Ecdysteroids in insects

In 2009 a retired British planktologist Donald I. Williamson published a controversial paper in the journal Proceedings of the National Academy of Sciences (via Academy member Lynn Margulis through a unique submission route in PNAS that allowed members to peer review manuscripts submitted by colleagues), wherein Williamson claimed that the caterpillar larval form originated from velvet worms through hybridogenesis with other organisms, giving rising to holometabolan species. [14] This paper was met with severe criticism, and spurred a heated debate in the literature.

During the period of metamorphosis, the fat body tissue undergoes extensive morphological, histological, biochemical, and organizational changes. These processes are triggered by the molting hormone on the background presence of extremely low levels of the JH. Such alterations have been thoroughly studied in dipterans and lepidopterans. Two major strategies for transforming the larval fat body into an adult tissue exist: (1) the histolytic pathway, in which the larval fat body adi-pocytes in dipteran species are completely histolyzed and the adult new tissue is formed from undifferentiated stem cells, and (2) the remodeling pathway, in which adipocytes in the larval stage of lepidopteran insects dissociate at metamorphosis into individual cells before being reassociated into the adult new tissue. In certain holometabolous insect species, a combination of the two processes takes place.
Dynamic exchanges of nutrients between fat body cells and the hemolymph compartments are evident throughout the life cycle of holometabolous insects (Fig. 1). Buildup of reserves and their partial

FIGURE 1 Exchange of stored reserves between fat body cells and hemolymph during the life cycle of holometabolous insects. Asterisk indicates that later, as stem cells are differentiated into adult fat body cells, a buildup of reserves occurs.
use at the molting periods are the characteristics of the larval stages. During the prepupal period, mass quantities of reserve material are accumulated in the fat body cells. Lysis of fat body cells in higher dipteran species at metamorphosis results in the discharge of stored reserves into the hemolymph. However, as the new adult fat body cells are formed, nutrients are reabsorbed. In contrast, in lepidop-teran species in which cell remodeling occurs, a status quo prevails. Fat body cells in adults are depleted of reserve materials, which are used for locomotion and reproduction

Ecdysteroids in insects

ecdysteroids in insects

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