Abstract
Ciliary and rhabdomeric photoreceptor cells represent two main lines of photoreceptor-cell evolution in animals. The two cell types coexist in some animals, however how these cells functionally integrate is unknown. We used connectomics to map synaptic paths between ciliary and rhabdomeric photoreceptors in the planktonic larva of the annelid Platynereis and found that ciliary photoreceptors are presynaptic to the rhabdomeric circuit. The behaviors mediated by the ciliary and rhabdomeric cells also interact hierarchically. The ciliary photoreceptors are UV-sensitive and mediate downward swimming in non-directional UV light, a behavior absent in ciliary-opsin knockout larvae. UV avoidance overrides positive phototaxis mediated by the rhabdomeric eyes such that vertical swimming direction is determined by the ratio of blue/UV light. Since this ratio increases with depth, Platynereis larvae may use it as a depth gauge during vertical migration. Our results revealed a functional integration of ciliary and rhabdomeric photoreceptor cells in a zooplankton larva.
Highlights
Bilaterian animals have two major photoreceptor cell-types, the rhabdomeric- and the ciliary-type photoreceptor cells (Arendt, 2003; Arendt et al, 2004; Eakin, 1979; Erclik et al, 2009)
The glutamatergic rPRCs of the adult eyes connect through three layers of interneurons to cholinergic motoneurons in the ventral head that innervate longitudinal trunk muscles
We identified six interneurons (INpreMN) that are postsynaptic to the RGW interneurons and presynaptic to the ventral motoneurons of the visual circuit (Figure 1E–G)
Summary
Bilaterian animals have two major photoreceptor cell-types, the rhabdomeric- and the ciliary-type photoreceptor cells (rPRC and cPRC, respectively) (Arendt, 2003; Arendt et al, 2004; Eakin, 1979; Erclik et al, 2009). Though if the larvae swim too deep, cyan light inhibits the ciliary photoreceptors and activates the rhabdomeric pigmented eyes. This makes the larvae swim upwards again. In Platynereis, non-pigmented brain cPRCs with ramified cilia express a ciliary type opsin (c-opsin1) (Arendt et al, 2004) and coexist with r-opsin-expressing rPRCs that are part of the pigmented visual eyes (adult eyes) and eyespots (Arendt et al, 2002; Jekely et al, 2008; Randel et al, 2014, 2013). The precise function of the cPRCs in Platynereis larvae and how they interact with rPRCs is still unknown
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