Larval Development The Intermolt

Because the exoskeleton places limits on growth, insect development occurs in stages, each ending with molting and cuticle shedding, or ecdysis. During the intermolt, which follows ecdysis, JH levels are maintained around 1 to 10 ng/ml in the blood (Fig. 2). It is presumed that these JH levels promote a high metabolic rate, active feeding behavior, synthesis of larval cuticle proteins, and continuous proliferation (but not differentiation) of imaginal discs. Growth during the immature stages is possible because the immature integument is predominantly unsclerotized procuticle, which is quite flexible compared with hard, sclerotized adult cuticle. Several mechanisms allow for larval cuticle expansion. The epidermal cells add new protein to the cuticle throughout the intermolt, increasing the surface area by intussusception. In addition, new cuticle in Manduca is deposited in vertical columns that are gradually reoriented during the feeding stage to allow for expansion. The increase in size during the larval stage can be quite impressive, as in fifth-instar Manduca, which increases its body weight from ~ 1 g on the first day of development to 15 g by the end of the instar, and its cuticular surface area by approximately fivefold just prior to pupation. Blood-feeding insects such as Rhodnius are known to release serotonin after a blood meal, which acts as a plasticizing agent, facilitating the enormous expansion of the body wall after a blood meal.

The Molt

At some point during each immature stage, growth results in a decision by the brain to initiate the molt. In Rhodnius, the simplest case known, stretch-receptor input from the abdomen to the brain causes release of PTTH, which induces synthesis and secretion of ecdysteroids from the prothoracic glands. In most insects, the decision to release PTTH is more complicated and less well understood, but it has to do with body weight, nutritional state, and time spent at that stage. The immediate effects of ecdysteroid elevation include cessation of feeding and apolysis, the detachment of the old cuticle from underlying epidermal cells. Apolysis of larvae results in head capsule slip, which occurs because the new head capsule is larger than the old one. This is the most visible sign that the molt has been initiated. If elevation of ecdysteroids occurs in the presence of JH, epidermal cells maintain the secretory program for immature phenotype, and larval cuticle is secreted (Fig. 2). Through the action of molting fluid, most components of the old cuticle are broken down and recycled into the new layer.

During the period of new cuticle synthesis, ecdysteroids also orchestrate gene expression crucial to the synthesis and action of peptide hormones that control ecdysis behaviors. Ecdysis is a complex process in which the old cuticle is shed not only from the surface of the animal, but also from the lining of the foregut, the hindgut, and the inner walls of the tracheal system. Success in this process depends on completion of new cuticular synthesis, attachment of the musculature to this new cuticle, and digestion of the old cuticle. In addition, the animal prepares for a sequence of Houdini-like escape behaviors necessary to shed the old cuticle. These consist of preecdysis and ecdysis behaviors. The ability to perform these behaviors depends on orchestration of a peptide signaling cascade involving the central nervous system and the epitracheal endocrine system.

For the ecdysis signaling cascade to be functional at the appropriate time, ecdysteroids orchestrate gene expression in four ways. Genes are activated in epitracheal glands to increase production of ecdysis triggering hormones (ETHs). Release of ETHs initiates ecdysis behaviors through direct action on the CNS. Although the CNS is not sensitive to ETHs during the feeding stage, acquisition of sensitivity occurs upon elevation of ecdysteroids, specifically around the time of apolysis. Third, the nervous system becomes competent to release EH, a peptide hormone that targets Inka cells to cause release of ETH. Finally, elevated ecdysteroids exert a negative influence on the secretory competence of Inka cells. As long as ecdysteroids remain high, Inka cells are unable to secrete ETHs in response to EH exposure. This latter effect of ecdysteroids, to block release of ETHs from Inka cells,

FIGURE 3 Hormonal regulation of development in postembryonic stages of the moth, M. sexta, exemplifying the Holometabola. Elevated JH levels occur throughout larval development in Manduca, in which metamorphosis is encoded by two ecdysteroid peaks. During the fifth instar, JH levels drop and an ecdysteroid pupal commitment peak (PC) signals a change in the way epidermal cells respond to the secretory program from larval to pupal phenotype. This peak also triggers cessation of feeding and wandering behavior. The next time epidermal cells are exposed to ecdysteroid and JH on day 6 leads to secretion of pupal cuticle. During the pupal stage, aE and 20HE rise in the complete absence of JH, signaling adult commitment. Sensitive periods of epidermal cell commitment are shown as L/P (larval vs pupal specification) and P/A (pupal vs adult specification of imaginal disc tissue). Fluctuating levels of ecdysteroid receptor subtypes (EcR- B1, EcR-A) and USP subtypes (USP-1, USP-2) are shown below. (Adapted from Riddiford and Truman, 2001, and Baker et al, 1987.)

FIGURE 3 Hormonal regulation of development in postembryonic stages of the moth, M. sexta, exemplifying the Holometabola. Elevated JH levels occur throughout larval development in Manduca, in which metamorphosis is encoded by two ecdysteroid peaks. During the fifth instar, JH levels drop and an ecdysteroid pupal commitment peak (PC) signals a change in the way epidermal cells respond to the secretory program from larval to pupal phenotype. This peak also triggers cessation of feeding and wandering behavior. The next time epidermal cells are exposed to ecdysteroid and JH on day 6 leads to secretion of pupal cuticle. During the pupal stage, aE and 20HE rise in the complete absence of JH, signaling adult commitment. Sensitive periods of epidermal cell commitment are shown as L/P (larval vs pupal specification) and P/A (pupal vs adult specification of imaginal disc tissue). Fluctuating levels of ecdysteroid receptor subtypes (EcR- B1, EcR-A) and USP subtypes (USP-1, USP-2) are shown below. (Adapted from Riddiford and Truman, 2001, and Baker et al, 1987.)

appears designed to ensure that ecdysis does not occur prematurely. The mechanism of block involves a crucial step in the EH-induced secretory mechanism of Inka cells. Declining ecdysteroid levels at the end of the molt provide the necessary signal permitting expression of one or more genes needed for secretory competence.

It is believed that initiation of ecdysis behaviors occurs as a result of an ongoing conversation between EH neurons and Inka cells. When Inka cells become sensitive to EH, ETH release initiates preecdysis behavior, which is thought to loosen the remaining connections between the new and the old cuticle. The transition from preecdysis to ecdysis occurs upon depletion of ETH from Inka cells. It is thought that the action of ETH activates a downstream cascade of peptide signaling within the CNS to regulate each unit of the behavioral sequence. Included in this cascade is a neuropeptide called crustacean cardioactive peptide (CCAP), named for its initial discovery and biological activity. In the context of insect ecdysis, CCAP appears to be an immediate chemical signal within the CNS for activation of peristaltic ecdysis behavior. Upon escaping the old cuticle, the animal is surrounded by a new soft cuticle and is therefore extremely vulnerable to injury. Release of the neuropeptide bursicon from neurosecretory cells of the CNS accelerates sclerotization of the cuticle.

In summary, ETH, EH, CCAP, and bursicon regulate ecdysis at all stages. This includes embryonic ecdysis in Manduca, and adult eclosion.

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