Homeotic (Hox) genes regulate the identity of structures

Many animal body plans are constructed of serially repeated structures that develop from initially equivalent fields of cells during embryogenesis. Serially homologous structures often attain different morphologies and functions within a single animal and changes in their number, form and function mark many important evolutionary differences between taxa (Carroll et al., 2001). Understanding this basic modular aspect of animal design and evolution requires detailed knowledge of the genetic and developmental mechanisms that regulate the formation and identity of body parts. In arthropods and vertebrates, selector genes play central roles in these processes and encode transcription factors that possess the distinct ability to direct the formation of specific tissues, such as the eye or heart and the differentiation of homologous body parts, such as segments, vertebrae and appendages. Selector proteins are proposed to regulate the expression of numerous target genes within regulatory networks that control the development of these tissues (Guss et al., 2001; Halder et al., 1998; Mann and Morata, 2000; Weatherbee et al., 1998). One class of selector genes, the Hox genes, encode homeodomain-containing proteins that regulate regional identity along the anteroposterior axis in animals and differentiate the identities of serially homologous structures such as vertebrae (Burke et al., 1995; Cohn and Tickle, 1999) and segments and appendages in arthropods (Abzhanov and Kaufman, 2000; Averof and Akam, 1995; Averof and Patel, 1997; Carroll et al., 1995; Grenier et al., 1997; Kaufman et al., 1990; Lewis, 1978; Panganiban et al., 1995; Rogers et al., 1997; Warren et al., 1994). Elucidating the mechanisms of Hox protein function is therefore critical to understanding the development and diversification of serially homologous structures. However, many facets of Hox target gene regulation are not well understood. Only a few direct genetic targets of Hox regulation have been identified, including cis-regulatory elements involved in Hox gene autoand cross-regulation (Appel and Sakonju, 1993; Beachy et al., 1993; Beachy et al., 1988; Bergson and McGinnis, 1990; Chan et al., 1994; Dessain et al., 1992; Ferretti et al., 2000; Frasch et al., 1995; Gould et al., 1997; Grieder et al., 1997; Haerry and Gehring, 1996; Jacobs et al., 1999; Li and McGinnis, 1999; Maconochie et al., 1997; Malicki et al., 1992; Pinsonneault et al., 1997; Popperl et al., 1995; Regulski et al., 1991; Thuringer et al., 1993; Zeng 3115 Development 129, 3115-3126 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV5020