During most of my career I have been interested in the developmental challenges of metamorphosis and the way that hormones orchestrate the underlying cellular and molecular events. In insects, molting and metamorphosis are regulated by two hormones: ecdysone, which causes molting and promotes metamorphosis, and juvenile hormone (JH), which allows larval molting but prevents metamorphosis. The insect epidermis makes the overlying cuticle or exoskeleton and in most insects is polymorphic, in that it must switch genetic programs at the time of metamorphosis to the pupa, then to the adult. Generation of the adult also depends on imaginal discs and primordia that grow during larval life, then differentiate when cued by the hormonal milieu at metamorphosis. Finally, larval-specific structures die at metamorphosis. Internally the central nervous system undergoes metamorphosis by a similar combination of remodeling of some neurons, development of new adult-specific neurons that were generated by neuroblasts during embryonic and larval development, and cell death of larva-specific neurons.
Juvenile hormone, a sesquiterpenoid, plays two roles in maintaining the larval state: (1) It allows the modulation of ongoing gene expression by ecdysone but prevents the activation of new genes and repression of active genes by this steroid hormone that are necessary for switching of genetic programs. (2) It permits proliferative growth but prevents morphogenesis of imaginal discs and primordia. This morphostatic action does not require ecdysteroid. My major goal is to understand the molecular mechanism of action of this unique hormone in these two different roles.
Juvenile Hormone and Metamorphosis
Our studies on the epidermis of Lepidoptera have shown that JH can prevent both the ecdysone-induced appearance of the transcription factor Broad that is necessary for specifying the pupal program and the ecdysone-induced disappearance of Broad that is necessary for the subsequent adult developmental program. In Drosophila, by contrast, JH does not prevent metamorphosis of the imaginal disc–derived structures but can prevent the adult differentiation of the imaginal abdominal epidermis derived from the histoblasts. In this case, JH treatment prevents the normal disappearance of Broad from the abdominal epidermis during adult differentiation, resulting in the formation of a pupal, rather than adult, cuticle.
A second key transcription factor is Kruppel homolog 1α, which is found primarily in the larva and may be critical for JH action in the larva. It also regulates aspects of the timing of the ecdysone-induced transcription factor cascade at the onset of metamorphosis. It is aberrantly up-regulated by JH in Drosophila pupae and, in turn, regulates the misexpression of Broad in parts of the developing adult abdominal epidermis. Thus, the regulation of both "switch genes" and temporal and spatial controllers induced by ecdysone can be influenced by JH.
The development of the optic lobe in Drosophila at the onset of metamorphosis is providing us a system in which to probe the cellular and molecular basis of JH action. JH is necessary for its normal prepupal development but must be absent during adult development to allow its normal differentiation. The developmental defects in the optic lobe caused by the loss of JH are mimicked by the loss-of-function of the Methoprene-tolerant gene, supporting its essential role in JH action. These studies are being conducted in collaboration with James Truman (HHMI, Janelia Farm Research Campus).
Hormones and Behavior
In Drosophila, JH is known to be necessary for egg maturation. Also, the onset of female receptivity after eclosion in Drosophila appears to be mediated by JH. We are exploring this possibility, with the aim of identifying the neuronal circuit(s) affected by JH. We are also studying the possible role of JH in male behavior.
At the onset of metamorphosis, insect larvae cease feeding and begin wandering to find a pupation site. In Lepidoptera, this switch to wandering behavior is initiated by ecdysone acting in the absence of JH on the brain. I am beginning a search for the neurons involved in this switch in behavior in Drosophila, and will then study how these neurons are regulated by ecdysone and JH.
As of July 07, 2010