Jeffrey Hall

Cryptochromes (CRYs) are blue-light-absorbing proteins involved in a variety of biological phenomena. In animals, CRYs exhibit a certain versatility with regard to these organisms’ circadian rhythms, as has been revealed by the effects of mutations and molecular manipulations. The rhythm system of
Drosophila uses one gene’s worth of CRY protein to transmit light into a circadian clock within the brain, which controls the fly’s sleep–wake cycles. In fact, the relevant pacemaking neurons are themselves circadian photoreceptive structures. In peripheral tissues and others located posterior to the brain,
Drosophila CRY may be a photoreceptive molecule and also part of the pacemaker mechanism. Mice have two CRY-encoding genes. They are expressed in many tissues, including the retina and a clock structure within the brain. In the former location, mouse CRY may play a circadian-photoreceptive role, along with that mediated by rhodopsins found elsewhere in the retina. In the latter tissue, the hypothalamic suprachiasmatic nucleus, mouse CRYs are closely connected to the multi-molecule murine clock mechanism.

Biological clocks are disturbed by certain mutations in
Drosophila and
Neurospora. It has recently been possible to isolate the genes defined by certain of these mutations in order to study their expression and the information that they encode. It is hoped that these investigationswill eventually shed light on the fundamental nature of timing mechanisms that exist in eukaryotic cells and nervous systems.

Male and female fruit flies learn and remember various aspects of their reproductive experiences. This ability can improve reproductive fitness. Such conclusions stem in part from the effects on courtship of mutations affecting learning and memory. Certain elements of experience-dependent reproductive behavior involve rhythmic components of the male's singing behavior. These song rhythms have entered the general arena of biological clocks via the effects of circadian rhythm mutations on an oscillating component of the courtship song. The behavioral genetic analysis of connections among these various kinds of complex behavior has recently been expanded to include molecular studies, aimed at manipulating and understanding the ‘learning’ and the ‘clock’ genes.