The bristles of a fruitfly demonstrate how the concept of 'prepatterns' governs biological development
in developmental biology, two philosophically distinct models have dominated the field of pattern formation. The subject matter of the field is, broadly speaking, the question of how spatial patterns arise during the course of transition of an egg into an adult. The patterns could be those formed by the stripes along a zebra's back or sensory hairs (bristles) on the thorax of insects. The two models ascribe different types of ultimate causes responsible for the final pattern.
One model goes by the name of 'prepattern'. As the word suggests, it implies that behind the final pattern there is a similar but more elementary pattern. For example, a prepattern for the zebra's stripes could be formed by a stripe-like distribution of a substance that can cause dark pigmentation.
The other model is associated with the phrase 'positional information'. It ascribes the final pattern to an act of creation as it were. It is based on the fact that even in a homogenous system, given the right sort of chemistry, order and pattern can emerge spontaneously. The theory of positional information is attractive because it does away with the need for a primary cause. As a concept, prepatterning is messy because it forces you to explain the origin of the prepattern itself, and then perhaps of a pre-prepattern, and so on. However, it appears that prepatterns, rather than systems of positional information, are the rule in development. This may be a pointer to the fact that living organisms are products of the process of evolution by natural selection.
Pat Simpson of the Institute of Genetics in Strasbourg, Germany, has put forward the latest example in this area: the pattern of sensory bristles in the fruitfly Drosophila melanogaster. As in other two-winged insects, the thorax of this fly has sets of large sensory bristles, known as macrochaete, arranged in rows. Comparative evidence indicates that the bristle pattern has been in existence for some 50 million years (see Figure 1). It has been known for some time that the differentiation of macrochaete in Drosophila depends on the activity of two genes, achaete ( ac ) and scute ( sc ). To begin with, these genes are active in small groups of cells -- the proneural clusters -- which give rise to the precursors of sensory organ. Later, their activity becomes restricted to the sensory organs themselves ( Current Biology , Vol 6, No 8).
In short, where the macrochaete develop depends on where the proneural clusters are formed, and that in turn depends on where the ac - sc set is active. Recent evidence indicates that two newly identified genes, iroquois ( iro ) and pannier ( pnr ), encode protein factors that influence the pattern of macrochaete. It turns out that the two genes have complementary domains of activity on the thorax of the fly and, accordingly, affect complementary sets of macrochaete (see Figure 2). Mutations in pnr can lead to an increase or decrease in bristle number. It has also been found that pnr can either activate or repress ac - sc (depending on the presence or absence of other gene products). Similarly, the products of the iro genes (there are two of them) have been found to regulate ac - sc .
It should be noted that both pnr and iro are expressed in longitudinal stripes along the thorax -- that is, in a pattern that is strikingly similar to that of the longitudinal bristle rows. An interesting hypothesis follows from this, and from the observed interactions of iro and pnr with ac - sc: the pattern of thoracic bristles found in the flies, the diptera (order of two-winged insects), might be guided by a prepattern of activity of genes such as iro and pnr .
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