Erigone atra
Erigone atra

Remember the paper I mentioned the other day, that showed how a symbiotic virus causes aphids to grow wings? Amazing at that may be, an equally amazing thing is that it’s not unique. A new paper1 points to a different species, and a different symbiont, that also flips the switch on flight.

There are some differences: the host is a spider, not an insect; the symbiont is a bacterium, not a virus; and the symbiont switches flight off, not on. But, you know, we’re talking about concepts, here, and conceptually this is remarkably similar.

The spider is Erigone atra, a “money spider”, an agriculturally important species (they control pests). They’re widespread and successful, and one of the reasons is their aeronautical ability. Like many other spiders, E. atra can travel many miles via “ballooning” — spinning a long, fine strand of silk that catches the breeze and takes them floating to a new habitat.

Also like many (if not all) arthropods, E. atra has a host of endosymbiotic bacteria. The most famous of the arthropod endosymbionts are the Wohlbachia family, which do utterly incredible things to to their hosts in order to spread their (the Wohlbachia’s) genes; but there are many other bacterial symbionts.

Ballooning baby lynx spider
Ballooning baby lynx spider

Treating the spiders with antibiotics (thus “curing” them of their endosymbionts — specifically, curing them of Rickettsia) changed their ballooning behavior: Treated, Rickettsia-free spiders were six times more likely to balloon. Ballooning is more than shooting out a web and hoping, it’s a complex behavior: “a stereotypical ‘tiptoeing’ posture, which is exclusively used for aerial dispersal (comprising leg stretching, abdomen raising and production of silk threads that are used as sails)“,1 and this behavior was strongly suppressed in Rickettsia-infected spiders.

Assuming that the bacteria are actually imposing this behavioral change on the spiders (the authors are appropriately cautious with their interpretation), why don’t the bacteria want to the spiders to balloon? Why are they the opposite of the densoviruses I mentioned earlier? 2

My own speculation here … One critical difference between the aphids and the spiders is that the spiders are much more versatile. Aphids grow on a tiny, constrained food source, and then have to fly to find a new food source (and, I believe, to breed) (Update: See comment #2 for more a informed explanation of aphid lifestyles). These spiders, on the other hand, can disperse locally as well as aeronatically; there are local communities of spiders that can be reached without ballooning, so they’re not dependent on flight to propagate the species. Ballooning takes the spiders to new, sparsely-settled regions, where there’s less competition for resources. But that’s also a region where there are fewer hosts for the bacteria. Building up a reasonably dense local spider population might be in the bacteria’s best interests, and suppressing ballooning might help do that.

  1. Microbial modification of host long-distance dispersal capacity.
    Sara L Goodacre, Oliver Y Martin, Dries Bonte, Linda Hutchings, Chris Woolley, Kamal Ibrahim, C.F. George Thomas and Godfrey M Hewitt.
    BMC Biology 2009, 7:32 doi:10.1186/1741-7007-7-32[][]
  2. In contrast to the densoviruses, it’s not a big stretch of imagination to imagine the bacteria doing this mechanistically. Bacterial endosymbionts do much more complex behavioral changes than this to their hosts.[]