Discovery

The discovery of juvenile hormones began in the 1930s, with a series of ingenious experiments conducted by Sir Vincent Wigglesworth aimed at elucidating the hormonal control of molting. Trained as a medical doctor, Wigglesworth dedicated his life to basic studies of insect physiology, believing that knowledge gained would hold the keys to controlling insect vector-borne disease and agricultural pests. As a model experimental insect, Wigglesworth chose the Chagas' disease vector Rhodnius prolixus, otherwise known as the "kissing bug" because of its habit of sucking blood from the lips of sleeping humans. The choice of Rhodnius was inspired, because its development is closely timed to its blood meals. This allowed Wigglesworth to precisely determine the physiological stage of the insects to coincide with his experimental manipulations. He found that 3 days after a blood meal, hormones are released into the Rhodnius system, stimulating the molt to the next stage. By performing a number of surgical procedures on the bug he demonstrated the source and timing of hormone release. Part of the advantage of working with insects as experimental animals is that they can survive for long periods without such seemingly vital organs as the brain, a fact that Wigglesworth took advantage of. He found that decapitation of animals prior to a 3-day critical period led to an arrest in development, even though the animal would remain alive for many months. If the brain was reimplanted, development resumed. He also found that the blood of a normally developing animal could reactivate development in the headless animal. This was achieved via a technique called parabiosis, in which the devel-opmentally arrested animal was joined to the normal one by means of a tube, which allowed blood from the two animals to mix. With these experiments, Wigglesworth demonstrated that hormones released from the brain trigger molting. This discovery actually had been made more than a decade earlier by Stefan Kopec, working with gypsy moth, but Wigglesworth's experiments revealed a new type of hormone, one that influenced the form taken by the animal after each molt.

Wigglesworth fundamentally changed the thinking about insect development, specifically the distinction between regulation of growth and regulation of form by separate hormones. Rhodnius passes through five nymphal stages before molting to the adult form. It is easy to tell the adults from the nymphs, because of differences in pattern and color of the cuticle, as well as the fact that only adults have wings. Wigglesworth found that parabiosis of a fifth (last)-stage nymph with a young nymph prevented the former's metamorphosis to the adult stage. Instead, the animal molted to a sixth-stage nymph, an extra immature stage that never occurs normally. A chemical in the blood of the young insect promoted continued expression of larval characters, and this factor came to be known as the "juvenile hormone." The source of the juvenile hormone was traced to a pair of small glands behind the brain called the corpora allata (Fig. 1). Surgical removal of the gland did not interfere with molting, but drastically altered the form taken after the molt, causing animals to become precocious adults. Reimplantation of the glands led to the return of larval characters.

While the corpora allata proved to be the sole source of juvenile hormone, only very small amounts were available from the gland for chemical studies. The short supply of JH greatly constrained experimentation, slowing the process of discovery considerably. A breakthrough came with the discovery of

FIGURE 1 Photomicrograph of the corpora allata (CA), paired, spherical glands that are the sole source of the juvenile hormones in insects. Also shown are the elongated, white corpora cardiaca (CC). The CC and CA are positioned behind the brain, where they release hormones into the blood. Structures shown were dissected from the cockroach, Periplaneta americana. (Photograph courtesy of Dr. S. J. Kramer.)

FIGURE 1 Photomicrograph of the corpora allata (CA), paired, spherical glands that are the sole source of the juvenile hormones in insects. Also shown are the elongated, white corpora cardiaca (CC). The CC and CA are positioned behind the brain, where they release hormones into the blood. Structures shown were dissected from the cockroach, Periplaneta americana. (Photograph courtesy of Dr. S. J. Kramer.)

large amounts of JH in abdomens of adult male silk moths by Carroll Williams. Ether extracts produced a dark orange material he called the golden oil. Such an abundance of juvenile hormone in a male adult at first was surprising, but already Wigglesworth had noted the essential gonadotropic role of JH associated with reproduction, that is, stimulation of egg and sperm development. The reason for enormous quantities of JH in male adult abdomens is likely caused by its inclusion in spermatophores, which contain sperm together with nutritive and hormonal stores and are provided to the female during mating for fertilization and nutrition of developing eggs. Williams is credited with stimulating the modern era of JH research, by making available enough of the natural hormone to conduct biological experiments on its modes of action in many types of insects. This work also provided quantities of starting material sufficient for the eventual isolation and chemical identification of the hormone, which occurred in the late 1960s and early 1970s.

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