One of the most magical and inspiring spectacles of the natural world is metamorphosis. We have all heard the familiar story The Very Hungry Caterpillar, by Eric Carle, in which a homely caterpillar eats voraciously before mysteriously disappearing into a cocoon and miraculously emerging as an elegant butterfly.
In fact, all holometabolous insects undergo metamorphosis. Holometabolous insects are ones that have four distinct stages of development: embryo, larva, pupa, and adult. The most widely studied holometabolous insect is the fruit fly species Drosophila melangogaster. For our purposes, we will analyze Drosophila development as a representation of all holometabolous insects.
So, how exactly is this phenomenon triggered?
It turns out, that hungry caterpillar was gorging himself for a reason! Nutrition plays a key role. In all holometabolous insects, a critical weight must be reached before metamorphosis can be initiated. Now, we pose the question:
How does an individual determine when they have reached critical weight?
A group of scientists from the University of Washington decided to test this very question, and published their results in an article titled “The Role of the Prothoracic Gland in Determining Critical Weight for Metamorphosis in Drosophila melangogaster,” by Christen Mirth, James W. Truman, and Lynn M. Riddiford.
The authors hypothesized that insulin-dependent growth of the prothoracic gland was part of a mechanism that assesses when critical weight has been achieved. To test their hypothesis, they screened for tissues that might be involved in size assessment. Specifically, they overexpressed PI3K or Dp110 to enlarge specific tissues relative to the larval body size. Conversely, they overexpressed PTEN to reduce their size. They predicted that if an organ were involved in size assessment, then enhancing its growth would produce small adults, whereas suppressing its growth would result in abnormally large adults.
To better understand the manipulations made, it is important to know a little about the Insulin signaling pathway in Drosophila. The best explanation I found about this topic was in a review written by the same authors called “Size assessment and growth control: how adult size is determined in insects.” by Christen K. Mirth and Lynn M. Riddiford. The Drosophila insulin receptor (InR) plays different roles in pre- and post- critical-weight larval growth. The InR cascade appears to be involved in regulating the development and is downstream of the size-assessment event occurring at critical weight. The Drosophila insulin-like peptides are the likely ligands for InR. InR in turn, activates the substrate which recruits Dp110 (PI3K) to the membrane via its adaptor protein p60. Dpp10 converts PIP2 to PIP3. The accumulation of PIP3 in the membrane results in increased cell growth. PTEN is a phosphatase that converts PIP3 to PIP2, thus suppressing cell growth. The balance between cellular levels of PIP3 and PIP2, regulated by Dp110 and PTEN, makes tissues larger or smaller.
What they found was that changing the size of the prothoracic glands produced changes in size that suggested that they might be involved in size assessment.
Larvae with prothoracic glands that had been enlarged by overexpressing PI3K had remarkably reduced critical weight and metamorphosed to tiny adults.
They concluded that the insulin-dependent growth of the prothoracic gland is involved in determining when critical weight has been reached. Furthermore, there is evidence that the signal involved in size assessment is ecdysteroid concentration. In larvae with enlarged glands, the individuals showed an increase in the transcription of two genes that encode enzymes involved in ecdysteroid synthesis: phantom and disembodied, and of the ecdysteroid -inducible gene E74B, indicating elevated ecdysteroid signaling.
In addition, larvae in which prothoracic gland growth was suppressed showed overgrowth phenotypes, but if they were fed food containing ecdysteroids, they metamorphosed at normal sizes. Most likely, ecdysteroid concentrations are important during size assessment.
Given the findings, the scientists proposed a model for size assessment in Drosophila:
The insulin pathway controls the growth of all tissues and can input directly on the size assessment system by controlling the growth of the prothoracic gland. Thus, the mechanism that controls adult size integrates both the pathways involved in controlling nutrition-dependent growth with those that regulate developmental transitions. Pre-critical weight, JH represses the secretion of prothoracicotropic hormone (PTTH), which supresses ecdysteroid synthesis by the prothoracic glands. After reaching critical weight, JH synthesis is shut off, allowing PTTH release. PTTH induces ecdysteroid secretion, which initiates the cessation of feeding and the onset of metamorphosis.
This article had many strengths. I found it to be clear and well-organized. The images of the flies and the glands were clearly focused and easy to see. The data visualizations, tables, and graphs of the fly measurements were relatively easy to interpret as well. Furthermore, the experimental procedures were explicitly documented at the end of the article, including the recipes of the flies’ diets, how the growth was measured, and the protocol for the enzyme immunoassay performed.
Mirth, C., Truman, J.W. and Riddiford, L.M. (2005). The role of the prothoracic land in determining critical weight for metamporphosis in Drosophila melangogaster. Curr.Biol. 15, 1796-1807.
Mirth, C. and Riddiford, L.M. (2007). Size assessment and growth control: how adult size is determined in insects. BioEssays. 29, 344-354.
Life Cycle of Drosophila melangogaster: http://3-b-s.eu/drosophila%20melanogaster%20lifecycle.html
Insulin-Signaling Pathway pictures (second image and third image): http://www.ncbi.nlm.nih.gov/pubmed/17373657
Drosophila pictures during development from the experiment: http://www.cell.com/current-biology/retrieve/pii/S0960982205010444