- Open Access
Big or fast: two strategies in the developmental control of body size
© Nijhout. 2015
- Published: 4 August 2015
Adult body size is controlled by the mechanisms that stop growth when a species-characteristic size has been reached. The mechanisms by which size is sensed and by which this information is transduced to the growth regulating system are beginning to be understood in a few species of insects. Two rather different strategies for control have been discovered; one favors large body size and the other favors rapid development.
- Juvenile Hormone
- Critical Weight
- Adult Body Size
The control of the timing of this ecdysone pulse was first investigated in Manduca [4, 5]. About half way through the fifth and final larval instar, larvae of Manduca pass what has been called the critical weight. This is the size at which the physiological events that lead to the secretion of ecdysone are set in motion. During this instar the secretion of ecdysone is repressed by juvenile hormone (JH), and at the critical weight the secretion of JH stops and expression of the enzyme JH-esterase (the principal catabolic enzyme for JH) is upregulated. Suppression of ecdysone secretion is relieved over a period of 24–60 hours (depending on the genetic background). Thus, the final size of the larva is set by the value of the critical weight and the duration of the period during which JH is eliminated, called the terminal growth phase (TGP).
The size-monitoring mechanism that senses the critical weight was quite unexpected, and involves oxygen limitation by a non-growing tracheal system that becomes unable to keep up with the ever-increasing oxygen demands of a growing body. The size at which oxygen becomes limiting corresponds to the critical weight . Rearing Manduca larvae under hypoxia lowers their critical weight and also lowers the size at which ecdysone is secreted and growth stops. A similar mechanism operates in Drosophila where larvae reared under hypoxia likewise yield miniature adult flies , presumably because the metamorphic molt is triggered prematurely. In both species the first step in the cascade that controls body size is oxygen restriction. In Manduca oxygen limitation defines the critical weight and triggers the decline in JH. In Drosophila there is no role for JH in regulating ecdysone secretion.
One of the most striking differences between Manduca and Drosophila is that Drosophila apparently has little or no JH in the third and final larval stage. And since in Manduca JH plays the key role in regulating body size, it was natural to assume that in Drosophila control of size must be quite different. This idea was enhanced by the discovery that interference with insulin signaling could have dramatic effects on adult body size in Drosophila. Suppression of insulin production or inactivation of the insulin receptor or receptor substrate resulted in miniature flies , whereas overexpression of insulin resulted in giant flies .
Another interesting difference between Manduca and Drosophila is their response to starvation after passing the critical weight. When Manduca larvae are starved after they have passed the critical weight their time to ecdysone secretion and pupation is identical to that of larvae that continue to feed normally . By contrast, when Drosophila larvae are starved their time to pupariation is accelerated . This premature metamorphosis has been called the bailout mechanism . It is a common adaptation in animals that live in evanescent habitats where finding a new source of food after the current one runs out is not possible, though the mechanism of the bailout response is still unknown.
Because of these differences in the endocrine biology of the two species, work on the regulation of size in Manduca has focused mostly on the physiology of the critical weight and the role of JH in controlling ecdysone secretion, while in Drosophila most work has focused on the role of insulin signaling and the relative roles of members of the insulin signaling network such as FOXO and TOR .
Although many aspects of the morphology and physiology of Drosophila are highly derived compared with other insects, it seemed difficult to understand why Drosophila would have evolved such a radically different control mechanisms over body size than what was found in Manduca. This puzzle might be resolved if there were only a way to make Drosophila more like Manduca, or Manduca more like Drosophila.
It is the latter that was accomplished in the paper by Hatem et al. . It turns out that there is a genetic strain of Manduca, the black strain, that, besides having black larvae and a much smaller adult body size, has a reduced level of JH during the larval stage . In this latter respect it resembles Drosophila.
What Hatem et al.  did was to feed wild-type and black larvae on a diet containing rapamycin, which inhibits TOR signaling, one of the terminal steps in the insulin pathway. They found that although growth was slowed down in both strains, as one might expect, the wild-type strain eventually grew to a normal size whereas the black strain grew much larger than normal. Interestingly, the critical weight was not altered in either strain when fed rapamycin. The increase in size in black larvae must therefore have been due to a lengthening of the TGP.
Another interesting property of the black strain is that when larvae that have been fed rapamycin are starved, their time to metamorphosis is accelerated, just as is observed in the bail-out response in Drosophila, but which does not happen in the wild-type strain of Manduca. Thus, in several respects the developmental physiology of the black strain is more like that of Drosophila than that of wild-type Manduca.
It is interesting to speculate which of these control systems is primitive. Since the JH sensitivity and the critical weight occur later in development than the nutrition-sensitive control and the MVW, it looks like it might have a later evolutionary origin. Alternatively, it could be that, driven by the adaptive significance of being able to bail out of a deteriorating environment, Drosophila abandoned control by JH.
Hatem et al.  suggest that the JH-controlled mechanism favors large body size, but at the cost of more prolonged development, whereas eliminating the repression of ecdysone secretion by JH favors rapid development time. Thus, the differences between the control mechanisms in Manduca and Drosophila are most likely adaptations for very different life histories.
Which of these strategies is likely to be the ultimate cause of the difference remains to be elucidated. In the meantime, Hatem et al.  have taken a significant step in resolving the puzzling differences between Manduca and Drosophila, and this work lays the foundation for novel experimental approaches to uncover more fully the details of the developmental physiology of body size regulation.
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