Ecdysone, role

At the physiological level it is well established that vital dyes such as nile blue, neutral red and methylene blue retard larval development under normal lighting conditions (12L/12D with source unspecified) (25 27). Female but not male pupal weights are also reduced. Unfortunately experiments were conducted without dark controls so that it is difficult to evaluate the role of photosensitization in these effects. As house flies and fire ants succumb to photosensitization, they lose motor control and become more excitable (28). This suggested a neurotoxic effect and investigation of fire ant acetylcholinesterase vitro revealed that this enzyme was sensitive to photo-oxidation. vivo results, however, revealed no effect on the enzyme which suggests another mode of action. Epoxldatlon of cholesterol and membrane lysis may be alternative primary sites. If this were the case ecdysone metabolism of insects would probably also be effected. 

Juvenile hormone plays a critical role in maintaining the juvenile or larval stage of insects, and if its secretion is not controlled, normal development to the adult stage is prevented. Use of hormones or substances with hormonelike activity to control insect populations is an area of intense research interest and activity.2 The secretion of juvenile hormone is controlled by other hormones originating in the brain (brain hormone) and the phthoracic gland (moulting hormone, ecdysone see Table 30-2). 

Structure of /3-ecdysone and juvenile hormone. These hormones play major roles in the growth and maturation of insects by controlling the timing for molting of the insect exoskeleton. 

A recent study differentiated the role of JH and ecdysteroids in the ant Streblognathus peetersi. Ecdysone was correlated with dominant ants and reproduction while JH was more commonly associated with low-ranking ants and sterility (Brent et al., 2006). 

The steroids known to play major regulatory roles in insect development and metamorphosis all fall into the class of polyhydroxylated ketosteroids called ecdysones (19-22). With the exception of Makisterone A (a C2 ecdysteroid identified from the milkweed bug Oncopeltus fasciatus) the known insect ecdysteroids constitute a group of eight or nine steroids that differ from one another... 

One hypothesis on themecKanlsm of action of JH In the Inhibition of ecdysis Implicates the active role of JH In the regulation of ecdysone tltres In the Insect body (17), possibly a consequence of an effect on the prothoradc glands (22). Another hypothesis Implicates an effect of JH on ecloslon hormone or Its action (20, 18). 

Our present meagre understanding of normal endocrine processes in invertebrates makes the assessment of chemical endocrine disruption in the field difficult (LeBlanc 1999). Steroid roles differ between species and sometimes sexes, and their influence may vary at different developmental stages. In most studies of invertebrates, endocrine disruption appears to involve androgenization rather than oestrogenic effects (see Box 7.2). Arthropods (crustaceans and insects), annelids and molluscs use ecdysteroids, terpenoids and vertebratelike sex steroids for endocrine control. For example, the ecdysteroid ecdysone is naturally converted to 20-hydroxyecdysone (Fig. 7.10), which induces moulting (ecdysis) in both insect larvae and crustaceans. 

In addition to its role in puparium formation, ecdysone also regulates reproduction by stimulating metamorphosis of the gonads. 

Although, ecdysteroids occur in helminth tissue and affect a number of aspects of development, the true hormonal role of ecdysone in helminths remains to be documented. Insect juvenile hormones (JH) have also been implicated as regulators of nematode development, particularly in relationship to molting and exsheathment, but there is little analytical evidence supporting the occurrence of JH in nematodes. However, exogenously applied JH and synthetic JH analogs have been reported to alter developmental processes of nematodes (71-73). A concerted effort to apply modern analytical methods could resolve the question of the presence of insect JH in nematodes. 

---