Abstract
BackgroundThe dopaminergic (DA) neurons present in the central brain of the Drosophila larva are spatially arranged in stereotyped groups that define clusters of bilaterally symmetrical neurons. These clusters have been classified according to anatomical criteria (position of the cell bodies within the cortex and/or projection pattern of the axonal tracts). However, information pertaining to the developmental biology, such as lineage relationship of clustered DA neurons and differential cell subtype-specific molecular markers and mechanisms of differentiation and/or survival, is currently not available.ResultsUsing MARCM and twin-spot MARCM techniques together with anti-tyrosine hydroxylase immunoreactivity, we have analyzed the larval central brain DA neurons from a developmental point of view and determined their time of birth, their maturation into a DA neurotransmitter phenotype as well as their lineage relationships. In addition, we have found that the homeodomain containing transcription factor Orthodenticle (Otd) is present in a cluster of clonally related DA neurons in both the larval and adult brain. Taking advantage of the otd hypomorphic mutation ocelliless (oc) and the oc2-Gal4 reporter line, we have studied the involvement of orthodenticle (otd) in the survival and/or cell fate specification of these post-mitotic neurons.ConclusionsOur findings provide evidence of the presence of seven neuroblast lineages responsible for the generation of the larval central brain DA neurons during embryogenesis. otd is expressed in a defined group of clonally related DA neurons from first instar larvae to adulthood, making it possible to establish an identity relationship between the larval DL2a and the adult PPL2 DA clusters. This poses otd as a lineage-specific and differential marker of a subset of clonally related DA neurons. Finally, we show that otd is required in those DA neurons for their survival.
Highlights
The dopaminergic (DA) neurons present in the central brain of the Drosophila larva are spatially arranged in stereotyped groups that define clusters of bilaterally symmetrical neurons
The stereotypical arrangement of their cell bodies in various groups has been used previously to define four clusters of DA neurons that occupy distinct anatomical positions in the central brain [23,24,25]: dorso medial 1 cluster (DM1: four cells), dorso medial 2 cluster (DM2: four cells), dorso lateral 1 cluster (DL1: seven cells) and dorso lateral 2 cluster (DL2: six cells). As judged by their projection and innervation patterns, DM2 and DL1 cell clusters contained apparently homogeneous populations of DA neurons. Both DM1 and DL2 clusters could be further subdivided into two subclusters according to their differential projection patterns
DM2 neurons projected ipsilaterally into the anterior part of the protocerebrum (Figure 1D), whereas DL1 neurons were characterized by dorsally projecting neurites that bifurcated into dorsal and ventral branches before crossing the midline to reach the contralateral brain lobe (Figure 1E)
Summary
The dopaminergic (DA) neurons present in the central brain of the Drosophila larva are spatially arranged in stereotyped groups that define clusters of bilaterally symmetrical neurons. The Drosophila adult brain is a highly organized and complex structure that contains thousands of neurons (in the order of 105) [1] exhibiting multiple cell-type identities, as characterized by various morphological, electrophysiological and molecular features. All these neurons arise from the mitotic activity of a small number of progenitor cells (neuroblasts (NBs)), which generate lineages of clonally related neurons via two proliferative phases of neurogenesis [2,3]. It has been proposed that neuron identity within a NB lineage depends on a combination of spatial and temporal cues provided, firstly, by the unique identity the NB acquires during its specification/delamination time [7,15] and, secondly, as a result of a birth time/order-dependent mechanism [16], whereby cell-type specification of the nascent post-mitotic neurons depends on the identity of the progenitor temporal transcription factor expressed by the NB at each particular time during lineage progression [17]
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