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.
When mammalian cells are deprived of glutamine, exogenous asparagine rescues cell survival and growth. Here we report that this rescue results from use of asparagine in protein synthesis. All mammalian cell lines tested lacked cytosolic asparaginase activity and could not utilize asparagine to produce other amino acids or biosynthetic intermediates. Instead, most glutamine-deprived cell lines are capable of sufficient glutamine synthesis to maintain essential amino acid uptake and production of glutamine-dependent biosynthetic precursors, with the exception of asparagine. While experimental introduction of cytosolic asparaginase could enhance the synthesis of glutamine and increase tricarboxylic acid cycle anaplerosis and the synthesis of nucleotide precursors, cytosolic asparaginase suppressed the growth and survival of cells in glutamine-depleted medium in vitro and severely compromised the in vivo growth of tumor xenografts. These results suggest that the lack of asparaginase activity represents an evolutionary adaptation to allow mammalian cells to survive pathophysiologic variations in extracellular glutamine.