Abstract

Tic-Toc Segmentation Clock Molecular oscillators are an essential component of vertebrate segmentation, but whether they exist in segmented animals in general has been controversial for almost a decade. Sarrazin et al. (p. 338, published online 8 March; see the Perspective by Roth and Panfilio) demonstrate the existence of a segmentation clock in the growth zone of insects. Microsurgical manipulation and embryo culture revealed cyclic expression of the segmentation gene Tc-odd in the beetle Tribolium castaneum, which suggests that segmentation clocks are a widely shared mechanism that mediates animal segmentation. Oscillating gene expression, a key feature of vertebrate segmentation, is shown to occur during segmentation in beetles. Vertebrate segmentation relies on a mechanism characterized by oscillating gene expression. Whether this mechanism is used by other segmented animals has been controversial. Rigorous proof of cyclic expression during arthropod segmentation has been lacking. We find that the segmentation gene odd-skipped (Tc-odd) oscillates with a two-segment periodicity in the beetle Tribolium castaneum. By bisecting embryos and culturing the two halves over different time intervals, we demonstrate that Tc-odd cycles with a period of about 95 minutes at 30°C. Using live imaging and cell tracking in green fluorescent protein–expressing embryos, we can exclude that cell movements explain this dynamic expression. Our results show that molecular oscillators represent a common feature of segmentation in divergent animals and help reconcile the contrasting paradigms of insect and vertebrate segmentation.