Mushroom Body is a Quadruple Structure

Mushroom Body is a Quadruple Structure - Figure 6

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Fig. 6. A single MBNb is sufficient to autonomously generate all of the MB structures. (partial ablation of MBNbs with hydroxyurea)

We performed partial ablation of the MBNbs and analysed the projection patterns of cells labelled with GAL4 enhancer trap in animals that have only one of the four MBNbs. To selectively ablate the MBNbs, newly hatched larvae were fed medium containing 3 mg/ml of hydroxyurea (HU) for three hours. Under these conditions, 20-30 % of examined brains had only one (or two) surviving MBNb(s) in either one or both hemisphere(s).

With all the MB subtype markers examined, the labelled structure of the surviving clones was essentially identical to the staining pattern of the intact brain. Considering that the escape from the ablation should occur randomly, this strongly suggests that a single MBNb, in the absence of the other MBNbs, can generate all the MB substructures labelled with these markers.

(oblique view from the anterior).

	click here for stereo view (red / green)
	click here for stereo view (red / green)

GAL 4 strain Mz 717 crossed to UAS-GFP S65T. This strain labels fibres in the dense alpha and beta lobes, as well as fibrous gamma and alpha' lobes. In the intact brain (A), four clusters of cell bodies are observed.

In the brain that has only one surviving MBNb clone in each hemisphere (B), only one cluster is seen in the cortex. Although the fibre number is greatly reduced, the clone forms essentially identical structures in the lobe region as would four clones in untreated animals. However, we found that the beta lobes of both sides often fuse together in the HU-treated flies (arrowhead in B).

	click here for stereo view (red / green)
	click here for stereo view (red / green)
	click here for stereo view (red / green)

(C-E) GAL4 strain Mz 984 crossed to UAS-GFP S65T. This strain labels fibres in the core region of only alpha and beta lobes. In the intact brain (C), the large round tip of the alpha lobe and the split tips of the beta lobe (two arrowheads) are labelled. (m bdl: median bundle)

The same characteristic structure can be observed in brains with single surviving clones (D). Again, the beta lobes of the HU-treated animals are fused with each other (arrowhead).

In a brain that has only one MB clone (E), the beta lobe fibres seem to divert after crossing the midline. It is important to note that only a defined substructure was affected by the HU treatment. Although the fibres in the upper tip of the beta lobe seem to cross the midline; the gamma lobe and the lower tip of the beta lobe look intact. Thus, we think that the fused lobe is a secondary outcome rather than a direct consequence of the HU-mediated ablation. Except for this fusion, the lobe and pedunculus structure of the single clones is essentially identical with that of the intact brain.

Three-dimensional reconstructions were made from about 160 confocal 1-um optical sections. Bar, 20 um.
Abbreviations: alpha, alpha', beta, gamma: lobes, ped: pedunculus, sp, spur, m bdl: median bundle

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Flybrain Development Mushroom Body Quadruple Structure Quadruple Structure - Figure 6

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