bacterium called Rickettsiella increased sharply in the next generation.

Broods containing the greatest amounts of Rickettsiella had the fewest feminised offspring. This relationship suggested a struggle for dominance between the two microbes.

Feminisation appeared to require more than simply having Wolbachia present: it also required Wolbachia to outweigh its rival. The researchers expressed this conclusion directly.

“A brief period of elevated temperatures disrupts the delicate competitive balance between these symbionts,” said the researchers.

“We observed that Rickettsiella appears to inhibit the feminization process, showing that reproductive manipulation requires not just the presence of Wolbachia, but its relative dominance within the microbial community.”

Safety in extra strains

The effects were not identical in every spider. Individuals carrying all three Wolbachia strains, rather than only the feminising strain, withstood heat much more effectively.

These more fully infected lineages maintained a steadier microbial balance across generations. They also returned to female-heavy broods more rapidly than relatives with fewer infections.

Having a broader collection of strains seems to protect the entire microbial community against a temperature shock. Co-infection becomes a source of resilience rather than an added burden.

In wild populations, however, the complete five-member community occurs in only about 19 percent of these spiders. Most spiders host a less extensive mix, making them much more vulnerable to a hot spell.

Heat restores male spiders

Viewed more broadly, the findings reveal a larger pattern. If Wolbachia always prevailed, a population could become so heavily female-biased that its long-term future would be unstable.

An occasional period of warmth may provide a natural adjustment. It brings males back into a population that might otherwise move towards producing only daughters.

The researchers described the wider significance as follows. “Our findings demonstrate how an organism’s environmental history shapes the evolutionary stability of its microbial communities and their induced phenotypes,” said the researchers.

In natural settings, where temperatures fluctuate daily and seasonally, these microbial changes are likely to influence how many males and females occur in each brood. Heat appears not merely to be a passive condition, but an active influence on the spider’s sex ratio.

What remains unanswered

Much about this system is still uncertain. The exact mechanism through which Rickettsiella prevents feminisation remains unidentified.

Current evidence shows correlation rather than confirmed cause and effect. Researchers have not yet investigated the effects of repeated or longer heat periods, nor established how bacteria carried by males fit into the system.

For now, the study offers an unusual real-time view of an internal ecosystem being reshaped by a single shift in temperature.