Shaping the growing wings of Drosophila melanogaster requires 'cell competition' — a process in which cells that proliferate too slowly, for whatever reason, are eliminated. A report in Nature now indicates that cells in the developing wing disc compete for the survival factor Decapentaplegic (Dpp) in order to escape death.

Cells with so-called Minute (M) mutations grow slowly as they are defective in producing ribosomal proteins, so they provide a useful tool for studying cell competition. In the wing disc, heterozygous Minute mutant (M−/+) cells expand less when wild-type M+/+ cells are present, which indicates that there is a control mechanism that compensates for the comparatively faster growth of M+/+ cells by eliminating the M−/+ cells.

But M−/+ cells, although slow to proliferate, are still viable, so how are they eliminated? One possibility was that they underwent apoptosis, and TdT-mediated dUTP nick-end labelling (TUNEL) assays showed this to be true. As c-Jun amino-terminal kinase (JNK) is known to be involved in wing-cell apoptosis, the authors studied whether it could be responsible for specifying the demise of M−/+ cells — and also showed that this was the case.

The authors then made the connection that the product of a gene called brinker ( brk ) activates JNK, but is itself repressed by Dpp, a protein that promotes growth in Drosophila wings. So, might the absence of Dpp cause an increase in Brk, which could then induce JNK-mediated apoptosis? This is indeed what the authors found. If brk is expressed inappropriately in regions in which it is normally absent or low, cells are eliminated. Consistent with this, M−/+ clones that express a brk-null allele are not eliminated — they continue to proliferate more slowly than M+/+ cells.

Co-expression of baculovirus p35 protein, which inhibits apoptosis, allows brk-overexpressing cells to survive, as does repression of JNK activity. Among the issues that remain to be addressed is how ectopic brk expression activates JNK signalling and how the Dpp signal becomes limiting in slow-growing cells. But the model for cell competition that is proposed by the authors extends beyond slow-growers, and Dpp is probably just one of several factors that cells fight for to survive.