Ecdysteroids metamorphosis

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... 

To date, interest in the ecdysteroids has focused primarily on their role in insect molting and metamorphosis and it is perhaps somewhat unfortunate that bioassays for ecdysteroid activity have been restricted almost exclusively to evaluating their effect on the molting process. It now appears probable that molting is just one of several important regulatory functions (e.g., embryogenesis, reproduction) performed by the ecdysteroids and that we are only just beginning to scratch the surface of an insect steroid system that is equally as complex as that in vertebrate species. 

The molting and other hormonal activities of the ecdysteroids are, of course, modulated by the titer of the insect juvenile hormone and the two materials typically function in close concert with one another in dictating insect molting and metamorphosis as well as in reproductive maturation (21). 

Knowledge of helminth endocrinology is rudimentary at present, though presence of a hormonal system similar to that of insects has been suggested [67]. It has been further pointed out that ecdysteroids and juvenile hormones, which control metamorphosis in insects, may also play a similar role in the growth of nematodes. This is based on the fact that ecdysteroids have been detected in nematodes, cestodes and trematodes [68-71],... 

The ecdysteroids are responsible for cellular programming in cooperation with the juvenile hormones secreted from the corpora allata. The juvenile hormones (JHs) regulate the result of a molt — whether a new larva or a pupa will be the next stage following the molt. When the amount of JH secreted is high, the epidermis is programmed for a larval molt. When the level declines, the cells are programmed for metamorphosis and pupa formations (in holometabolous insects). 

Ecdysteroids are the steroid hormones of insects, where they regulate moulding and metamorphosis. As ecdysteroids are essential to the normal development of insects, it was presumed that some plant secondary metabolites might have the ecdysteroid antagonistic activity and be capable of affecting insect development. Such compounds would be useful... 

Similarly, parasitism of P. separata, by A. kariyai or injection of the calyx fluid or virus material from the parasitoid, caused a decline in the host ecdysteroid titer and arrested metamorphosis of the host (54). Again, injection of exogenous 20-hydroxyecdysone reversed the developmental arrest. In this case, administration of prothoracicotropic hormone caused a reactivation of the prothoracic glands in the treated host. These results showed that the virus material inhibited the synthesis or secretion of prothoracicotropic hormone and also lowered the ecdysteroid level in the host. Also in this case, a mixture of both venom and calyx fluid were needed to obtain the full prolongation of the larval stage in the host (110). 

In another example, metamorphosis of the beetle Xyleb-orus ferrugineus will occur only if the larva receives the metabolic assistance of the fungus Fusarium solani. The latter organism has the ability to introduce a double bond into the nucleus of dietary sterols such functionality is an essential feature of all ecdysteroids. Larvae without the symbiotic fungus will metamorphose only if provided with er-gosterol (39) (Fig. 23.17), a sterol that has this double bond (Kircher, 1982). 

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