( Many baffled parents have wondered whet...)Researchers find gland that tells fruit flies when to stop growing

Now University of Washington biologists studying the physiology of Drosophila melanogaster, the common fruit fly, have discovered an organ that assesses the size of the juvenile and signals when it has reached a critical weight to begin metamorphosis into an adult.

The team, led by postdoctoral researcher Christen Mirth, found that the prothoracic gland, a major endocrine organ situated just in front of the brain, assesses the fly's size as it grows during the larval stage. The gland then sends hormonal signals when it senses the fly has reached a size appropriate to enter adulthood.

The scientists found they could use the pathway that sends insulin to a fly's cells to genetically manipulate the size of the prothoracic gland, part of a more complex structure called the ring gland, and send false signals about a fly's weight. Enlarging the gland by increasing insulin signaling triggered metamorphosis at smaller sizes than usual. Suppressing the gland's growth by decreasing insulin signaling allowed larvae to grow larger than usual before entering the pupal stage that precedes adulthood.

Mirth and her colleagues, UW biology professors Lynn Riddiford and James Truman, surmised that size assessment had to be accomplished through a major endocrine organ, so they screened all fruit fly endocrine glands, enlarging or reducing them and studying the effect on body size.

"The only thing that gave us the size shifts that we had hypothesized was changing the size of the prothoracic gland. Enlarging the organ made the animals small, and vice versa," Mirth said. "It seems to be a nutrition-related phenomenon. You sort of trick the fruit fly into thinking it is bigger than it really is."