Evolution of circadian organization in vertebrates

Abstract

Circadian organization means the way in which the entire circadian system above the cellular level is put together physically and the principles and rules that determine the interactions among its component parts which produce overt rhythms of physiology and behavior. Understanding this organization and its evolution is of practical importance as well as of basic interest. The first major problem that we face is the difficulty of making sense of the apparently great diversity that we observe in circadian organization of diverse vertebrates. Some of this diversity falls neatly into place along phylogenetic lines leading to firm generalizations: i) in all vertebrates there is a "circadian axis" consisting of the retinas, the pineal gland and the suprachiasmatic nucleus (SCN), ii) in many non-mammalian vertebrates of all classes (but not in any mammals) the pineal gland is both a photoreceptor and a circadian oscillator, and iii) in all non-mammalian vertebrates (but not in any mammals) there are extraretinal (and extrapineal) circadian photoreceptors. An interesting explanation of some of these facts, especially the differences between mammals and other vertebrates, can be constructed on the assumption that early in their evolution mammals passed through a "nocturnal bottleneck". On the other hand, a good deal of the diversity among the circadian systems of vertebrates does not fall neatly into place along phylogenetic lines. In the present review we will consider how we might better understand such "phylogenetically incoherent" diversity and what sorts of new information may help to further our understanding of the evolution of circadian organization in vertebrates. Key wordsOrganisms, from unicellulars to vertebrates, are structured in time as well as in space. Many, if not most, biochemical, physiological and behavioral parameters exhibited by organisms show daily fluctuations and most of these daily rhythms persist in constant conditions, thus demonstrating that they are driven by endogenous oscillators. The rhythms that persist in constant conditions with periods close to 24 h are called circadian rhythms.By circadian organization we mean the way in which the entire circadian system above the cellular level is put together physically, and the principles and rules that determine the interactions among its component parts. Circadian organization extends both broadly and deeply into the physiology and

The effects of light pollution on biological rhythms of birds: an integrated, mechanistic perspective

“…The greatest difference is in the degree of involvement of the separate components of the circadian systems. In mammals, the master circadian clock is located in a specific set of cells of the hypothalamus, the suprachiasmatic nuclei (SCN) (Menaker et al 1997 ), which receives light stimuli from a subset of ganglion cells called ipRGCs (see above) (Berson et al 2002 ), while the pineal gland is less involved in circadian timing. In birds, non visual light stimuli are mainly received in the retina, the hypothalamus (via the retinal hypothalamic tract, RHT), and in the pineal gland (Gwinner and Brandstätter 2001 ).…”

Interactions between endocrine and circadian systems

“…Unlike mice and humans, many non-mammalian vertebrates can perceive photic information by extraretinal photoreceptors (Menaker et al 1997, Foster & Soni 1998, e.g. in the pineal.…”

“…in the pineal. The pineal gland is an ancient organ that exists in most vertebrates (Menaker et al 1997). In mammals, it is buried deep beneath the skull and lies within the furrow of the two hemispheres.…”

E-box function in a period gene repressed by light

“…This defining property is thought to ensure optimal entrainment by zeitgebers (2). In vertebrates, the circadian clock was originally thought to reside in a small number of specialized pacemakers: the suprachiasmatic nucleus, the retina, and in lower vertebrates, the pineal gland (3,4). However, rhythmic clock gene expression was encountered subsequently in vivo in most cell types (5,6) and shown to persist in vitro (7,8).…”