The evolution of neural patterning and the achaete-scute complex. S.R. Wheeler 1, S.J. Brown 2, J.B. Skeath 1. 1) Dept Genetics, Washington University, St Louis, MO; 2) Kansas St Univ, Manhattan, KS.
Most animals share a common set of genes and regulatory networks that govern similar developmental events in different organisms. Thus, alterations to the number, regulation or function of members of this genetic toolkit likely underlie much of the morphological diversity found in animals. The evolution of HOX genes are an important model, but modification to HOX genes may be restricted by their central role in development. We have begun a comparative analysis of the achaete-scute complex (AS-C) that largely determines the nervous system pattern in invertebrates. Like Hox genes, the AS-C genes are located in a complex and encode transcription factors. However, alterations to the AS-C display only localized effects on nervous system patterning not global effects on body plan organization. Thus, the AS-C may provide an more plastic model to uncover the genetic basis of morphological diversity. The AS-C is divided into two activities encoded by the proneural and neural precursor genes. Proneural genes instruct ectodermal cells in choosing the neural precursor fate making them the primary determinant of neural patterning. Once neural precursors form they activate neural precursor gene expression which aids in their differentiation. Drosophila has three proneural genes, achaete, scute, and lethal of scute and one neural precursor gene, asense. We have extended the study of AS-C genes to two other insect species; Tribolium castaneum (Coleoptera) and Thermobia domestica (Apterygota). We find two AS-C genes in Tribolium, one proneural and one neural precursor, that are closely linked forming the Tribolium AS-C. In Thermobia we find only a single AS-C gene which, at the amino acid level, has characteristics of both the proneural and neural precursor genes. These data are consistent with the model that the Drosophila AS-C arose by serial duplication and subfunctionalization of a single ancestral AS-C gene that performed both the proneural and neural precursor functions.