Innocently spreading by mycelial growth, Pilobolus species faithfully secrete their extracellular enzymes, breaking down herbivore dung and recycling its nutrients. Upon maturation, Pilobolus species produce tiny fluid-filled vesicles, atop each of which a spore packet called a sporangium is poised to shoot. That sporangium is destined to be tossed away in a burst of fluid, which surely ranks among the most impressive feats of any fungus. It is doing the job that nature intended, breaking down its substrate and propagating its genes. In the meantime, unbeknownst to it, its reproduction is aiding the spread of a disease of grazing animals: parasitic bronchitis is caused by lungworms in the nematode genus Dictyocaulus. The lungworm takes advantage of Pilobolus in a commensal relationship that neither benefits nor harms the fungus directly.
Quicktime 5+ movie
Time lapse video of Jack’s isolate of Pilobolus by Kent Loeffler. Details below.4
A successful lungworm passes twice through a cow. Let’s begin with the mommy lungworm illustrated here—she’s about 7 cm long (yes, almost 3 inches). She lives in the lungs of an unfortunate host with a ticklish cough, and lays her eggs there. The eggs hatch in the lungs and the larvae swim about, causing parasitic bronchitis, until they are coughed up in a gob of frothy mucus. Some lucky larvae and eggs then get swallowed by the animal, and pass unscathed through the gastrointestinal tract. Those swallowed larvae are then excreted in the feces, where they develop into a different, infective larval stage within about a week. During this stage, if the larvae are eaten by an animal, the larvae will then enter through the digestive system and work their way through the blood and lymph system, eventually arriving at the lungs. There, the larvae mature, mate, and start the reproductive cycle again.2,3
Although most of the lungworm’s life cycle was known, its specific means of dispersal was unknown for some time. Infective lungworm larvae are motile but can’t move far. And grazing animals avoid foraging near feces. These two observations suggested there might be an intermediary between the lungworm and the animal. Pilobolus was a likely culprit.
Pilobolus species aren’t animal pathogens, but they have the same problem as the infective larvae—they need to get far from the offending dung heap to get a herbivore to eat them. With their far-shooting ability (sporangia land up to 3 meters from the dung), Pilobolus sporangia might be the perfect transportation for lungworms. To test this speculation, an experiment was designed to test what effect Pilobolus has on the dispersal of lungworm. The study done by Jorgensen1 revealed that pastured cattle were more prone to lungworm when both Pilobolus and lungworm were present in feces, than when feces contained only lungworms. Cattle raised in a pasture without Pilobolus had about seven times fewer lungworms than cattle raised in a pasture with Pilobolus.1
Pilobolus gives lungworms a lift just by being itself. Pilobolus has a phototrophic spore shooting mechanism to prevent its sporangia from landing back on the poop. The fruiting bodies track light, and shoot the sporangia in the early morning, when the angle of the sun is low—they’re going for distance. Lungworms take advantage of this shooting mechanism. They are most active when the sun is shining. During this time, they actively seek any vertical structure. Upon reaching the sporangium, they attach themselves to it, chill out, and wait to be shot off. By landing far away from the feces, the lungworms have a greater chance of getting eaten by another grazing animal, which is Pilobolus‘s goal too.3
The result of lungworm infection depends on the animal. In dairy cows, it can cause a decrease in milk production. In animals that are raised for meat, it can cause a decrease in quality and amount of meat. Young animals are especially susceptible, but can develop some immunity after mild infection. In both wild and domesticated animals, lungworm infections can ultimately cause death through pneumonia or weakened immune system.2,3
More research is needed to see just how much effect Pilobolus has on lungworm distribution. Does the hitch-hiking nematode alter the trajectory of the shot spores? Or their ability to germinate? The routine use of dewormers like ivermectin in livestock management has significantly decreased the impact of lungworms—will organic practices result in a resurgence? So many questions…
- Jorgensen, RJ, H Ronne, C Helsted, AR Iskander. (1982) Spread of infective Dictyocaulus viviparus larvae in pasture and to grazing cattle: experimental evidence of the role of Pilobolus fungi. Vet Parasitol 10(4):331-9.
- Anonymous. Lungworm. University of Aberdeen, Organic Website. Accessed Nov 30, 2006.
- Robinson, J. (1962) Pilobolus spp. and the Translation of the Infective Larvae of Dictyocaulus viviparus from Faeces to Pastures. Nature 193, 353 – 354.
- We grew Jack’s Pilobolus isolate on a cube of autoclaved horse poop (thanks to Dave Kalb) under an overhead lamp. The movie begins at 4PM on November 15, 2006. At about 10:30PM the yellow pigment had migrated to the tops of the developing sporangia. At about 4AM the yellow pigment had turned black. The sporangia started to fly at about 6AM and continued until all had shot by 7:30AM. Kent took a photo every two minutes, and compiled them into this cool video.
- Edited to add a link to A Blog Around the Clock, where there is a very nice piece about light-sensing and circadian clocks in Pilobolus. [Feb 28 2007]
Time lapse video by Kent Loeffler (of the fungus that the author isolated from horse dung); lungworm photo courtesy of Dr. E.T. Lyons, Professor of Veterinary Science at the University of Kentucky’s Gluck Equine Research Center.