Japanese scientists have discovered how a parasitic worm can “control” its host, the praying mantis, so that the worm itself can complete its life cycle in the water. To this end, the worm appears to have incorporated a whole package of genes from its host into its own DNA, a phenomenon known in evolutionary biology as ‘horizontal gene transfer’. This gives the worm the keys to manipulate the behavior of the host in which it lives. The researchers reported their findings this week in the scientific journal Current Biology.
Horsehair worms are thin, aquatic worms that grow from larva and cyst to adult worm as a parasite in insects or crustaceans. Crickets, grasshoppers and praying mantises often serve as definitive hosts for the worm. When the worm reaches adulthood, it induces its host to engage in strange jumping behavior, often causing the insect to end up in the water. As soon as this happens, the long worm crawls out of its victim to live freely in the water in search of a mating partner and produce a new generation of worms.
Very dangerous
Two years ago discovered by the same group of Japanese biologists Led by Takuya Sato of Kobe University, mantises infected with adult worms are attracted to horizontally polarized light, such as that reflected from a water surface in the sun. This explains why the infected insects so often go to the water, which is life-threatening for them because of hunting fish, while it is only beneficial for their parasite. Somehow the worm gets the mantis so crazy.
In their new publication, the Japanese have found an indication of how the worm does this, namely through genes obtained by deception. A genetic analysis showed that the horsehair worm (Chordodes fukui) has inherited as many as 1,400 genes from its definitive host, the praying mantis (Tenodera angustipennis). That is one of the largest horizontal gene transfers biologists have ever observed. Given the very high similarity in the nucleotide sequence of these genes (with 1,342 genes, the similarity between worm and grasshopper is more than 94.5 percent), this takeover of genes must have occurred recently from an evolutionary perspective.
Things got interesting when the Japanese started looking at the activity of these stolen genes during different stages of the worm’s life. During the phase in which the worm manipulates the behavior of its host, many of these genes suddenly appeared to be remarkably active. This may also include genes that promote the host to be attracted to the reflection of light on the water. They identify at least two candidates for this. But whether this actually works needs to be proven in a subsequent experiment.