Deconstructing Hunting Behavior Reveals a Tightly Coupled Stimulus-Response Loop

Animal behavior often forms sequences, built from simple stereotyped actions and shaped by environmental cues. A comprehensive characterization of the interplay between an animal’s movements and its environment is necessary to understand the sensorimotor transformations performed by the brain. Here, we use unsupervised methods to study behavioral sequences in zebrafish larvae. We generate a map of swim bouts, revealing that fish modulate their tail movements along a continuum. During prey capture, larvae produce stereotyped sequences using a subset of bouts from a broader behavioral repertoire. These sequences exhibit loworder transition dynamics and immediately respond to changes in visual cues. Chaining of prey capture bouts is disrupted in visually impaired (lakritz and blumenkohl) mutants, and removing the prey stimulus during ongoing behavior in closed-loop virtual reality causes larvae to immediately abort the hunting sequence. These results suggest that the continuous integration of sensory information is necessary to structure the behavior. This stimulus-response loop serves to bring prey into the anterior dorsal visual field of the larvae. Fish then release a capture strike maneuver comprising a stereotyped jaw movement and tail movements fine-tuned to the distance of the prey. Fish with only one intact eye fail to correctly position the prey in the strike zone, but are able to produce the strike itself. Our analysis shows that short-term integration of binocular visual cues shapes the behavioral dynamics of hunting, thus uncovering the temporal organization of a goal-directed behavior in a vertebrate.

Projections of the Diencephalospinal Dopaminergic System to Peripheral Sense Organs in Larval Zebrafish (Danio rerio)

Dopaminergic neurons of the descending diencephalospinal system are located in the posterior tuberculum (PT) in zebrafish (Danio rerio), and correspond in mammals to the A11 group in hypothalamus and thalamus. In the larval zebrafish, they are likely the only source of central dopaminergic projections to the periphery. Here, we characterized posterior tubercular dopaminergic fibers projecting to peripheral sense organs, with a focus on the lateral line neuromasts. We labeled and identified catecholaminergic neurons and their projections by combining two immunofluorescence techniques, (i) using an antibody against Tyrosine hydroxylase, and (ii) using an antibody against GFP in transgenic zebrafish expressing in catecholaminergic neurons either membrane-anchored GFP to track fibers, or a Synaptophysin-GFP fusion to visualize putative synapses. We applied the CLARITY method to 6 days old whole zebrafish larvae to stain and analyze dopaminergic projections by confocal microscopy. We found that all lateral line neuromasts receive direct innervation by posterior tubercular dopaminergic neurons, and tracked these projections in detail. In addition, we found dopaminergic fibers projecting to the anterior and posterior lateral line ganglia, and extensive central dopaminergic arborizations around the terminal projection field of the lateral line afferent neurons in the hindbrain medial octavolateralis nucleus (MON). Therefore, dopaminergic innervation may affect lateral line sense information at different processing stages. Additional dopaminergic fibers innervate the trigeminal ganglion, and we observed fine catecholaminergic fibers in the skin with arborization patterns similar to free sensory nerve endings. We also detected potentially dopaminergic fibers innervating inner ear sensory epithelia. Therefore, the diencephalospinal A11-type dopaminergic system may broadly modulate peripheral senses. We also briefly report peripheral sympathetic catecholaminergic projections labeled in our experiments, and their innervation of the developing intestine, swim bladder and abdominal organs.

Orthopedia Transcription Factor otpa and otpb Paralogous Genes Function during Dopaminergic and Neuroendocrine Cell Specification in Larval Zebrafish

The homeodomain transcription factor Orthopedia (Otp) is an important regulator for specification of defined subsets of neuroendocrine cells and dopaminergic neurons in vertebrates. In zebrafish, two paralogous otp genes, otpa and otpb, are present in the genome. Neither complete loss of Otp activity nor differential contributions of Otpa and Otpb to specification of defined neuronal populations have been analyzed in detail. We characterized zebrafish embryos and early larvae mutant for null alleles of otpa, otpb, or both genes to determine their individual contributions to the specification of th expressing dopaminergic neuronal populations as well as of crh, oxt, avp, trh or sst1.1 expressing neuroendocrine cells. otpa mutant larvae show an almost complete reduction of ventral diencephalic dopaminergic neurons, as reported previously. A small reduction in the number of trh cells in the preoptic region is detectable in otpa mutants, but no significant loss of crh, oxt and avp preoptic neuroendocrine cells. otpb single mutant larvae do not display a reduction in dopaminergic neurons or neuroendocrine cells in the otp expressing regions. In contrast, in otpa and otpb double mutant larvae specific groups of dopaminergic neurons as well as of crh, oxt, avp, trh and sst1.1-expressing neuroendocrine cells are completely lost. These observations suggest that the requirement for otpa and otpb function during development of the larval diencephalon is partially redundant. During evolutionary diversification of the paralogous otp genes, otpa maintained the prominent role in ventral diencephalic dopaminergic and neuroendocrine cell specification and is capable of partially compensating otpb loss of function. In addition, we identified a role of Otp in the development of a domain of somatostatin1-expressing cells in the rostral hindbrain, a region with strong otp expression but so far uncharacterized Otp function. Otp may thus be crucial for defined neuronal cell types also in the hindbrain.

Deep Brain Photoreceptors Control Light-Seeking Behavior in Zebrafish Larvae

Most vertebrates process visual information using elaborately structured photosensory tissues, including the eyes and pineal. However, there is strong evidence that other tissues can detect and respond to photic stimuli [1–3]. Many reports suggest that photosensitive elements exist within the brain itself and influence physiology and behavior; however, a long-standing puzzle has been the identity of the neurons and photoreceptor molecules involved [4, 5]. We tested whether light cues influence behavior in zebrafish larvae through deep brain photosensors. We found that larvae lacking eyes and pineal perform a simple light-seeking behavior triggered by loss of illumination (‘‘dark photokinesis’’). Neuroanatomical considerations prompted us to test orthopedia (otpa)-deficient fish, which show a profound reduction in dark photokinesis. Using targeted genetic ablations, we narrowed the photosensitive region to neurons in the preoptic area. Neurons in this region express several photoreceptive molecules, but expression of the melanopsin opn4a is selectively lost in otpa mutants, suggesting that opn4a mediates dark photokinesis. Our findings shed light on the identity and function of deep brain photoreceptors and suggest that otpa specifies an ancient population of sensory neurons that mediate behavioral responses to light.

Y Chromosomal Variation Tracks the Evolution of Mating Systems in Chimpanzee and Bonobo

The male-specific regions of the Y chromosome (MSY) of the human and the chimpanzee (Pan troglodytes) are fully sequenced. The most striking difference is the dramatic rearrangement of large parts of their respective MSYs. These nonrecombining regions include ampliconic gene families that are known to be important for male reproduction,and are consequently under significant selective pressure. However, whether the published Y-chromosomal pattern of ampliconic fertility genes is invariable within P. troglodytes is an open but fundamental question pertinent to discussions of the evolutionary fate of the Y chromosome in different primate mating systems. To solve this question we applied fluorescence in situ hybridisation (FISH) of testis-specific expressed ampliconic fertility genes to metaphase Y chromosomes of 17 chimpanzees derived from 11 wild-born males and 16 bonobos representing seven wild-born males. We show that of eleven P. troglodytes Y-chromosomal lines, ten Y-chromosomal variants were detected based on the number and arrangement of the ampliconic fertility genes DAZ (deleted in azoospermia) and CDY (chromodomain protein Y)—a so-far never-described variation of a species’ Y chromosome. In marked contrast, no variation was evident among seven Ychromosomal lines of the bonobo, P. paniscus, the chimpanzee’s closest living relative. Although, loss of variation of the Y chromosome in the bonobo by a founder effect or genetic drift cannot be excluded, these contrasting patterns might be explained in the context of the species’ markedly different social and mating behaviour. In chimpanzees, multiple males copulate with a receptive female during a short period of visible anogenital swelling, and this may place significant selection on fertility genes. In bonobos, however, female mate choice may make sperm competition redundant (leading to monomorphism of fertility genes), since ovulation in this species is concealed by the prolonged anogenital swelling, and because female bonobos can occupy high-ranking positions in the group and are thus able to determine mate choice more freely.

Evolutionary analysis of the highly dynamic CHEK2 duplicon in anthropoids

Background: Segmental duplications (SDs) are euchromatic portions of genomic DNA (≥ 1 kb) that occur at more than one site within the genome, and typically share a high level of sequence identity (textgreater90%). Approximately 5% of the human genome is composed of such duplicated sequences. Here we report the detailed investigation of CHEK2 duplications. CHEK2 is a multiorgan cancer susceptibility gene encoding a cell cycle checkpoint kinase acting in the DNA-damage response signalling pathway. The continuous presence of the CHEK2 gene in all eukaryotes and its important role in maintaining genome stability prompted us to investigate the duplicative evolution and phylogeny of CHEK2 and its paralogs during anthropoid evolution. Results: To study CHEK2 duplicon evolution in anthropoids we applied a combination of comparative FISH and in silico analyses. Our comparative FISH results with a CHEK2 fosmid probe revealed the single-copy status of CHEK2 in New World monkeys, Old World monkeys and gibbons. Whereas a single CHEK2 duplication was detected in orangutan, a multi-site signal pattern indicated a burst of duplication in African great apes and human. Phylogenetic analysis of paralogous and ancestral CHEK2 sequences in human, chimpanzee and rhesus macaque confirmed this burst of duplication, which occurred after the radiation of orangutan and African great apes. In addition, we used inter-species quantitative PCR to determine CHEK2 copy numbers. An amplification of CHEK2 was detected in African great apes and the highest CHEK2 copy number of all analysed species was observed in the human genome. Furthermore, we detected variation in CHEK2 copy numbers within the analysed set of human samples. Conclusion: Our detailed analysis revealed the highly dynamic nature of CHEK2 duplication during anthropoid evolution. We determined a burst of CHEK2 duplication after the radiation of orangutan and African great apes and identified the highest CHEK2 copy number in human. In conclusion, our analysis of CHEK2 duplicon evolution revealed that SDs contribute to inter-species variation. Furthermore, our qPCR analysis led us to presume CHEK2 copy number variation in human, and molecular diagnostics of the cancer susceptibility gene CHEK2 inside the duplicated region might be hampered by the individual-specific set of duplicons.