1. From molecules to morphologyMetazoan development is often viewed as a series ofinformation processing events in which the genomecomputes where and when to synthesize particular geneproducts. Importantly, development is also a feat ofengineering in which genetic information is used to changecell shape and cell–cell interactions to generate form. Oneof the major players in the path from molecules tomorphogenesis is the actin cytoskeleton. Actin filamentstogether with myosin motors help control the shape of alleukaryotic cells and, in multi-cellular animals, generate theforces required for many morphogenetic processes; fromcytokinesis, axon path-finding and muscle contraction toepithelial folding. In each case, the precise form, locali-sation and mechanical properties of the requisite actinfilament network are regulated by the action of a distinct setof actin-binding proteins. Thus, for many processes, actin-regulatory proteins can be considered the ‘architects’ of thecell. In this review, we use examples taken from Drosophiladevelopment to explore ways in which the complement ofactin-regulatory proteins expressed by a cell together withlocal environmental cues shape the cytoskeleton to bringabout animal morphogenesis.2. The actin cytoskeletonActin is a globular protein that exists in dynamicequilibrium, cycling between monomeric and filamentousstates(Fig.1A,B).Ourunderstandingofthekineticsofactinfilament formation and disassembly owes much to bio-chemical studies investigating the properties of purifiedactin and actin-binding proteins (Higgs and Pollard, 2001;Pollard and Borisy, 2003). In vitro, the rate-limiting step inthe polymerisation of a new actin filament from monomersis nucleation (Mullins et al., 1998). By analogy, the rate ofnew actin synthesis in cells is thought to be dominated bythe availability of non-actin templates to which actinmonomers can be added and by the availability of freeactin filament ends that are generated by the uncapping orsevering of existing filaments. Once initiated, actinfilaments rapidly extend in a polarised fashion as actin-ATP monomers are preferentially added to the growingfilament end (Fig. 1B). Commonly known as the ‘barbed’ orplus end. As the filament ages, the ATP moeity ishydrolysedandphosphateisreleased(CarlierandPantaloni,1988). The resulting ADP-actin filaments are disassembledthrough the loss of actin monomers from the filament‘pointed’ (minus) end and by the action of actin-severingproteins (Bamburg et al., 1999). This cycle of actinpolymerization and disassembly is extremely rapid, so thatwhile the total numbers of actin filaments remainsapproximately constant in a migrating cell, most individualfilaments have but a fleeting existence (Watanabe andMitchison, 2002). The hydrolysis of ATP accompanyingfilament turnover providing the free energy to powerdirectional filament growth and the rapid re-mobilizationof actin-based structures.A further consideration is that many organisms expressmultiple actin isoforms (Fyrberg et al., 1980; Goldstein andGunawardena, 2000; Lovato et al., 2001; Mounier et al.,1992). In Drosophila, the six actin proteins are expressed indistinct patterns (Fyrberg et al., 1983), which, althoughdifferinginamino-acidsequenceatonlyaveryfewresidues,cannot substitute for one another (Fyrberg et al., 1998). It is