Guess what is similar to the dull wings of a fruitfly and the colourful ones of a butterfly
in the course of evolution, the same set of genes may have acquired new targets and caused the same fundamental structure -- the insect wing -- to develop into very differently patterned entities: the sombre wing of the fly and the spectacularly decorated wing of the butterfly. This is part of the findings of S D Weatherbee and colleagues at the University of Wisconsin, usa ( Current Biology , Vol 9, No 3).
A striking phenomenon, exhibited by a variety of insects and other invertebrates, is homeosis, the replacement of the one body part by another; for example a wing growing in place of a limb. Homeosis mostly results from a genetic mutation, but it can also follow the accidental loss of a body part. The process of regeneration misfires: instead of replacing the missing part, an extra copy of a body part already exisiting arises.
A well-known case of homeosis is the bithorax mutant in the fruitfly ( Drosophila melanogaster ) . In these mutant flies, the normal pattern of two wings and two balancers (halteres) is replaced by one of four wings: the balancers develop into wings. The gene that mutates is called ultrabithorax ( Ubx ). Research has shown that Ubx is a homeotic gene characterised by the homeobox, a unique sequence of dna . It is believed that proteins encoded in homeobox genes bind to other dna sequences and affect them. The nature of these 'target' genes varies. One possibility is that the same homeobox gene has the same target in all organisms. If this is true, there may well be a deep similarity in the principles on which all animals are constructed.
Weatherbee's team tested the hypothesis in the case of a butterfly homeotic mutant known as Hindsight . The effect of the mutation is to cause part of the normal pattern in the lower portion of the hindwing to be replaced by the corresponding part of the pattern on the forewing. The replacement is accompanied by the loss of activity of the Ubx gene product from patches of cells in the hindwings. The question was: did the absence of Ubx activity in the butterfly lead to malfunctioning of the same target genes as would have malfunctioned in Drosophila ? The answer was a clear no. Ubx represses a whole lot of genes in the Drosophila haltere but leaves them unaffected in the butterfly.
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