Clonal unit architecture of the adult fly brain.

During larval neurogenesis, neuroblasts repeat asymmetric cell divisions to generate clonally related progeny. When the progeny of a single neuroblast is visualized in the larval brain, their cell bodies form a duster and their neurites form a tight bundle. This structure persists in the adult brain. Neurites deriving from the cells in this duster form bundles to innervate distinct areas of the brain. Such clonal unit structure was first identified in the mushroom body, which is formed by four nearly identical clonal units each of which consists of diverse types of neurons. Organised structures in other areas of the brain, such as the central complex and the antennal lobe projection neurons, also consist of distinct clonal units. Many clonally related neural circuits are observed also in the rest of the brain, which is often called diffused neuropiles because of the apparent lack of dearly demarcated structures. Thus, it is likely that the clonal units are the building blocks of a significant portion of the adult brain circuits. Arborisations of the clonal units are not mutually exclusive, however. Rather, several clonal units contribute together to form distinct neural circuit units, to which other clones contribute relatively marginally. Construction of the brain by combining such groups of clonally related units would have been a simple and efficient strategy for building the complicated neural circuits during development as well as during evolution.