This slip of cuteness -- for, yes, they are cute, a favorite pet of children, worthy of a place in the teddy bear stores -- this fragile slip of cuteness survives New England's deep freeze, one of the hardy insects that winter over in the larval stage. In spring, it wakes, has a bite to eat, then rolls itself into a pupa, using its hairs to make the cocoon, lacing them together with silk. Two weeks later, an Isabella tiger moth emerges, presto-chango, like a magician's trick.That was a decade ago. The brain-secreted hormone that triggers the transformation had been known for a long time. Now, in the 4 December issue of Science, researchers at the University of Minnesota and the University of North Carolina identify the receptor of the hormone and its signaling cascade, taking us closer to unraveling the chemistry of what would appear to be an inexplicable mystery. Here is the abstract of their report. Don't worry about understanding it; I don't. But we get the drift. Drosophilia, by the way, is the ever-helpful fruit fly.
A black-and-brown woolly bear goes into the box -- a wave of the wand -- a yellow-winged tiger moth emerges. Somehow, the creature has managed to remake itself, rearranging its atoms, from crawling fuzzball to airborne angel. In few insects is the transformation so stunning, so complete. An insatiable leaf-eating machine becomes a sex-obsessed nectar-sipper. Shape, color, internal organs, mode of transportation -- all changed. It's as if an elephant became a swan, or a rattlesnake became a parakeet.
Of course, the totality of the transformation is to some extent illusory. What remains constant through all the stages of metamorphosis is information. It's all there, at the heart of every cell, in the DNA, blueprints for making a woolly bear and a tiger moth. There are clusters of cells in the larval caterpillar that are destined to become anatomical features of the adult moth, dormant, awaiting a chemical signal that will make them surge into activity. The warmth of spring releases hormones from glands in or near the brain. These cause the caterpillar to build a chrysalis and begin metamorphosis. Previously dormant adult cells begin to multiply. They take their nutrients from superceded larval cells, which are transformed into a kind of nutrient soup for the benefit of the growing adult organs. The woolly bear's six stumpy front feet are turned into the tiger moth's slender legs. Four bright wings develop, as do reproductive organs. Chewing mouth parts become adapted for sucking. In two weeks, the rearrangement of atoms is complete. The chrysalis breaks.
There's no way to think about this without gasping for breath. It's one thing to understand the biology, at least that part of it that we know something about: DNA, hormones, gene expression, and all that. But knowing the biology only makes the metamorphosis all the more breathtaking. Not magic at all, but a fierce, inextinguishable force driving the universe, Dylan Thomas' "green fuse," permeating every atom of matter, soaking nature the way water soaks a sponge. Call it life, call it God, call it an inch-and-a-half of black and brown fur. It can't be ignored when you hold it curled in your hand, a gram of divinity.
Holometabolous insects undergo complete metamorphosis to become sexually mature adults. Metamorphosis is initiated by brain-derived prothoracicotropic hormone (PTTH), which stimulates the production of the molting hormone ecdysone via an incompletely defined signaling pathway. Here we demonstrate that Torso, a receptor tyrosine kinase that regulates embryonic terminal cell fate in Drosophila, is the PTTH receptor. Trunk, the embryonic Torso ligand, is related to PTTH, and ectopic expression of PTTH in the embryo partially rescues trunk mutants. In larvae, torso is expressed specifically in the prothoracic gland (PG), and its loss phenocopies the removal of PTTH. The activation of Torso by PTTH stimulates extracellular signal–regulated kinase (ERK) phosphorylation, and the loss of ERK in the PG phenocopies the loss of PTTH and Torso. We conclude that PTTH initiates metamorphosis by activation of the Torso/ERK pathway.Does understanding the chemistry of metamorphosis disenchant the world. In a sense, yes. But in a more important sense, no. It replaces one enchantment with another -- bigger, more encompassing. The metamorphosis of the woolly bear becomes part of the biochemical fire that animates the world. The two blockquoted passages above -- from Natural Prayers and Science -- do not stand in opposition. They complement each other. Mind and heart. Knowledge and feeling. Science and poetry go hand in hand.