PHOTOPERIOD CONTROL OF WINTER DIAPAUSE IN THE FRESH-WATER CRUSTACEAN, DAPHNIA

¹ Department of Biology, State University of New York at Albany, Albany, New York 12203

1. The initiation of sexual reproduction and duration of diapause in the resulting embryo was studied in a population of Daphnia pulex Leydig that restricts such to the autumn. Initiation is under the control of a density associated stimulus that is permitted to function only in short-day photoperiods. In short days the proportion of sexual broods is linearly related to the log of culture density.

2. Critical photoperiod (50% sexual broods) at 19° C. was L:D 12 3/4:11 1/4, and only 15 minutes longer at 12° C. The prenatal life of the mother is the more sensitive to photoperiodic induction of sexual reproduction. Exposure to short days after birth may permit a large percentage of parthenogenic broods in which the necessary males are produced, thereby providing males in advance of sexual reproduction in the population. In long days the mothers quickly revert to parthenogenesis, and the extent of reversal is apparently related to age of the grandparent when the mother was born.

3. Sexually derived embryos from a natural population are in diapause from inception in October until the following April when held in the source lake or in the laboratory at 4° C. The embryos complete phase 1 at the end of a refractory period, requiring low temperature for completion. Termination of the diapause follows phase 1, provided that the embryos are in an aerated microenvironment. A limbo, or phase 2 period may be slightly influenced by photoperiod, although termination is only slightly delayed in short daylength as compared to controls in constant dark. The influence of light or short daylength is reversed when the microenvironment is permitted to become stagnant. Under stagnant conditions, phase 2 may persist if the embryos are maintained in constant dark. In other words winter diapause, as exemplified by embryos from Paul Lake, has no obligate need of light for termination, and this is in contrast to the light requirement of other populations of Daphnia that have been studied. The exception is noted when diapause is "shock" terminated in autumn shortly after inception.

4. The control of diapause termination and hatching of embryos in the source (Paul) lake are consistent with laboratory observations. Caged in transparent and opaque containers, the embryos terminated diapause at all depths after having spent the winter in position. The rate of hatching was most rapid in cages suspended near the surface, which is where the embryos remain naturally during winter. In the dark diapause terminated more slowly when water exchange was impeded by the container, and more rapidly than in light when water flow was comparable in transparent and opaque cages and when intensity, and probably quality, of light was reduced below the intensity at the surface of the lake.

5. The two-stimulus control of sexual reproduction and diapause in Daphnia pulex is examined and analogies between it and terrestrial and other aquatic organisms are described. Also noted is an apparent relationship of the stimuli necessary to induce sexuality and another seasonal polymorphism (cyclomorphosis). Finally, the alternative of embryonic or reproductive diapause is discussed in the context of a population that must survive the winter and at the same time maximize its intrinsic capacity for increase at the onset of the next period of population increase.

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