Pax group III genes paired, gooseberry and gooseberry- neuro in short germ flour beetles and grasshoppers. During Drosophila embryogenesis, paired acts as one of several pair-rule genes

We currently know a great deal about how the Drosophila embryo becomes progressively subdivided into its future body segments. Gradients of maternal information act at the top of a genetic hierarchy that involves the sequential activation of the zygotic gap, pair-rule, and segment polarity genes. In phylogenetically derived long germ insects such as Drosophila, this genetic hierarchy functions to define and pattern all segments almost simultaneously within the blastoderm. In short germ insects, however, only segments of the head are defined in the initial blastoderm, while the remaining segments of the thorax and abdomen form progressively from a posterior growth zone (Sander, 1976). Thus, crucial questions arise as to which components of the Drosophila segmentation hierarchy are shared by different short germ insects and how they might function in the short germ context. To better understand segmentation in short germ insects, we have chosen here to focus on homologs of the pair-rule and segment polarity class of segmentation genes in flour beetles and grasshoppers. Segment polarity genes were originally defined by their loss-of-function phenotypes in Drosophila, which reveal patterning defects within each segment of the embryonic cuticle (Nüsslein-Volhard and Wieschaus, 1980). Consistent with their phenotypes, most of these genes are expressed in Drosophila just before and throughout the morphologically segmented germ band stage in a segmentally reiterated pattern. The segment polarity genes that have been most widely studied in other insects are engrailed (en) and wingless (wg). In Drosophila, each is expressed as a single ectodermal stripe within each individual segment, defining the anterior and posterior boundaries, respectively, of each parasegment. These same patterns have thus far been found in all insects examined (reviewed by Patel, 1994a; Dearden and Akam, 2001), suggesting that segment polarity genes constitute part of the ancestral insect segmentation system. Indeed the role of these genes in patterning segments is likely to be ancient, as they are also expressed in segmental stripes in embryos of non-insect arthropods such as crustaceans (reviewed by Patel, 1994b; Nulsen and Nagy, 1999) and spiders (Damen et al., 1998). In contrast to segment polarity genes, pair-rule homologues tend to exhibit more divergent patterns. Pair-rule genes were also originally defined by their loss-of-function phenotypes in Drosophila, in which regions of the embryonic cuticle are deleted with a two-segment periodicity (Nüsslein-Volhard and 3445 Development 128, 3445-3458 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV5934