Parasegmental organization of the spider

The arthropod body consists of metameric units that become manifest as the segments at the germband stage (Anderson, 1973; Scholtz, 1997). The molecular mechanisms that underlie the segmentation process have been best studied in the insect Drosophila, where segmentation genes act in a hierarchic cascade; as a result, the metameric embryo is formed. A remarkable feature of Drosophila segmentation is that the fundamental developmental units are not the segments, but the parasegments that are defined by functional compartment boundaries (Martinez-Arias and Lawrence, 1985; Lawrence, 1988; Patel, 1994). The crustacean body is also initially built from units that resemble the parasegmental modules in insects (Patel et al., 1989a; Patel et al., 1989b; Patel, 1994; Dohle and Scholtz, 1988; Scholtz, 1997). The subdivision of the anteroposterior body axis in the insect Drosophila results from the successive action of the maternal, gap, pair rule and segment polarity genes (Ingham, 1988; St Johnston and Nüsslein-Volhard, 1992; Pankratz and Jäckle, 1993). The pair rule genes delimit the parasegments, the initial metameric units in Drosophila, and define the domains that will express the segment polarity genes, such as engrailed and wingless (Lawrence et al., 1987; DiNardo and O’Farrell, 1987; Ingham, 1988; DiNardo et al., 1988; Baker, 1988). The engrailed gene encodes a homeobox-containing protein that is involved in establishing and maintaining the parasegmental boundaries in the Drosophila embryo. The anterior domain of the parasegment that expresses engrailed corresponds to the future posterior part of the segment in Drosophila as well as in other insects (Rogers and Kaufman, 1996; Schmidt-Ott et al., 1994; Patel et al., 1989a; Patel, 1994). Drosophila embryos in which engrailed is expressed uniformly are unsegmented (Lawrence et al., 1996). In addition, embryos that lack both wingless and engrailed function are unsegmented. The alternation of cells that express engrailed and non-expressing cells is essential for segmentation, and determines how these cells respond to morphogens (Lawrence et al., 1996). In malacostracan crustaceans, Engrailed is expressed in the newly forming segments in the most anterior row of four rows of cells that form a genealogical unit (Patel et al., 1989a; Patel, 1994; Scholtz et al., 1994; Scholtz, 1995; Scholtz and Dohle, 1239 Development 129, 1239-1250 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV7918