The Evolution of the Neural Basic Helix-Loop-Helix Proteins
Basic Helix-Loop-Helix (bHLH) transcription factors control various aspects of the formation of the nervous system in the metazoans. In Drosophila some bHLH (such as the achaete-scuteatonal, and amos genes) act as proneural genes, directing ectodermal cells toward a neural fate. Their vertebrate orthologs, however, probably do not assume such a neural determination function, but rather control the decision made by neural precursors to generate neurons and not glial cells, as well as the progression of neuronal precursors toward differentiation into mature neurons. The proneural function of Drosophila bHLH genes may be an innovation that occurs in the evolutive lineage that leads to arthropods. In addition, although neural bHLH appear to be involved in the specification of neuronal identities, they probably do not confer by themselves neuronal type-specific properties to the cells. Rather, neural bHLH allow neural cells to correctly interpret specification and positional cues provided by other factors. Although bHLH genes are often expressed in complementary subsets of neural cells and/or expressed sequentially in those cells, the coding regions of the various neural bHLH appear largely interchangeable. We propose that the specific expression patterns have been acquired, following gene duplications, by subfunctional-ization, i.e., the partitioning of ancestral expression patterns among the duplicates and, by extension, we propose that subfunctionalization is a key process to understand the evolution of neural bHLH genes.