A Structural Atlas of the Developing Zebrafish Telencephalon Based on Spatially-Restricted Transgene Expression.

Zebrafish telencephalon acquires an everted morphology by a two-step process that occurs from 1 to 5 days post-fertilization (dpf). Little is known about how this process affects the positioning of discrete telencephalic cell populations, hindering our understanding of how eversion impacts telencephalic structural organization. In this study, we characterize the neurochemistry, cycle state and morphology of an EGFP positive (+) cell population in the telencephalon of Et(gata2:EGFP)bi105 transgenic fish during eversion and up to 20dpf. We map the transgene insertion to the early-growth-response-gene-3 (egr3) locus and show that EGFP expression recapitulates endogenous egr3 expression throughout much of the pallial telencephalon. Using the gata2:EGFP bi105 transgene, in combination with other well-characterized transgenes and structural markers, we track the development of various cell populations in the zebrafish telencephalon as it undergoes the morphological changes underlying eversion. These datasets were registered to reference brains to form an atlas of telencephalic development at key stages of the eversion process (1dpf, 2dpf, and 5dpf) and compared to expression in adulthood. Finally, we registered gata2:EGFPbi105 expression to the Zebrafish Brain Browser 6dpf reference brain (ZBB, see Marquart et al., 2015, 2017; Tabor et al., 2019), to allow comparison of this expression pattern with anatomical data already in ZBB.

Nucleus Isthmi Is Required to Sustain Target Pursuit during Visually Guided Prey-Catching.

Animals must frequently perform a sequence of behaviors to achieve a specific goal. However, the neural mechanisms that promote the continuation and completion of such action sequences are not well understood. Here, we characterize the anatomy, physiology, and function of the nucleus isthmi (NI), a cholinergic nucleus thought to modulate tectal-dependent, goal-directed behaviors. We find that the larval zebrafish NI establishes reciprocal connectivity with the optic tectum and identify two distinct types of isthmic projection neuron that either connect ipsilaterally to retinorecipient laminae of the tectum and pretectum or bilaterally to both tectal hemispheres. Laser ablation of NI caused highly specific deficits in tectally mediated loom-avoidance and prey-catching behavior. In the context of hunting, NI ablation did not affect prey detection or hunting initiation but resulted in larvae failing to sustain prey-tracking sequences and aborting their hunting routines. Moreover, calcium imaging revealed elevated neural activity in NI following onset of hunting behavior. We propose a model in which NI provides state-dependent feedback facilitation to the optic tectum and pretectum to potentiate neural activity and increase the probability of consecutive prey-tracking maneuvers during hunting sequences.

An interhemispheric neural circuit allowing binocular integration in the optic tectum.

Binocular stereopsis requires the convergence of visual information from corresponding points in visual space seen by two different lines of sight. This may be achieved by superposition of retinal input from each eye onto the same downstream neurons via ipsi- and contralaterally projecting optic nerve fibers. Zebrafish larvae can perceive binocular cues during prey hunting but have exclusively contralateral retinotectal projections. Here we report brain activity in the tectal neuropil ipsilateral to the visually stimulated eye, despite the absence of ipsilateral retinotectal projections. This activity colocalizes with arbors of commissural neurons, termed intertectal neurons (ITNs), that connect the tectal hemispheres. ITNs are GABAergic, establish tectal synapses bilaterally and respond to small moving stimuli. ITN-ablation impairs capture swim initiation when prey is positioned in the binocular strike zone. We propose an intertectal circuit that controls execution of the prey-capture motor program following binocular localization of prey, without requiring ipsilateral retinotectal projections.

Identification and expression analysis of two novel members of the Mesp family in zebrafish.

Mesp proteins play crucial roles in the formation of heart, vasculature and somites during vertebrate embryogenesis. We have used phylogenetic and genomic analysis, combined with qRT-PCR and in situ hybridization, to characterize two novel additional mesp genes in zebrafish, mesp-ab and mesp-bb, and describe their expression pattern in wild type and segmentation mutants. Both mesp-ab and mesp-bb are expressed in early mesoderm with mesp-ab expression starting during late blastula stages and mesp-bb expression initiating later, at the end of gastrulation. During somitogenesis, both mesp genes are expressed dynamically in the anterior presomitic mesoderm. mesp-ab is expressed in presumptive somites S-I and S-II, while mesp-bb is detected in S-I, S-II and S0, with expression restricted to the rostral compartment of presumptive somites. We show that the segmentation clock program regulates expression of these newly identified zebrafish mesp genes in a similar manner to their ohnologs, mesp-aa and mesp-ba. We also present evidence that zebrafish, minnow and salmon retained these additional mesp genes after the teleost whole genome duplication, while medaka, stickleback, fugu and tetraodon did not. Finally we show that although expression and regulation of zebrafish mesp genes appears highly comparable, there is no conservation in non-coding regions with other teleosts. In this study we have completed the description of the Mesp family in zebrafish, which will enable correct genome annotation and facilitate further functional studies on the role of these proteins in zebrafish.