Diapause: diverse states of developmental and metabolic arrest THOMAS H. MACRAE  Department of Biology, Dalhousie University, Halifax, N.S. B3H 4JI Canada Abstract Diapause, characterized by developmental arrest and metabolic inhibition, is a widespread phenomenon, occurring either in response to environmental changes or as genetically predetermined life history stages. Organisms undergoing diapause experience increased longevity, coordinated reproduction, and enhanced stress resistance, adaptive traits that promote survival under harsh conditions often not conducive to life. Diapause is studied from several perspectives, such as identification of regulatory cues, detection of hormonal signals appearing in response to external influences, differential gene expression, and gene flow within populations. Signaling pathways regulating environmentally induced dauer in Caenorhabditis elegans have been studied extensively and are described in the review. Hormonal control of gene expression during insect diapause, although less well understood at molecular/biochemical levels than C. elegans dauer, is considered, as is the remarkable stress resistance exhibited by encysted Artemia embryos during and after diapause, and the potential role of a small heat shock protein in the process. A profile of delayed implantation in certain mammalian embryos completes the review. Attention is given to the recent literature and to experimental advantages offered by divergent model systems as they relate to particular aspects of diapause.   < back .