Internal Controls in Developing Systems

An important discovery is that biological systems are capable of regulating their development to ensure that all of the parts are in the correct relative positions. An excellent example is seen in an experiment performed by Barry Sinervo (Sinervo, B. and Huey, R.B. 1990. Allometric engineering: testing the causes of interpopulation differences in performance. Science 248:1106-1109.): he removed material from one lizard egg and injected it into another lizard egg, thereby producing one egg that was much smaller than normal, and one that was much larger than normal. The lizards that hatched from these eggs were normally-formed lizards, but smaller or larger than normal. This kind of experiment (manipulating eggs or embryos) has been performed with many different species, and leads to the very clear conclusion that developing systems can control their relative proportions. That is, removing half of the material in an egg does not result in a lizard with half of its body parts. It produces a normal-looking, but small, lizard.

A more recent finding shows the importance of this kind of "internal control," as illustrated in the figure below. A genetic change that alters the length of a fish jaw does not just produce fish with jaws that are too short in an otherwise normal head. Instead, the entire head changes shape to accommodate the shorter jaw. This is somewhat surprising, since the jaw and the other structural elements of the head develop from different groups of cells. Apparently, the changes in the head result from the same kind of developmental plasticity as was seen with the too-large and too-small lizards.

This is important because it illustrates that a mutation that alters one piece of a very complex structure need not cause the structure to fail, because developmental controls can often accommodate the change.

There is a general misconception, or perhaps "worry," that a mutation that changes the size of one body part relative to others would disrupt the entire organism. The pieces wouldn't fit. We now know from experimental manipulations and from observations of naturally-occurring mutations, that the pieces do fit. Developmental controls don't just make the pieces; they make the pieces and the connections among the pieces, so that the overall organism works.

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Last updated: 31 December 2005
Comments: Jose Bonner, OSO
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