February 25, 2009

Aphids Devoted To Repairing Galls

According to findings that have been published in Proceedings of the Royal Society B, an aphid species has "soldiers" who often die in the process of repairing damage to galls, the homes that aphids force their plant to grow.

The aphids gather around a hole in the gall and squeeze out a fluid comprising two-thirds of their body size, using their legs to mix it and form a "scab".

The presence of the aphids after the scab formed is the only possible way for full recovery of the plant tissue to take place.

Many soldier aphids, of the species Nipponaphis monzeni, die from loss of body mass.

Others get stuck in the fluid and fail to escape, becoming a physical part of the building work.

Utako Kurosu of the Tokyo University of Agriculture in 2003 first noted the aphids' self-sacrifice, and called it "the most elaborate social behavior so far known among aphids."

The fate of the galls remained unclear after N. monzeni's emergency repair mission.

The gall's survival is tied to that of the creatures, which mature and escape fully-grown in the autumn, as a living part of the plant and a food source for the growing aphid.

The progress was followed by Takema Fukatsu of the University of Tokyo and colleagues, which found a number of damaged galls after the short-term fluid fix were performed.

The team found that the galls that were left unpatched were more likely to die.

Also, the soldier aphids' efforts did not stop by just plugging the gap; they tended to cluster around the damaged area for weeks afterwards.

"After the hole is plugged by solidified body fluid, soldier nymphs manipulate the growth and regeneration of plant tissue nearby the breach in an intricate manner, which leads to complete sealing of the hole by plant tissue," Fukatsu told BBC News.

The regeneration did not occur when aphids inside the gall were prevented from clustering around the damage after the initial fix.

The evolutionary path that led to the repair behavior is hard to unpick, because N. monzeni is the first species to be observed exhibiting it, said Fukatsu.

Currently, research is underway to better understand the mechanism that the soldiers' body fluid forms the paths.  According to Fukatsu, the fluid contains several molecules that are involved in the clotting when insects themselves get injured.

"Once the gall is actually growing with the aphids inside, you'd think it'd be in the plant's interest not to help them out," said Peter Smithers, an entomologist at the University of Plymouth."

"This is the aphids fooling the plant into doing something that it doesn't need or want to do. It's an interesting evolved set of behaviors and physiologies that are closely linked, that have co-evolved."

"But the aphids are calling the shots."


Image Courtesy Wikipedia


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Proceedings of the Royal Society B

Tokyo University of Agriculture

University of Tokyo