The transition from immature to adult is signaled by the elevation of ecdysteroid levels in the absence of JH. This is a one-step process in hemimetabolous insects. During the last nymphal instar of the cockroach Nauphoeta, JH levels fall from 5 to 10 ng/ml to less than 1 ng/ml prior to the next ecdysteroid peak (Fig. 2). Appearance of ecdysteroids at this low JH level signals a commitment to an adult gene expression pattern. Some examples of cellular responses to this adult commitment peak include mitosis in wingpad tissue, development of flight muscles, competence of gonadal accessory glands to differentiate, formation of external genitalia, and reorganization of the nervous system to accommodate these new adult structures. Also included are more subtle alterations, such as the relative proportions of body parts and addition of secondary sexual characteristics such as acoustic organs for communication.
In the Holometabola, complete metamorphosis requires an intervening pupal stage for remodeling of the larva into an adult. During the last larval instar of Manduca, ecdysteroids rise on two occasions, first in the absence of JH and later in its presence (Fig. 3). The first ecdysteroid pulse is a small one during days 3 to 4, referred to as the pupal commitment peak. This is the first time in the life history of the animal that a peak of 20HE occurs in the absence of JH. This triggers cessation of feeding and a wandering behavior aimed at locating a suitable site for pupation. The pupal commitment peak prepares the genome for its response to the next ecdysteroid peak. Although the second pulse of ecdysteroids occurs in the presence of JH, the commitment peak has changed the response of epidermal cells from a larval to a pupal secretory program. Similarly, imaginal discs respond to this peak by differentiating into adult tissues, something not observed in the previous larval stages.
In the Lepidoptera, the new hormonal milieu that triggers metamorphosis produces striking changes in the CNS and musculature. These tissues must be drastically altered during construction of the adult body form. Simultaneously, undifferentiated cells in imaginal discs proliferate and differentiate. For these tissues, the pupal commitment peak sets the stage for three types of cellular responses to the subsequent ecdysteroid peak: programmed cell death (apoptosis), cellular remodeling, or differentiation of imaginal discs. For example, some motoneurons that innervate larval-specific structures such as prolegs die shortly after pupal ecdysis. Others persist because of their involvement in the motor patterns involved in adult eclosion and die soon thereafter. Most larval neurons survive, but are remodeled to play roles in adult behavior.
The precise mechanisms governing cellular responses to the pupal commitment peak remain obscure, but the identification of EcR and USP subtypes has allowed monitoring of their expression during metamorphosis, for example, the response of epidermal cells of Manduca to the ecdysteroid peak during days 2 to 3 of the fourth instar by up-regulation EcR-B1, no change in EcR-A, and down-regulation of USP-1 (Fig. 3). However, during the fifth instar, the pupal commitment peak is correlated with sharply increased EcR-B1 and EcR-A expression, an altered USP-1 response. These altered patterns of expression apparently encode a change in downstream gene expression, leading to a pupal phenotype as well as imaginal disc differentiation during this stage.
During the pupal stage, ecdysteroid levels rise in the complete absence of JH (Fig. 3). This signals commitment to the adult phenotype and accelerated development of imaginal discs. It is remarkable that aE begins to rise on day 1 of the pupal stage, well before elevation of 20HE, and this is correlated with increases in both EcR-B1 and EcR-A expression. This suggests that aE may have a hormonal role itself in programming the adult stage. Elevation of 20HE occurs in two phases, one beginning on day 3 and a second, steeper rise on day 7. The slow gradual rise coincides with the adult commitment phase, whereas the steeper rise beginning on day 7 is associated with differentiation of new tissues. This latter phase coincides with a rapid rise of EcR-B1 and USP-1 expression.
The hormonal signaling mechanisms governing metamorphosis are complex and include a diversity of hormones, receptors, and varying temporal patterns of hormone release and receptor expression. Ecdysteroid signaling in the presence or absence of JH can set the stage for changes in the programming of target tissues, such as epidermal cells that secrete cuticle. Depending on the responses of EcR and USP subtypes, qualitatively different cellular programs are initiated.
Ecdysteroids • Embryogenesis • Imaginal Discs • Juvenile Hormone • Metamorphosis • Mating Behaviors • Temperature, Effects on Development and Growth
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