What's the difference between mice and men?
It's often said that a man shares 30% of his genes with a banana, rather more with a fruit fly, and yet more with a mouse; so why are these organisms so different if many of their genes are so similar? In this issue of PLoS Biology, Pat Simpson and her colleagues for Cambridge University use fruit flies to shed some light on the problem.
Genes have two parts: a bit that codes for proteins (this part is similar in different organisms) and a part that regulates the gene, which determines when, where, and how strongly the genes are turned on. To the layman, one fruit fly looks much like another, but there are, in fact, nearly 4,000 distinct species differing in characteristics such as the pattern of veins on the wings or the position of bristles on the thorax. These differences have evolved over millions of years, since the first ancestral fruit fly arose--and it's not just flies. Every species--man, mouse (and bananas)--has evolved a unique form, correlated with subtle differences in their genes, but where in the genes do these subtle differences lie? By comparing genes between two closely related species of flies, one with two rows of bristles on the thorax and one with four rows, these workers homed in on a short part of the control region of one particular gene that appeared to be different. By swapping the bit around, they could turn a two rows fly into a four rows fly.
These results show that slight changes in the control regions of genes lead to slight changes in the organism, and accumulation of such small differences may result in the creation of a new species. While this is widely accepted by evolutionary biologists, actual examples directly linking small changes in regulatory parts of genes to morphological differences have been scarce. While it may not be possible to swap regulatory sequences to turn a man into a banana, or even into a mouse, examination of these sequences may tell us how different organisms evolved.
Citation: Marcellini S, Simpson P (2006) Two or four bristles: Functional evolution of an enhancer of scute in Drosophilidae. PLoS Biol 4(12): e386. DOI: 10.1371/journal.pbio. 0040386.
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