The two-spotted spider mite avoids bacterial infections not by relying on an immune response or physical protection, but by being a fussy eater, according to new research.
In a paper published on the preprint site BioRxiv, Flore Zele and colleagues from the Universidade de Lisboa in Portugal found that the microscopic Tetranychus urticae makes up for its poor immune protection by choosing to avoid possible sources of infection, such as tainted food.
While it might sound basic, the scientists suggest it points to an infection-avoidance mechanism for the mite that has been selected for after the failure of its other protective measures.
T. urticae has a notoriously poor immune system. When scientists sequenced its genome, they discovered that most of the immunity genes conserved across the majority of arthropod species were absent.
Furthermore, Zele and colleagues earlier found that when the mite was injected with bacteria, it had no immune response at all. And that was bad news, leading to a rapid death.
However, in nature an injection of bacteria is unlikely to occur, so the scientists turned to investigating other ways the mite could be infected, such as being sprayed with bacteria or ingesting contaminated foods.
No matter how they were infected, the spider mites died when exposed to Pseudomonas putida, a pathogenic bacteria. Such is their sensitivity, they even died when infected with far less pathogenic Escherichia coli.
While infection by ingestion or topical spraying did lead to mites dying, the numbers were lower than for direct injection. This, suggest the authors, shows that while the gut epithelium and cuticles provide some protection to the spider mites, they’re still not able to provide complete protection.
This raises an interesting conundrum – how would a species is so susceptible to bacterial infection neither become extinct, nor evolve immune protective mechanisms?
Zele and the team established that rather than an active immune system, the spider mite has developed alternative, behavioural defensive strategies.
The researchers found that the spider mites are particularly picky eaters – they’ll prevent infection by avoiding infected food, such as bacteria-infested plants.
The food avoidance, however, does not rely on olfactory signals, either directly from the bacteria or as a result of their interaction with the plants. Instead, the scientists believe, the mites reject rotten food after an initial taste. After a nibble, they make a call on whether the food is safe to be consumed.
However, rejecting sources can be costly if food is scarce, say the researchers. They found that the spider mites would turn down food infected with the highly pathogenic P. putida more often than stuff covered in the less dangerous E. coli. This in turn suggests they have some way of discriminating between types of contamination.
The researchers add that the avoidance behaviour is not necessarily just to escape infection. Instead, they suggest the spider mites could use the presence of bacteria as a proxy for low-quality, rotten food.
The strategy ultimately critical to species survival.
The life of any individual spider mite drastically reduced when bacteria penetrate its body. And, as a carrier in a high-density population, it puts others at risk of subsequent infection – producing, in extreme cases, colony extinction.