As the genomes of more and more organisms are completed, the power of comparative genomics is becoming increasingly apparent. Cell biologists can use the wealth of genomic data to integrate knowledge gained from genetically tractable organisms, such as the fruitfly Drosophila melanogaster, with their understanding of cell biology — often gained from cells or organisms whose genomes are harder to manipulate.

Take limb development, for example, described in a Review by Ginés Morata on page 89. Many years of work on the development of appendages in fruitflies have revealed a general developmental programme for making an appendage, albeit with variations depending on the appendage's position. At least parts of this programme have been conserved from insects to mammals, and the framework laid down by Drosophila biologists is now being used to work out the details of vertebrate limb development.

A Highlight on page 84 provides another example of the utility of species hopping: a screen that aimed to identify Drosophila mutants with cell-motility defects identified taiman, which is related to a mammalian steroid receptor co-activator produced in excess in some breast and ovarian cancers. Work in the humble fruitfly therefore reveals a new function for steroid receptor signalling that could be relevant to tumour invasiveness.

If a pathway works, it tends to be maintained throughout evolution, even if it's not perfect. This phenomenon is discussed in a Perspective on page 147 by Anthony Poole, David Penny and Britt-Marie Sjöberg. Swapping uracil for thymine when life was first evolving solved the problem caused by cytosine's tendency to deaminate to uracil: DNA repair enzymes evolved to recognize uracil in DNA and repair the fault. However, when methylation evolved, we were back to square one: methylcytosine deaminates to thymine, which cannot be differentiated from the thymines that are meant to be there. This double-edged sword might have speeded up evolution though; without such imperfections, perhaps we would still be fruitflies.