Photoreceptor specialization and the visuomotor repertoire of the primitive chordate Ciona - PubMed (original) (raw)

Photoreceptor specialization and the visuomotor repertoire of the primitive chordate Ciona

Priscilla Salas et al. J Exp Biol. 2018.

Abstract

The swimming tadpole larva of Ciona has one of the simplest central nervous systems (CNSs) known, with only 177 neurons. Despite its simplicity, the Ciona CNS has a common structure with the CNS of its close chordate relatives, the vertebrates. The recent completion of a larval Ciona CNS connectome creates enormous potential for detailed understanding of chordate CNS function, yet our understanding of Ciona larval behavior is incomplete. We show here that Ciona larvae have a surprisingly rich and dynamic set of visual responses, including a looming-object escape behavior characterized by erratic circular swims, as well as negative phototaxis characterized by sustained directional swims. Making use of mutant lines, we show that these two behaviors are mediated by distinct groups of photoreceptors. The Ciona connectome predicts that these two behavioral responses should act through distinct, but overlapping, visuomotor pathways, and that the escape behavior is likely to be integrated into a broader startle behavior.

Keywords: Behavior; Connectome; Phototaxis.

© 2018. Published by The Company of Biologists Ltd.

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Conflict of interest statement

Competing interestsThe authors declare no competing or financial interests.

Figures

Fig. 1.

Fig. 1.

Phototaxis and dimming responses in wild-type and pristine (prs) Ciona robusta mutants. (A) Coomassie Blue-stained wild-type C. robusta juveniles attached to a 10 cm Petri dish. The majority are clustered at the far edge (dashed oval), away from the indicated directional light. (B) Dependency of negative phototaxis in C. robusta on directional light intensity. Graph shows the percentage of juveniles attaching to the distal third (away from the light source) of the Petri dish from three independent trials as a function of directional light intensity (lx). Points are the mean from three trials (±s.d.). (C) Top panel: C. robusta wild-type larvae with an ocellus pigment cell and arrestin-positive photoreceptor cells indicated; bottom panel: homozygous prs mutant larva that is positive for arrestin but lacking pigment. (D,E) Homozygous prs larvae do not show negative phototaxis. (D) In a phototaxis assay, no clustering on the distal side of the Petri dish (away from the light source) was observed. (E) Percentage of prs larvae attaching to the distal and proximal sides of Petri dishes. Bars indicate means from three trials (±s.d.). (F) Homozygous prs larvae do respond to light dimming. Blue bars indicate the percentage of larvae swimming in a 5 s period prior to light dimming, while red bars indicate the percentage of larvae swimming in a 5 s period after light dimming. Shown are the means (±s.d.) from three movies, with 20 larvae assessed for each movie. *P<0.05, **P<0.01 (Student's _t_-test). wt, wild-type larvae.

Fig. 2.

Fig. 2.

Development of negative phototaxis in C. robusta. (A,B) Representative images showing the beginning and end points, respectively, of a negative phototaxis time-lapse recording. Yellow arrows indicate the direction of light, and red arrows in B indicate the accumulation of larvae at the distal side of the dish. (C) Graph of negative phototaxis. The percentage of larvae in the distal halves of Petri dishes measured at 30 min intervals starting at 21.2 h post-fertilization (hpf) is shown. Points indicate means from three movies (±s.d.). (D) Angles (Θ) and lengths of swims plotted in the angular and radial axes, respectively, for larvae at 21 and 25 hpf. The direction of light is indicated. (E) Tortuosity of the swims analyzed in D. For D and E: *P<0.05, **P<0.01 (Wilcoxon signed rank test).

Fig. 3.

Fig. 3.

Developmental time course of dimming response in C. robusta larvae. Blue bars indicate the percentage of larvae swimming in a 5 s period prior to light dimming, while red bars indicate the percentage of larvae swimming in a 5 s period after light dimming. Shown are the means (±s.d.) from three movies with 45 larvae assessed for each movie.

Fig. 4.

Fig. 4.

Swim types in C. robusta phototaxis. (A,B) Swim paths in ∼28 hpf C. robusta larvae responding to direction of light (indicated). Lines show all swims recorded in a 1 min capture session at 10 frames s−1, and classified as sustained swims (A), or tail flicks (B). (C) Quantification of swims classified as either sustained or tail flicks for wild-type (wt) and homozygous pristine mutant (prs) larvae either with directional (light) or far-red (dark) illumination. Bars indicate means from three movies (±s.d.). (D) Total swim events. These numbers were generated by summing sustained swims and tail-flick counts from data in C. The total number of larvae screened for each condition (n) is given in parentheses in C and D. *P<0.05, **P<0.01 (Student's _t_-test).

Fig. 5.

Fig. 5.

Phototaxis and dimming behaviors have different swim patterns. (A) Five-second projection images of a field of C. robusta larvae in the presence of directional light (arrow). Swims appear as white trails. (B) Same field of view as in A, but showing a 5 s projection of swims following light dimming. Scale bars: (A,B) 1 mm. (C) Selected swim paths of 11 larvae in the 10 s following light dimming. Arrows indicate the orientations of the larvae relative to the indicated directional light at the moment of dimming (anterior at the arrowhead). (D) Quantification of swim tortuosity of larvae before (light) and after (dim) dimming of directional light. The number of swims analyzed (n) is indicated. **P<0.01 (Wilcoxon signed rank test).

Fig. 6.

Fig. 6.

Dimming response in pristine (prs) larvae. (A,B) Ten-second projection images of swims induced by light dimming in wt and prs larvae, respectively. (C) Quantification of tortuosity in swims induced by light dimming in wt and prs larvae randomly selected from four movies. The total number of swims analyzed (n) is indicated. *P<0.05 (Wilcoxon signed rank test).

Fig. 7.

Fig. 7.

Trajectory reversal induced by light dimming in swimming larvae. (A) Traces of swims from five larvae in

Movie 7

. Orange lines indicate swims before lights-off, and blue lines indicate swims after lights-off. Yellow arrows indicate trajectory reversals that immediately follow lights-off. (B) Collection of 22 swims from five movies. Swims on the left of the dashed line show trajectory reversals. Turns highlighted in yellow are counter-clockwise and those in purple are clockwise.

Fig. 8.

Fig. 8.

Minimal visuomotor pathway from 21 hpf C. intestinalis larvae. PR-I: Group I photoreceptor; PR-II: Group II photoreceptor; prIN: photoreceptor interneuron; prAMG-RN: photoreceptor-ascending MG neuron relay neuron; prRN: photoreceptor relay neuron; MGIN-L and MGIN-R: motor ganglion interneuron (left and right); AMG: ascending MG peripheral interneuron; MN-L and MN-R: motor neuron (left and right); PNS, peripheral nervous system. The numbers in parentheses indicate the number of cells in that class.

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