A map of brain neuropils and fiber systems in the ant Cardiocondyla obscurior

Abstract

A wide spectrum of occupied ecological niches and spectacular morphological adaptations make social insects a prime object for comparative neuroanatomical studies. Eusocial insects have evolved complex societies based on caste polyphenism. A diverse behavioral repertoire of morphologically distinct castes of the same species requires a high degree of plasticity in the central nervous system. We have analyzed the central brain neuropils and fiber tract systems of the worker of the ant Cardiocondyla obscurior, a model for the study of social traits. Our analysis is based on whole mount preparations of adult brains labeled with an antibody against Drosophila-Synapsin, which cross-reacts strongly with synapses in Cardiocondyla. Neuropil compartments stand out as domains with a certain texture and intensity of the anti-Synapsin signal. By contrast, fiber tracts, which are composed of bundles of axons accompanied by glia and are devoid of synapses, appear as channels or sheaths with low anti-Synapsin signal. We have generated a digital 3D atlas of the Cardiocondyla brain neuropil. The atlas provides a reference for future studies of brain polymorphisms in distinct castes, brain development or localization of neurotransmitter systems.

Keywords: Cardiocondyla obscurior; fiber tracts; hymenoptera; neuroanatomy; neuropile compartements.

Figure 1

Overview of the brain neuropil of DrosophilaCardiocondyla obscurior (C,D; labeled with anti-Synapsin). (A) Cross section at level of central complex (CCX); (B,D) Sagittal section at level of peduncle (PED; anterior to the right). Section was derived by digital 90 deg tilt of the original confocal stack. Peduncle and calyx (CX) of mushroom body (MB) are shaded magenta. Hatched lines demarcate boundaries between dorsal layer, middle layer, and ventral layer of neuropile. (C–F) Consecutive cross sections of part of Cardiocondyla brain labeled with anti-Synapsin. Compartment boundaries stand out by low Synapsin signal. Boundaries between SMP and IP (black arrow), SMP and SLP (red arrow), SLP and VLP (green arrow). (G–J) Comparison of anti-Synapsin staining with anti-Acetylated-tubulin staining. (G,I) Cross section of the Cardiocondyla brain labeled with anti-Acetylated-tubulin (green). (H,J) Cross section at corresponding level, labeled with anti-Synapsin (green). Neuronal nuclei are labeled by DAPI (G,H; magenta). Synapsin-negative fiber bundles (PLF, MEF and l-ALT in I,J) are positively labeled by anti-Acetylated-tubulin (I). For abbreviations see Table 1. Bar: 50 µm.

Figure 2

Neuropil compartments of the Cardiocondyla brain and axon tracts associated with brain compartments. Panels (A–F) show z-projections of 5–8 contiguous confocal sections of adult brain hemispheres labeled with anti-Synapsin (Cardiocondyla), demarcating neuropile compartments (shaded in different colors). Panels are ordered from anterior (A) to posterior (F). (A) level of antennal lobe (AL); (B) level of MB medial lobe (ML); (C) level of the central body lower unit (CBL) ; (D) level of the central body upper unit (CBU) and great commissure (arrowhead); (E) level around posterior boundary of central body; (F) level of protocerebral bridge (PB). For abbreviations see Table 1. Panels (G–J) show individual confocal sections of Cardiocondyla obscurior stained with anti-Synapsin. Scalebar: 50 µm (A–F) and (G–J), (A–F) and (G–J) are at same scale.

Figure 3

3D digital model of the adult brain compartments of the ant Cardiocondyla obscurior. Colors of compartments correspond to those used in Figure 2. (A,B) Entire brain with the optic lobes; tilted anterior view (A) and posterior view (B). Note the proximal-distal arrangement of the lobula complex, medulla and lamina. (C–H) Individual compartments of the three layers of the cerebrum; (C,E,G) antero-lateral view, (D,F,H) postero-lateral view. (C,D) Dorsal layer of neuropil, with the two compartments SMP and SLP. (E,F) Middle layers harboring the CCX (green) and mushroom bodies (magenta), and the inferior protocerebrum (IP; brown). Note massive MB calyces which are tilted dorsally and anteriorly, to cover much of the dorsal neuropil surface. (G,H) Ventral layer, composed of the lateral accessory lobe (LAL), ventromedial cerebrum (VMC), ventrolateral protocerebrum (VLP), posterolateral protocerebrum (PLP), AL, antennal mechanosensory and motor center (AMMC) and suboesophageal ganglion (SOG). For further abbreviations see Table 1.

Figure 4

Mushroom bodies neuropil complex of the ant Cardiocondyla obscurior. (A,B) 3D digital model of the MB showing the relative position of each of its elements, (C,D) 3D model showing each of its elements isolated; note the bipartite organization of the calyx, the size of the different elements are not in scale. (E–J) show the MB elements on different Z-sections from anterior to posterior. Blue arrowheads show fringes between the lobes (medial and vertical) and Peduncle (G). Pink arrowheads show the fringe between Lip and Collar. Yellow arrowheads indicate the bright hallmark continuous (G,H) between the VL and the Ped. White arrowhead (F,H,I) show the transition of brighter and less bright layers ML/VL and Peduncle. See discussion for more details. Scale bar: 50 µm. Abbreviations: MB: mushroom bodies, ML: medial lobe, VL: vertical lobe, Ped: peduncle, lCx: lateral calyx, mCx: medial calyx, Li: lip, Co: collar, mLi: medial lip, lLi: lateral lip, mCo: medial collar, lCo: lateral collar.

Figure 5

Central complex neuropils of the ant Cardiocondyla obscurior. (A,B) 3D digital model of the different neuropil compartments of the CCX. (C–H) Confocal sections of the CCX, (C–F) Central body: central body upper unit (Fan-shaped body) and central body lower unit (ellipsoid body). (G) transition between the Fan-shaped body and the Protocerebral bridge. (H) Protocerebral bridge. Blue arrowheads in (D) show the fringe between the CBU and CBL. Yellow arrowheads in E tentatively label the subregions of the CBU/CBL. Red arrowhead in (G) and (H) show the endpoint of the MEF under the PB. Arrows in F show the layering of the underlying medial lobe. Scale bar: 50 µm.

Figure 6

3D digital rendering of the long axon fascicles. Longitudinal fascicles of dorsal neuropil: green; transverse fascicles of dorsal neuropil: magenta; longitudinal fascicles of middle neuropil: yellow (LEF, MEF), orange (PLF); longitudinal fascicles of ventral neuropil: blue; AL tracts: cyan; ventral commissures: red. (A) Anterior view of all fascicles. For left hemisphere, neuropile compartments (gray, semi-transparent) are superimposed upon fascicles. Boxed inset: anterior view of AL tracts in relationship to MB. (B) Dorsal view of fascicles of dorsal neuropil. (C) Anterior view of fascicles of middle and ventral neuropil layers. For further abbreviations see Table 1.