THE INTERACTION OF PHOTOPERIOD AND TEMPERATURE IN DIAPAUSE TIMING - ACOPEPOD EXAMPLE

In many organisms, photoperiod and temperature are thought to be the m ost significant token cues for seasonally timed life history events, i ncluding diapause in arthropods. A common pattern in many species of t errestrial insects and several copepod species is the existence of a c ritical daylength on one side of which the animals do not enter diapau se and on the other side of which they do. Temperature plays a seconda ry role as modifier of the critical daylength. In some species, howeve r, including the freshwater copepod Diaptomus sanguineus, the fraction of females making subitaneous eggs (eggs that hatch immediately) unde rgoes a very gradual transition as daylength changes over the natural range of photoperiods experienced in nature. Here we show that tempera ture is as important as photoperiod in cuing diapause timing in a popu lation of D. sanguineus living in Bullhead Pond, Rhode Island. When ec ologically relevant photoperiod and temperature cues are provided in t he laboratory, the copepods rapidly switch from producing subitaneous eggs to producing diapausing eggs in a way that is typical of the seas onal switch seen in the pond. We provide a graphical model that illust rates how copepod sensitivities to photoperiod and temperature interac t to produce an abrupt transition, and we discuss how natural selectio n should act on D. sanguineus diapause response to produce the variati on in diapause timing seen within and between natural populations.