Hot Sex Means No Sex at All

Hot sex means better sex, right?

But for some, hotter could result in no reproduction at all...

An overheated pea aphid won't be able to reproduce.

In fact, it's not the insect, but its bacterial symbiont that fails.

And all is on a single gene.

"It's the first time a mutation in a symbiont has been shown to have a huge impact on host ecology," said Nancy A. Moran, Regents' Professor of ecology and evolutionary biology at The University of Arizona in Tucson.

"One version of the gene is good if the aphids experience heat, and the other version is good if they are in cool conditions."

The Buchnera aphidicola bacterium, totally dependent on the insect, provides the aphids, Acyrthosiphon pisum, with essential nutrients, as its food and plant sap are poor on many crucial biochemicals.

Aphids reproduce by parthenogenesis throughout the summer: virgin females lay eggs from which hatch clones of their mother.

The bacteria are transmitted from mother to offspring.

The heat-intolerant gene was discovered when the team was investigating bacteria's ibpA gene, which encodes the heat-shock protein, with protective role against thermal damage.

When the researchers heated for four hours the same colony, formed by aphid clones carrying the same DNA, some of the symbiotic bacteria synthesized roughly 100 times more heat-shock protein than others.

A DNA stretch named promoter turns on or off the ibpA gene.

Some bacteria had a disabling mutation in their promoters that impeded them to synthesize the heat protective protein, being killed in the experiment.

Then the team exposed to 35 C heat aphids whose bacteria carried the mutated promoter, the regular promoter and strains from Tucson aphids.

The gene's activity was assessed by the RNA amount, as this is an intermediary molecule in protein synthesis.

Tucson genes synthesized three times more RNA than the normal genes and about 50 times more than the heat sensitive mutation.

Aphids colonies flourish in the summer, and when they are attacked by predators, they spread pheromones that makes them all jump off the plant.
The temperature difference can be over 12 degrees C compared to the cool, shady plant.

The team exposed juvenile aphid to 35 C for four hours.

Those carrying the heat sensitive bacteria did survive, but did not reproduce, as they lacked essential nutrients for reproduction.

Instead, these aphids reproduced faster at 15 - 20 C than other aphids.

These differences may be determined by the insects' ecology: Tucson aphids' bacteria, from hot arid environments, all presented heat-tolerant promoters, while those from cooler Michigan or New York state aphids were five to seven times more likely to present the mutated variant.

Pea aphids are alien invaders from Europe.

"Such easy-to-arise mutations in symbiotic bacteria could explain how some insect pests can expand into new environments," said Moran.