There is an interesting consequence to the theory that the evolutionary basis of diversity within the bilaterians is cooptive change in developmental gene regulatory networks. This is that the morphological effect of an evolutionary change in network linkages depends entirely on where in the network the linkage is, rather than on other things; for example, the effect of the change cannot be predicted by the identity of the transcription factor or signaling system affected by it. Given transcription factors and signaling systems participate in multiple developmental regulatory networks, which are required to build different parts of the body plan. But often they also participate at different levels within the same network. The box genes again provide an example that is right to the point. Though their more famous functions are in the early-mid stages of the development of given body parts, they are also involved in numerous terminal developmental programs that control small details of morphogenesis.
Some examples are reproduced in Fig. 5.6 that illustrate the role of ¿oxgenes in terminal aspects of development. Evolutionary changes that affect these particular box gene functions would cause small morphological or physiological changes, of the sort that distinguish species from one another. For instance, in Fig. 5.6A we see that the ubx gene, the same which globally represses the wing patterning network in the haltere imaginal disc (Fig. 5.2), and that carries out so many other upstream developmental processes, also determines the fine spatial distribution of hairs (trichomes) on the 2nd femur of the Drosophila leg (Stern, 1998). In D. melano-gaster there is a small patch of naked cuticle lacking trichomes on the posterior side of the femur (Fig. 5.6A1); in D. simulans the naked patch is much larger; in D. virilis it is not there at all (Fig. 5.6A4). The naked patch depends on localized
FIGURE 5.6 Some effects of hox genes on species-specific, terminal mor-phogenetic functions. (A) Species-specific trichome pattern in Drosophila depends on ubx expression. (A I) Patch of naked cuticle (between arrow and arrowhead) on posterior side of the 2nd femur of the 2nd leg of D. melanogaster. Trichomes cover the
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