Short-day photoperiods in conjunction with a second stimulus were required to induce sexual reproduction (and diapause) in an autumnally diapausing strain of Daphnia pulex. The second stimulus was associated with the unalkalized portion of a small lake and with density of the population in laboratory cultures. Diapause was induced at 12 degrees and 19 degrees C.
The light and thermal requirements for diapause development and release in Daphnia pule.r were determined for the ephippia from a pseudo-sexual strain cultured in the laboratory and from an autumnal diapausing, bisexual strain in Paul Lake, Michigan.Light was essential for termination of diapause in the laboratory-cultured strain regardless of the temperature or duration of ephippia storage. Ephippia from the lake population were activated by light, but prolonged storage in constant dark eliminated the requirement for light, and thereby implicated photoperiodic control of diapause release.The laboratory population completed diapause development within a period of 3 to 6 weeks when stored in constant dark at 22°C. Storage at 3.5°C in constant dark prolonged diapause. In the Paul Lake strain, low temperature was a requirement for diapause development, and at 3.5°C the eggs were in diapause for a period of 5 or 6 months.The contrasting light and thermal requirements are discussed in the context of environments regulating the duration of diapause in summer and winter diapausing populations of Daphnia.
Females of Daphnia, as of all Cladocera, may be reproductively polymorphic, and produce two structurally and functionally different types of eggs, one of which later enters an embryonic diapause. Expression or display of the polymorphism has been claimed to be under the control of two stimuli (Stress and Hill, 1965;. One of the stimuli is an inductively short photoperiod. The second is unidentified, but its effect may be proportional to density of culture. The indicated function of the second stimulus is to trigger expression when daylength is permissively short. The hypothesis of dual stimulation is further examined in this
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