Triple-labelled confocal image of a field of Drosophila neuroblasts that are cell-cycle arrested by a mutation in the string (cdc25) gene. Neuroblasts are stained for Krüppel (green), Pdm1 (blue) and Eagle (red). krüppel is transiently expressed early in neuroblast lineages, where it specifies the birth order identity of neuroblast progeny, but in this embryo most neuroblasts remain Krüppel positive because of cell-cycle arrest. Photograph by Bret Pearson, University of Oregon, USA.

Just as a person's character is believed to be shaped by their birth order relative to their siblings, so the identity of many neurons is determined by the sequence in which they are generated. The mechanisms that control this temporal mode of patterning are just beginning to be understood, and as reported in Cell, Isshiki et al. have gained important new insights into a molecular mechanism that enables neurons in Drosophila to 'remember' their birth order.

The Drosophila neural precursor cells, or neuroblasts, undergo a series of asymmetric cell divisions, budding off smaller ganglion mother cells (GMCs), most of which divide once more to generate two neurons. Neurons that are born early populate the deep cell layers and develop long axonal processes, whereas later-born neurons colonize increasingly more superficial layers and develop shorter processes. Birth order is also reflected in the expression of transcription factor genes; early-born neurons express the hunchback gene (hb), neurons in the middle layers express pdm and late-born neurons express castor (cas). In addition, Isshiki et al. have identified a population of cells between the hb- and pdm-expressing layers that express the Krüppel (Kr) gene.

The authors analysed the expression of these four genes in neuroblasts during development, and showed that, irrespective of neural lineage, they are almost invariably expressed transiently in neuroblasts, following the sequence hb → Kr → pdm → cas. The GMCs and their neuronal progeny inherit and stabilize the gene expression profile of the neuroblast at the time of their birth. In loss- and gain-of-function experiments, Isshiki et al. showed that the expression of birth order genes in GMCs is both necessary and sufficient to specify birth order identity (based on gene expression, neurotransmitter expression and/or axon morphology). However, although the fate of cells expressing specific genes is invariant in a given lineage, there is considerable variation between lineages. For example, hb-expressing GMCs can give rise to motor neurons, interneurons or even glia, depending on the neuroblast from which they arise.

This study raises some intriguing questions. For example, how does the expression of the genes become stabilized in the GMCs to create a birth-order 'memory'? It also remains to be seen how the expression sequence is regulated. Isshiki et al. found evidence of regulation within the pathway, with each factor able to activate the next gene in the sequence while repressing the next-but-one gene. However, this is unlikely to be the main controlling mechanism, as inactivation of any one gene causes only minor perturbations in the expression sequence. The authors suggest the involvement of an additional clock mechanism that is linked to the cell cycle.

In certain regions of the vertebrate nervous system, including the cortex and the retina, cells are generated in a highly stereotyped temporal sequence. Mammalian homologues of some of the Drosophila birth order genes have already been identified, and future studies should show whether temporal patterning in vertebrates is controlled by a similar mechanism to that described by Isshiki and colleagues.