Effects of photoperiods and melatonin on planarian asexual reproduction

Abstract

Abstract Asexual planarians of the species Dugesia dorotocephala exhibit a distinct day‐night rhythm of fissioning under the influence of normal photoperiod; fissioning occurs only at night. This rhythm is broken by continuous illumination, continuous darkness, or by decapitation. The fissioning rate increases when planarians are exposed to light for less than 1 hr/day or when they are decapitated. These results suggest that the planarian head, probably brain, releases a substance that inhibits fissioning, and that the synthesis and release of this substance are closely related to the environmental photoperiod. Furthermore, fissioning of decapitated planarians is suppressed by continuous treatment with melatonin in their culture water. Fissioning resumes when these are returned to normal culture water. Interestingly, fissioning occurs at night when decapitates are treated with melatonin in the daytime, while it is observed in the daytime with night‐time melatonin treatment. These results are consistent with the hypothesis that melatonin release from brain mediates the influence of environmental photoperiods on fissioning.