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 stage. 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 elongation. This difference is discernible at the molecular level in the pattern of initiation of the segment polarity gene engrailed, and the homeotic gene abdominal-A (ref. 5). For example, in both types of insects, engrailed is expressed by the highly conserved germband stage in a pattern of regularly spaced stripes, one stripe per segment. 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 pattern that reflects its known control by pair-rule genes such as even-skipped. In contrast, in the grasshopper, the engrailed stripes appear one at a time after the blastoderm stage as the embryo elongates. 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.