The evolutionary physiology of photoperiodism in vertebrates

The capacity to measure day length (photoperiod) is a trait subject to intense evolutionary pressure, and the circadian system has become an important part of the photoperiodic machinery. With the exception of mammals, vertebrates possess multiple sites of photosensitivity within the central nervous system through which light responses may be coordinated. Of these, deep brain photoreceptors play a special role in photoperiodism in nonmammalian vertebrates, independent of either retinal or pineal pathways. In mammals, the pineal hormone, melatonin has assumed a function orthologous to that of deep brain photoreceptors in other vertebrates. Contrasting with this dichotomy in photoperiodic input pathways, downstream signal processing to produce switch-like seasonal responses is largely conserved and appears to center on photoperiodic control of thyroid hormone bioavailability within the hypothalamus. Recent studies implicate the clock-controlled gene eyes absent 3 in linking the circadian clock to photoperiodic responses, dictating the seasonal level of expression of thyroid-stimulating hormone by a specialized population of photoperiodically sensitive cells in the pituitary stalk which in turn govern thyroid hormone metabolism in the basal hypothalamus. This ancestral control pathway appears to have originated prior to the divergence of the mammalian and avian vertebrate lineages.