Merlin Lab
Department of Biology
Texas A&M University
Photoperiodism:
Linking the clock to the seasons
The circadian clock is thought to provide an internal time reference for measuring day length, or photoperiod, which enables organisms living at temperate latitudes to predict and prepare for the changing seasons. The molecular, genetic and cellular mechanisms by which photoperiodic changes are sensed and translated into seasonal changes in physiological and behavioral processes in animals have not been fully deciphered. With strong and ecologically relevant photoperiodic responses, the monarch is ideally suited for the study of animal photoperiodism. For instance, shortening day length in the fall, which coincides with the start of the monarch migration, also contributes to their reproductive dormancy (i.e., diapause). This reproductive diapause is broken at the end of the overwintering period when monarchs are exposed to increasing daylength and temperature. Leveraging the fact that females respond to photoperiod changes in laboratory conditions by producing less mature oocytes in short photoperiod, we demonstrated that a functional circadian clock is necessary for photoperiodic responses.
Monarchs housed under long photoperiods (LP) produce a large quantity of eggs, similar to summer butterflies, while monarchs housed under short photoperiods (SP) produce fewer eggs, similar to wild-caught migrant butterflies (above). Loss-of-function mutants for circadian activators and repressors lose the ability to discriminate between LP and SP.
Interestingly, we also found the vitamin A pathway is under seasonal and clock regulation in the monarch brain. In vivo disruption of the rate-limiting enzyme (ninaB), which converts beta-carotene into retinal, abolishes photoperiodic responses. Thus, the vitamin A pathway is essential for monarch photoperiodic responses. The mechanisms of the clock-controlled vitamin A pathway in photoperiodism still remain unclear however. The vitamin A pathway produces both retinal and retinoic acid, and either could contribute to photoperiod sensing and/or regulation. Retinal is a chromophore which binds to opsin visual pigments to make them light sensitive; retinal might activate a deep brain photoreceptor that senses daylength. Alternatively, retinoic acid is a key ligand for transcription factors, and could regulate a seasonal transcriptional program that regulate photoperiodic response.
We are currently teasing apart the mode of action of vitamin A in photoperiodic responses.
Vitamin A pathway in insects.
The circadian clock or clock genes in the brain are involved in the induction of reproductive diapause exhibited by autumn migrants. The brain clock helps monarchs distinguish long photoperiods (LP) in the summer from short photoperiods (SP) in the fall. The brain clock affects photoperiodic responsiveness by regulating, in a photoperiod-dependent fashion, the expression of genes involved in the vitamin A pathway. β-Carotene is transported into extraretinal neural cells of the adult brain via SANTA MARIA and converted to retinal by the rate-limiting enzyme NINA B. Retinal can either be interconverted into retinol by a retinol dehydrogenase (RDH) or converted into retinoic acid (RA) by a retinaldehyde dehydrogenase (RALDH). RA binds to retinoid receptors to regulate transcription of target genes. Functional disruption of the clock and of the vitamin pathway disrupts photoperiod responsiveness. The connection between vitamin A and juvenile hormone deficiency, characteristic of diapausing monarchs, remains unknown.