The Drosophila brain develops from the procephalic neurogenic region of the ectoderm. About 100 neural precursor cells (neuroblasts)delaminate from this region on either side in a reproducible spatiotemporal pattern. We provide neuroblast maps from different stages of the early embryo(stages 9, 10 and 11, when the entire population of neuroblasts has formed),in which about 40 molecular markers representing the expression patterns of 34 different genes are linked to individual neuroblasts. In particular, we present a detailed description of the spatiotemporal patterns of expression in the procephalic neuroectoderm and in the neuroblast layer of the gap genes empty spiracles, hunchback, huckebein, sloppy paired 1 and tailless; the homeotic gene labial; the early eye genes dachshund, eyeless and twin of eyeless; and several other marker genes (including castor, pdm1, fasciclin 2, klumpfuss, ladybird,runt and unplugged). We show that based on the combination of genes expressed, each brain neuroblast acquires a unique identity, and that it is possible to follow the fate of individual neuroblasts through early neurogenesis. Furthermore, despite the highly derived patterns of expression in the procephalic segments, the co-expression of specific molecular markers discloses the existence of serially homologous neuroblasts in neuromeres of the ventral nerve cord and the brain. Taking into consideration that all brain neuroblasts are now assigned to particular neuromeres and individually identified by their unique gene expression, and that the genes found to be expressed are likely candidates for controlling the development of the respective neuroblasts, our data provide a basic framework for studying the mechanisms leading to pattern and cell diversity in the Drosophilabrain, and for addressing those mechanisms that make the brain different from the truncal CNS.

BackgroundThe heads of annelids (earthworms, polychaetes, and others) and arthropods (insects, myriapods, spiders, and others) and the arthropod-related onychophorans (velvet worms) show similar brain architecture and for this reason have long been considered homologous. However, this view is challenged by the 'new phylogeny' placing arthropods and annelids into distinct superphyla, Ecdysozoa and Lophotrochozoa, together with many other phyla lacking elaborate heads or brains. To compare the organisation of annelid and arthropod heads and brains at the molecular level, we investigated head regionalisation genes in various groups. Regionalisation genes subdivide developing animals into molecular regions and can be used to align head regions between remote animal phyla.ResultsWe find that in the marine annelid Platynereis dumerilii, expression of the homeobox gene six3 defines the apical region of the larval body, peripherally overlapping the equatorial otx+ expression. The six3+ and otx+ regions thus define the developing head in anterior-to-posterior sequence. In another annelid, the earthworm Pristina, as well as in the onychophoran Euperipatoides, the centipede Strigamia and the insects Tribolium and Drosophila, a six3/optix+ region likewise demarcates the tip of the developing animal, followed by a more posterior otx/otd+ region. Identification of six3+ head neuroectoderm in Drosophila reveals that this region gives rise to median neurosecretory brain parts, as is also the case in annelids. In insects, onychophorans and Platynereis, the otx+ region instead harbours the eye anlagen, which thus occupy a more posterior position.ConclusionsThese observations indicate that the annelid, onychophoran and arthropod head develops from a conserved anterior-posterior sequence of six3+ and otx+ regions. The six3+ anterior pole of the arthropod head and brain accordingly lies in an anterior-median embryonic region and, in consequence, the optic lobes do not represent the tip of the neuraxis. These results support the hypothesis that the last common ancestor of annelids and arthropods already possessed neurosecretory centres in the most anterior region of the brain. In light of its broad evolutionary conservation in protostomes and, as previously shown, in deuterostomes, the six3-otx head patterning system may be universal to bilaterian animals.

In the Drosophila embryo, studies on CNS development have so far mainly focused on the relatively simply structured ventral nerve cord. In the trunk, proneural genes become expressed in small cell clusters at specific positions of the ventral neuroectoderm. A lateral inhibition process mediated by the neurogenic genes ensures that only one cell within each proneural cluster delaminates as a neural stem cell (neuroblast). Thus, a fixed number of neuroblasts is formed, according to a stereotypical spatiotemporal and segmentally repeated pattern, each subsequently generating a specific cell lineage. Owing to higher complexity and hidden segmental organisation, the mechanisms underlying the development of the brain are much less understood. In order to pave the way towards gaining deeper insight into these mechanisms,we have undertaken a comprehensive survey of early brain development until embryonic stage 11, when all brain neuroblasts have formed. We describe the complete spatiotemporal pattern of formation of about 100 brain neuroblasts on either side building the trito-, deuto- and protocerebrum. Using 4D-microscopy, we have uncovered various modes of neuroblast formation that are related to specific mitotic domains of the procephalic neuroectoderm. Furthermore, a detailed description is provided of the dynamic expression patterns of proneural genes (achaete, scute, lethal of scute, atonal)in the procephalic neuroectoderm and the individual neuroblasts. Finally, we present direct evidence that, in contrast to the trunk, adjacent cells within specific domains of the procephalic neuroectoderm develop as neuroblasts,indicating that mechanisms controlling neuroblast formation differ between head and trunk.

The insect brain is traditionally subdivided into the trito-, deuto-and protocerebrum. However, both the neuromeric status and the course of the borders between these regions are unclear. The Drosophila embryonic brain develops from the procephalic neurogenic region of the ectoderm, which gives rise to a bilaterally symmetrical array of about 100 neuronal precursor cells, called neuroblasts. Based on a detailed description of the spatiotemporal development of the entire population of embryonic brain neuroblasts, we carried out a comprehensive analysis of the expression of segment polarity genes (engrailed, wingless, hedgehog, gooseberry distal, mirror) and DV patterning genes (muscle segment homeobox, intermediate neuroblast defective, ventral nervous system defective) in the procephalic neuroectoderm and the neuroblast layer (until stage 11, when all neuroblasts are formed). The data provide new insight into the segmental organization of the procephalic neuroectodem and evolving brain. The expression patterns allow the drawing of clear demarcations between trito-, deuto-and protocerebrum at the level of identified neuroblasts. Furthermore, we provide evidence indicating that the protocerebrum (most anterior part of the brain) is composed of two neuromeres that belong to the ocular and labral segment, respectively. These protocerebral neuromeres are much more derived compared with the trito-and deutocerebrum. The labral neuromere is confined to the posterior segmental compartment. Finally, similarities in the expression of DV patterning genes between the Drosophila and vertebrate brains are discussed.

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