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Published on: July 12, 2014
A molt timer is involved in the metamorphic molt in Manduca sexta larvae
Yuichiro Suzuki1, Takashi Koyama, Kiyoshi Hiruma
1Department of Biology, University of Washington, Seattle, WA 98195-1800, USA.
Abstract:
Manduca sexta larvae are a model for growth control in insects, particularly for the demonstration of critical weight, a threshold weight that the larva must surpass before it can enter metamorphosis on a normal schedule, and the inhibitory action of juvenile hormone on this checkpoint. We examined the effects of nutrition on allatectomized (CAX) larvae that lack juvenile hormone to impose the critical weight checkpoint. Normal larvae respond to prolonged starvation at the start of the last larval stage, by extending their subsequent feeding period to ensure that they begin metamorphosis above critical weight. CAX larvae, by contrast, show no homeostatic adjustment to starvation but start metamorphosis 4 d after feeding onset, regardless of larval size or the state of development of their imaginal discs. By feeding starved CAX larvae for various durations, we found that feeding for only 12-24 h was sufficient to result in metamorphosis on day 4, regardless of further feeding or body size. Manipulation of diet composition showed that protein was the critical macronutrient to initiate this timing. This constant period between the start of feeding and the onset of metamorphosis suggests that larvae possess a molt timer that establishes a minimal time to metamorphosis. Ligation experiments indicate that a portion of the timing may occur in the prothoracic glands. This positive system that promotes molting and the negative control via the critical weight checkpoint provide antagonistic pathways that evolution can modify to adapt growth to the ecological needs of different insects.
Insights
Insect larvae use a critical weight checkpoint and a molt timer to regulate metamorphosis. Nutrition, especially protein, influences this timing, overriding size and development cues in hormone-deficient larvae.
Area of Science:
- Insect physiology
- Developmental biology
- Endocrinology
Background:
- Manduca sexta larvae are a model for insect growth control.
- Critical weight is a threshold for metamorphosis, influenced by juvenile hormone.
- Allatectomized (CAX) larvae lack juvenile hormone, imposing the critical weight checkpoint.
Purpose of the Study:
- To examine nutrition's effects on CAX larvae lacking juvenile hormone.
- To understand the interplay between critical weight and nutritional status.
- To identify the mechanisms controlling the timing of metamorphosis.
Main Methods:
- Larval starvation and refeeding experiments.
- Diet composition manipulation (protein focus).
- Ligation experiments to assess tissue-specific timing mechanisms.
Main Results:
- CAX larvae initiate metamorphosis 4 days after feeding onset, irrespective of size.
- Short feeding durations (12-24h) trigger metamorphosis timing.
- Dietary protein is critical for initiating metamorphosis timing.
- Prothoracic glands may contribute to a larval molt timer.
Conclusions:
- Insect metamorphosis timing is regulated by antagonistic pathways: a critical weight checkpoint and a molt timer.
- Nutritional cues, particularly protein, can initiate metamorphosis timing.
- This regulatory system allows adaptation of insect growth to ecological demands.
