John and George visited the scrub near Stuart, Florida, yesterday on a torrid humid afternoon topped off with donner and blitzen. Even the bugs seemed hot and bothered. We usually go for botanical purposes, but today’s trip took an arthropod bend, beginning with spiders, big ones, all over the place. But let’s turn to the ants in your plants. What caught our eye were ant shenanigans on oak galls.
Galls are growths on plants, most often caused by various arthropods forcing the plant to create a home for the creature’s larva. Two plant groups especially prone to this are members of the Aster Family and oaks. Most oak galls are the work of little wasps who lay their eggs into the bud, leaf, or stem. Lotsa wasp species, lotsa galls types.
On sand live oaks (Quercus geminata) are spherical stem galls crawling with at least two ant species. These galls probably house larvae from a small oak-loving wasp, Disholcaspis quercusvirens (or related species). Why ants on a wasp’s larval home? A larva trapped in a gall is a sitting duck for parasitoid insects to lay eggs in you. The vulnerable larva needs defense. Our wasp not only can force an oak to launch a gall, but even better, galls with benefits to lure hungry ants who deter those pesky parasitoids. The galls make ant food. At least four species all intertwined: mighty oak, tricky wasp, odious parasitoid, and all those ants. Not to mention multiple ant species, and there are probably “inquilines,” tagalong insects who use the gall as secondary guests. It’s a tiny ecosystem in and on an organ the size of a grape.
Does the tree benefit? Although the plant is traditionally cast as a passive victim, a breath of preliminary experimental evidence has shown the gall experience to enhance sometimes a plant’s stress resistance.
A second gall we inexpertly identify as the Horned Oak Gall (starring the wasp Callirhytis cornigera or related species) on Myrtle Oak is another ant playpen. Ants push their faces into the tips of the horns like a dog drinking from the toilet. The horns are exit-holes for the baby wasps and also sap drinking fountains, although the relationship in time between larval departure and snack time is a little mysterious. CLICK and CLICK AGAIN
The ants do not stay the outside galls. Oak Apple Galls on the inside have a nearly hollow capsule with the larval crib suspended on a network of strands, shock-resistant, temperature-insulated, and out of reach of probing parasitoid ovipositors. Occasionally an old dried Oak Apple Gall becomes an ant nest, a lively surprise when you pluck the gall for a close look or photograph. The ants use the wasp exit hole as the door.
That an insect can induce a plant to make a device to perform the bug’s bidding is remarkable, one species directing development in another, how very odd. The interspecific genetic control implications boggle the mind.
Plant hormones are no doubt important in galls, especially cytokinins. Cytokinins are involved in tissue growth and differentiation, and are strongly implicated in gall formation. That an insect could cause or mimic cytokinin production seems plausible, and might be a satisfying answer if galls were mere unstructured tumors. The mystery is in the complexity: how the insect directs the plant to form nectar-secreting horns, and little honeydew bite-off chunks, and reinforced packaging spheres. The intricacy goes beyond mere structure: gall insects can suppress the natural defenses of host plant species, and they can force the plant to make nutritional material as well as enzymes to help digest the food. This is sequential complex development.
So now the tough questions. Would a plant evolve genes to build the larval home for an insect? Not likely. Does the insect transmit genetic information to the plant? Likewise improbable, although not beyond the imagination. Most likely by far, the insect manipulates what’s already in the plant’s genes.
As an example, galls sometimes provide nutritional protein to the larva within. This fancy achievement is easier to explain knowing that the protein is already coded in the plant’s genetic library, normally expressed in feeding the plant’s embryo in the seed. Somehow the wasp activates the “on” switch to nourish its own little interloper.
To extend the theme of repurposing the plant’s existing genome, a plant has the genes to make sugary nectar, merely redirected onto galls. The spongy tissue in the Oak Apple gall doesn’t seem a genetic stretch for a plant. Even the horns on Horn Galls might be misdirected “buds.”
With all that said, galls must have an astounding gene control story. Check in again in 20 years and maybe the story will be told.
Our peek in the gall development window is superficial, entailing vague references to Waddington’s epigenetic-landscape model*, DNA methylation, and histone modification. That all sounds spiffy but tells us about as much as, “how did Kurt Vonnegut create Slaughterhouse-Five? With a typewriter. “
Potentially relevant, one family of plant growth-stimulating hormones called brassinosteroids is biochemically similar to animal hormones. They are the only steroid hormones in plants. These have been shown to sabotage herbivorous insects, but could it work the other way around…can the animal use its steroids to influence plant growth? Hints are in the air but far from anything solid. Time to quit, because we’re already out on a gall-infested limb beyond any actual knowledge.
*Waddington’s model uses a downhill series of valleys and ridges like a branching river delta to describe the progressively branching and narrowing genetic options of developing tissues—or of developing students sorting into colleges, then academic majors, then careers. DNA methylation is a means of gene suppression during development where methyl groups added to genes inhibit or prevent their expression. Histones are proteins associated with DNA. Enzymatic modifications to histones regulate expression of their associated genes.