A Brief History of Circannual Time

Abstract

The tenet of this review is that innate circannual timing is an ancestral trait that first evolved in free‐living eukaryotic cells some 2000My ago. Marine algae of the genus Allexandrium provide a living unicellular model. This species shows the primitive trait of 'alternation of generations' where the organism alternates between fast replicating vegetative cells in summer, and a dormant cystic cell over winter. The resistant cysts sink into the cold ocean sediments. Remarkably, excystment in spring is governed by an endogenous circannual timing mechanism. Thus, a tiny, short‐lived unicell can utilize a circannual clock as part of the species' life history program. Innate timing allows for major adjustments in physiology and behavior in anticipation of the seasons, and provides an internalized sense of seasonal time for the many species where standard environmental cues are weak or ambiguous. This is a highly adaptive strategy irrespective of the organism's size and longevity. Circannual rhythms are expressed by a diverse range of organisms from flowering plants,to mammals, interwoven into each species life‐history program – the consequence of fore‐ever living in a periodic world. In complex vertebrates, the early division of the zygote potentially carries circannual timer genes into all progeny cells and into all tissues. This supports the concept of a 'clock shop' where cell‐autonomous long‐term rhythms are generated in each tissue, orchestrated by a central circannual pacemaker system. This is analogous to the organization of the circadian timing system. For the circannual time‐scale, specialized thyrotrope cells located in the pars tuberalis (PT) of the pituitary gland and adjacent tanycyte cells located in the ependymal wall of the third cerebral ventricle (3rdV) of the brain act as putative central circannual pacemakers. At a molecular level epigenetically regulated, cyclical remodeling of chromatin, that determines whether specific circannual timer genes are transcriptionary active, or not, is thought to drive the oscillation between the summer and winter phenotypes.

This article is protected by copyright. All rights reserved.

http://bit.ly/2Spza8H