PHOTOPERIOD CONTROL OF DIAPAUSE IN DAPHNIA. III. TWO-STIMULUS CONTROL OF LONG-DAY, SHORT-DAY INDUCTION

¹ Department of Biological Sciences, State University of New York at Albany, Albany, New York 12203

The reproductive polymorphism in Daphnia pulex may be under the control of photoperiod and a second stimulus whose effect is proportional to culture density. Originally tested with a monocyclic strain, the hypothesis is extended to describe control in two other strains of D. pulex.

The dicyclic strain which displays the polymorphism in both long and short-day photoperiod ls may be only short-day inductive by usual standards. Long-day expression results when photoperiod control is thermally uncoupled. Display of the polymorphism is then under the control of culture density.

The acyclic strain, which in nature omits the reproductive polymorphism, may be induced in the laboratory. It is the acyclic strain which demonstrates the simultaneous requirement of inductively short photoperiod and the density proportional stimulus. The three types of seasonal cycles in D. pulex are presumed to be genetically distinct responses to the two control stimuli. The dicyclic strain may be the most sensitive to thermal uncoupling of photoperiod control, and the acyclic strain may be least sensitive to the density stimulus.

An embryonic diapause results from display of the polymorphism. The dicyclic strain may function by virtue of a much shorter refractory interval which, for the summer diapause, was found to be approximately two weeks at 4° C. Light is an absolute requirement for termination of the diapause, and it is sugested to be more effective when presented as a long-day photoperiod.

In an attempt to relate the requirement for dual stimulation, the reproductive polymorphism of Daphnia is suggested to belong to a class of polymorphisms which requires external assistance in the temporal positioning of a photo-inducible phase. Expression of the polymorphism is presumed to occur when a photo-inducible phase is displaced into the dark period of the environment by a second stimulus. Several lines of evidence, which are taken from earlier studies of sex induction in the Cladocera, seem to support the idea.