Appendix 8: Examples of the extended phenotype in the context of a parasite’s ability to manipulate its host[1]
[1] Source|: Carl Zimmer, “Meet nature’s nightmare: Mindsuckers”, National Geographic, November 2014, 36-55.
- A ladybird is stung by the wasp Dinocampus coccinellae, leaving behind a single egg. After the egg hatches, the larva begins to eat its host from the inside out. When ready, the parasite emerges and spins a cocoon between the ladybug’s legs. Though its body is now free of the tormentor, the bug remains enslaved, standing over the cocoon and protecting it from potential predators. It will continue to play this role for a week, until an adult wasp cuts a hole through the cocoon with its mandibles, crawls out and flied away. Only then do most of the ladybug zombies die, their service to their parasite overlord complete.
- The jewel wasp Ampulex compressa stings a cockroach transforming it into a passive zombie. It snakes its stinger into the roach’s brain, sensing its way to the regions that initiate movements. The wasp douses the neurons with a cocktail of neurotransmitetrs, which work like psychoactive drugs, taming down the activity of neurons that normally respond the danger by prompting the cockroach to escape. The roach is perfectly capable of movement. It just lacks any motivation to move on its own behalf. This enables the wasp to walk its drugged victim into its burrow by the roach’s antenna, like a dog on a leash. The wasp lays an egg on the roach’s underside, and the roach simply stands there as the wasp larva emerges from the egg and digs into its abdomen. The genes of the wasp that encode the nenom molecules enlist the cockroach in the wasp’s survival plan by providing an ideal nursery for the wasp’s young.
- Baculoviruses infect the caterpillar of gypsy moths and a number of other species of moths and butterflies. When this happens, the caterpillar continues eating but the food is not resulting in new caterpillar tissue. Instead it becomes new baculoviruses. When the virus is ready to leave its host, the caterpillar becomes frenzied, eating without rest, and then beginning to climb, remaining on the tops of leaves where it is easily visible to predators. The bacoluviruses merge with the caterpillar cells turning the caterpillar into goo. As the caterpillar dissolves, clumps of viruses shower down onto the leaves below, to be ingested by new caterpillar hosts. The climbing behaviour of the caterpillars becomes an exquisite example of an extended phenotype. By causing their hosts to move up in trees, the baculoviruses increased their chances if infecting a new host down below.
- Similarly with the Amazonian ant Dinoponera longipes afflicted by the mindsccker Ophiocordyceps. When spores of the fungus land on an ant, they penetrate its exoskeleton and enter its brain, compelling the host to leave its normal habitat on the forest floor and scale a nearby tree. Filled to bursting with fungus, the dying ant fastens itself into a leaf or another surface. Fungal stalks burst from the ant’s husk and rain spores onto ants below to begin the process again.
- Larvae of the horsehair worm parasite Paragordus varius invade the house cricket Acheta domesticus as the latter scavenges dead insects. The larvae then grows inside it. The cricket is terrestrial, but the adult stage of the worm’s life style is aquatic, so when the mature worm is ready to emerge, it alters the brain of its host, driving the cricket to abandon the safety of land and take a suicidal leap into the nearest body of water. As the cricket drowns, an adult worm emerges, sometimes a foot in length.
- Compare the examples of the loa loa eye burrowing worm, a nematode transmitted to humans through fly bites, which burrows through the eyeballs of children in east Africa, blinding them, and the Ommatokoita elongata crustacean which permanently attaches itself to the front of the eyes of the Greenland Shark, permanently damaging their corneas and in the process rendering 90% of the species blind, noted at /the-selfish-gene.html