Wayne Pereanu

Neural Lineages of the Drosophila Brain: A Three-Dimensional Digital Atlas of the Pattern of Lineage Location and Projection at the Late Larval Stage

The late larval brain consists of embryonically produced primary neurons forming a deep core cortex, surrounded at the surface by ∼100 secondary lineages. Each secondary lineage forms a tract (secondary lineage tract) with an invariant and characteristic trajectory. Within the neuropile, tracts of neighboring lineages bundle together to form secondary tract systems. In this paper, we visualized secondary lineages by the global marker BP106 (neurotactin), as well as green fluorescent protein-labeled clones and thereby establish a comprehensive digital atlas of secondary lineages. The information contained in this atlas is the location of the lineage within the cortex, the neuropile compartment contacted by the lineage tract, and the projection pattern of the lineage tract within the neuropile. We have digitally mapped the expression pattern of three genes, sine oculis, period, and engrailed into the lineage atlas. The atlas will enable us and others to analyze the phenotype of mutant clones in the larval brain. Mutant clones can only be interpreted if the corresponding wild-type clone is well characterized, and our lineage atlas, which visualizes all wild-type lineages, will provide this information. Secondly, secondary lineage tracts form a scaffold of connections in the neuropile that foreshadows adult nerve connections. Thus, starting from the larval atlas and proceeding forward through pupal development, one will be able to reconstruct adult brain connectivity at a high level of resolution. Third, the atlas can serve as a repository for genes expressed in lineage-specific patterns.

The development of the Drosophila larval brain.

In this chapter we will start out by describing in more detail the progenitors of the nervous system, the neuroblasts and ganglion mother cells. Subsequently we will survey the generic cell types that make up the developing Drosophila brain, namely neurons, glial cells and tracheal cells. Finally, we will attempt a synopsis of the neuronal connectivity of the larval brain that can be deduced from the analysis of neural lineages and their relationship to neuropile compartments.

Identifying Neuronal Lineages of Drosophila by Sequence Analysis of Axon Tracts

The Drosophila brain is formed by an invariant set of lineages, each of which is derived from a unique neural stem cell (neuroblast) and forms a genetic and structural unit of the brain. The task of reconstructing brain circuitry at the level of individual neurons can be made significantly easier by assigning neurons to their respective lineages. In this article we address the automation of neuron and lineage identification. We focused on the Drosophila brain lineages at the larval stage when they form easily recognizable secondary axon tracts (SATs) that were previously partially characterized. We now generated an annotated digital database containing all lineage tracts reconstructed from five registered wild-type brains, at higher resolution and including some that were previously not characterized. We developed a method for SAT structural comparisons based on a dynamic programming approach akin to nucleotide sequence alignment and a machine learning classifier trained on the annotated database of reference SATs. We quantified the stereotypy of SATs by measuring the residual variability of aligned wild-type SATs. Next, we used our method for the identification of SATs within wild-type larval brains, and found it highly accurate (93–99%). The method proved highly robust for the identification of lineages in mutant brains and in brains that differed in developmental time or labeling. We describe for the first time an algorithm that quantifies neuronal projection stereotypy in the Drosophila brain and use the algorithm for automatic neuron and lineage recognition.

Development-based compartmentalization of the Drosophila central brain

The neuropile of the Drosophila brain is subdivided into anatomically discrete compartments. Compartments are rich in terminal neurite branching and synapses; they are the neuropile domains in which signal processing takes place. Compartment boundaries are defined by more or less dense layers of glial cells as well as long neurite fascicles. These fascicles are formed during the larval period, when the approximately 100 neuronal lineages that constitute the Drosophila central brain differentiate. Each lineage forms an axon tract with a characteristic trajectory in the neuropile; groups of spatially related tracts congregate into the brain fascicles that can be followed from the larva throughout metamorphosis into the adult stage. Here we provide a map of the adult brain compartments and the relevant fascicles defining compartmental boundaries. We have identified the neuronal lineages contributing to each fascicle, which allowed us to compare compartments of the larval and adult brain directly. Most adult compartments can be recognized already in the early larval brain, where they form a “protomap” of the later adult compartments. Our analysis highlights the morphogenetic changes shaping the Drosophila brain; the data will be important for studies that link early‐acting genetic mechanisms to the adult neuronal structures and circuits controlled by these mechanisms. J. Comp. Neurol. 518:2996–3023, 2010.

Lineage‐based analysis of the development of the central complex of the drosophila brain

Most neurons of the central complex belong to 10 secondary (larvally produced) lineages. In the late larva, undifferentiated axon tracts of these lineages form a primordium in which all of the compartments of the central complex can be recognized as discrete entities. Four posterior lineages (DPMm1, DPMpm1, DPMpm2, and CM4) generate the classes of small‐field neurons that interconnect the protocerebral bridge, fan‐shaped body, noduli, and ellipsoid body. Three lineages located in the anterior brain, DALv2, BAmv1, and DALcl2, form the large‐field neurons of the ellipsoid body and fan‐shaped body, respectively. These lineages provide an input channel from the optic tubercle and connect the central complex with adjacent anterior brain compartments. Three lineages in the posterior cortex, CM3, CP2, and DPMpl2, connect the posterior brain neuropil with specific layers of the fan‐shaped body. Even though all of the compartments of the central complex are prefigured in the late larval brain by the axon tracts of the above‐mentioned lineages, the neuropil differentiates during the first 2 days of the pupal period when terminal branches and synapses of secondary neurons are formed. During this phase the initially straight horizontal layers of the central complex bend in the frontal plane, which produces the characteristic shape of the fan‐shaped and ellipsoid body. Our analysis provides a comprehensive picture of the lineages that form the central complex, and will facilitate future studies that address the structure or function of the central complex at the single cell level. J. Comp. Neurol. 519:661–689, 2011.

Modeling the Developing Drosophila Brain: Rationale, Technique, and Application

ABSTRACT Digital three-dimensional models, besides representing helpful didactic tools, play an important role in the analysis of brain function and development. The fundamental idea of this approach is that patterns of neural connectivity and activity, pathological lesions, or gene expression are transferred into a single in silico structure: the digital atlas model. This article focuses on recent and ongoing work to build digital models of the developing Drosophila brain, which is formed by an invariant set of approximately 100 neural lineages. Lineages represent key elements in the emerging models of the fly brain: aside from their common origin, which is reflected in the shared expression of numerous developmental control genes, neurons belonging to a given lineage share many morphological characters, including axonal projection and dendritic arborization.