How do developing organisms establish their basic body plan, and how well conserved are these mechanisms? In insects, the stereotypical segmentation of the anterior-posterior body axis can arise in two apparently distinct ways: some species, known as long-germ insects, pattern all their segments simultaneously, whereas others – short-germ species – do so sequentially. Using the fruit fly Drosophila melanogaster (long-germ) and the beetle Tribolium castaneum (short-germ), Erik Clark and Andrew Peel now show that these two modes of segmentation may actually be more similar than has previously been appreciated. The authors demonstrate that the expression patterns of three key transcriptional regulators (Caudal, Dichaete and Odd-paired) follow the same temporal sequence, but with different spatiotemporal patterns according to the underlying logic by which segments are formed – simultaneously or sequentially. Of these three regulators, Caudal and Dichaete have previously been implicated in segmentation in both long- and short-germ insects, but a role for Odd-paired had only been demonstrated in Drosophila. Clark and Peel now show that Odd-paired is also essential for segmentation in Tribolium. With these findings, the authors propose a series of steps – involving a heterochronic shift in segmentation gene expression – by which the ancestral mode of sequential segmentation might have evolved into the simultaneous mode observed in the fruit fly.
