Changing role of even-skipped during the evolution of insect pattern formation

THE development of Drosophila is typical of the so-called long germband mode of insect development, in which the pattern of segments is established by the end of the blastoderm stage1,2. Short germband insects, such as the grasshopper Schistocerca americana, by contrast, generate all or most of their metameric pattern after the blastoderm stage by the sequential addition of segments during caudal elongation3. This difference is discernible at the molecular level in the pattern of initiation of the segment polarity gene engrailed4, and the homeotic gene abdominal-A (ref. 5). For example, in both types of insects, engrailed is expressed by the highly conserved germband stage4,6 in a pattern of regularly spaced stripes, one stripe per segment7–9. In Drosophila, the complete pattern is visible by the end of the blastoderm stage, although engrailed appears initially in alternate segments in a pair-rule pattern9–10 that reflects its known control by pair-rule genes such as even-skipped11*15. In contrast, in the grasshopper, the engrailed stripes appear one at a time after the blastoderm stage as the embryo elongates4. To address the molecular basis for this difference, we have cloned the grasshopper homologue of the Drosophila pair-rule gene even-skipped and show that it does not serve a pair-rule function in early development, although it does have a similar function in both insects during neurogenesis later in development.