Abstract
"For every behavioral observation, there is an equal and opposite observation." S. Benzer Photoreception is an important component of rhythm systems and is involved in adjusting circadian clocks to photic features of daily cycles. In Drosophila, it has been suggested that there are three light input pathways to the clock that underlie rhythms of adult behavior: One involves the eyes; the other two extraocular photoreception through a structure called the Hofbauer-Buchner (H-B) eyelet and light reception carried out by pacemaker neurons themselves, mediated by a substance called cryptochrome. All photoreceptor cells including the H-B eyelet have been surmised to be removed by glass-null mutations. Mutations in the no-receptor-potential-A (norpA) gene cause the compound eyes and ocelli to be non-functional and may also affect the eyelet's function. The one cryptochrome mutant known (cryb) harbors an amino-acid substitution in the blue-light absorbing protein encoded by this gene. With regard to adult locomotor rhythms, all single mutants (gl60j, norpAP41, and cryb) re-entrained to altered light:dark (LD) cycles in which the L phase involved relatively intense light. Dropping light levels ca. 10 or ca. 30-fold permitted small percentages of doubly-mutant gl60j cryb flies clearly to re-synchronize their behavior. The marginal re-entrainability in the lowest-light situation nevertheless involved superior responsiveness of the gl60j cryb type, compared with that observed previously using a different re-entrainment protocol. Furthermore, transgenic types in which rhodopsin-expressing cells within the H-B eyelet were ablated or suffered from the effects of tetanus-toxin also entrained with behavior similar or superior to that of gl60j cryb at a low light level. Light inputs that are necessary to synchronize periodic adult emergence can be inferred (from previous studies) to involve a cry-dependent pathway and perhaps also a norpA-dependent one, so that combining mutations in these two genes would cause cultures to be unentrainable. The current results were that each singly-mutant type eclosed rhythmically; flies emerging from norpAP41;cryb cultures also (on balance) exhibited solid eclosion rhythmicity. The ensemble of these behavioral and adult-emergence results suggest that additional light-to-clock pathways function within the system; alternatively, that rhythm assays employed here have teased out residual function of the mutated CRY protein.