A pair of research papers published this week report findings thatincrease our understanding of how an organism's body size is determinedand how the speed of its development is controlled. In particular, thework sheds light on the molecular and cellular pathways that act toconvey information about a growing organism's size, as well as onpathways that use that information to correctly time criticaltransitional events during development.
The two studies are reported in Current Biology online on September22 by Dr. Philip E. Caldwell and colleagues of Rice University and Dr.Christen Mirth and colleagues of the University of Washington.
Previous work had shown that there is a close linkage betweenthe final body size of an organism and the length of its developmentalstages: Elephants are larger and develop more slowly than mice.However, the mechanisms by which body size and developmental rate arecontrolled remain incompletely understood.
Both studies examine the control of larval development in thefruit fly Drosophila. Fruit flies undergo three successive larvalstages and molts before undergoing metaphorphosis and emerging as adultflies. In insects, it was previously found that the release of thehormone ecdysone from an endocrine organ called the prothoracic glandtriggers larval molting and, ultimately, metamorphosis. Thus,researchers have speculated that the timing of ecdysone release iscritical in determining both the final body size and developmental rateof an insect.
In their new work, Philip Caldwell, Magdalena Walkiewicz, andMichael Stern manipulated the timing and amount of ecdysone releaseduring development of the fruit fly. They induced precocious ecdysonerelease by specifically expressing an activated form of the signalingmolecule Ras in the endocrine prothoracic gland. This precociousecdysone release caused flies to develop more rapidly and exhibit amuch smaller body size than normal. In contrast, inhibiting Ras in theprothoracic gland prevented ecdysone release and delayed development,creating flies that are much larger than normal. On the basis of theirfindings, the investigators conclude that Ras activity in theprothoracic gland regulates body size and developmental rate byregulating ecdysone release.
In the second study, Christen Mirth, James W. Truman, and LynnM. Riddiford address how developing flies sense that they have reachedthe proper size to initiate a new phase of development. Their newfindings show that the prothoracic gland'the organ that releasesecdysone'itself acts as a size-sensing tissue. The researchers foundthat by manipulating the growth of specific cells within the gland,they were able to control the timing of metamorphosis and the body sizeof adult flies. They showed that artificial enlargement of theprothoracic gland appeared to cause an overestimation of the larvalflies' overall body size, prompting the initiation of metamorphosisbefore the flies surpassed the minimal viable weight necessary tosurvive pupation. On the basis of their findings, the authors proposethat under normal conditions, growth of prothoracic gland duringdevelopment helps larval flies determine when a critical body weighthas been reached and when metamorphosis should be initiated.
Mirth et al.: 'The Role of the Prothoracic Gland in DeterminingCritical Weight for Metamorphosis in Drosophila melanogaster.'Publishing in Current Biology online on September 22, 2005. DOI10.1016/j.cub.2005.09.017 www.current-biology.com.
Caldwell et al.: 'Ras Activity in the Drosophila ProthoracicGland Regulates Body Size and Developmental Rate via Ecdysone Release.'Publishing in Current Biology online on September 22, 2005. DOI10.1016/j.cub.2005.09.011 www.current-biology.com.
The researchers include Christen Mirth, James W. Truman, andLynn M. Riddiford of University of Washington in Seattle. This projectwas funded by the Royalty Research Fund and the Virginia and a PrenticeBloedel Professorship.
The researchers include Philip E. Caldwell, MagdalenaWalkiewicz, and Michael Stern of Rice University in Houston. This workwas supported by a National Institutes of Health grant.
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