Abstract
In order to grow, immature insects must periodically synthesize a new cuticle and shed the old one (a process of molting and ecdysis) until they have reached a stage permitting metamorphosis to the reproductive adult. Each molt is induced by a pulse of ecdysteroids, but the nature of the molt is determined by the juvenile hormones (JHs) [1]. These are sesquiterpenoids, synthesized from three isoprene units via the mevalonate pathway and decorated by an epoxide group on one end and a methyl ester on the other [1]. The JHs maintain larval characters, a status quo effect that is lifted when the level of JH drops. A threshold size must be attained for each molt and for metamorphosis to occur, and this size may relate to the limit of oxygen supply by the tracheal system [2], [3]. The number of molts varies among and sometimes within species; it is influenced by nutrition and by environmental and genetic signals. Yet, the mechanisms that “measure size” or “count the larval instars” are overridden by the experimental depletion of JHs. It has long been a tenet of insect endocrinology that removal of the corpora allata (CA), the endocrine glands that produce the JHs, causes precocious metamorphosis [1]. However, bringing the JH titer down experimentally is not as trivial as it may seem.
Highlights
In this issue, Daimon et al [4] utilize the genetic resources of the silkworm Bombyx mori to explore why larvae of the dimolting mutant strain undergo metamorphosis early, giving miniature pupae and adults after just three instars rather than the normal five (Figure 1)
The work of Daimon et al is forceful in its elegance, and it brings a surprise: the corpora allata (CA) of mod mutants do not produce the juvenile hormones (JHs) normally needed for the status quo of larval development, yet the mod larvae do not commit to become pupae until the third larval instar (Figure 1)
Tarrant and Cupp [10] noted that the first-instar larvae of the true bug Rhodnius prolixus were ‘‘quite refractory’’ to chemical allatectomy with precocene
Summary
Daimon et al [4] utilize the genetic resources of the silkworm Bombyx mori to explore why larvae of the dimolting (mod) mutant strain undergo metamorphosis early, giving miniature pupae and adults after just three instars rather than the normal five (Figure 1). The work of Daimon et al is forceful in its elegance, and it brings a surprise: the CA of mod mutants do not produce the JHs normally needed for the status quo of larval development, yet the mod larvae do not commit to become pupae until the third larval instar (Figure 1).
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