Abstract
We investigate here the mechanism of allochronic resumption of development by the dormant forms in organisms inhabiting temporary habitats. The cohorts of resting eggs of a short living freshwater crustacean Daphnia magna collected in two temporary waters at two occasions (spring and autumn) were exposed after different storage periods (0–16 weeks) spent either in wet or dry conditions to a given set of hatching stimuli announcing appearance of favourable conditions. Freshly formed resting eggs did not hatch or hatched occasionally. The resting eggs formed in autumn hatched more eagerly than the spring ones when exposed to favourable conditions after wet storage. The hatching proportion increased linearly up to 68–82% in autumn resting eggs while to 33–44% in the spring ones over 16 weeks of storage that might have covered several generations of the active forms. Dry storage of the resting eggs reduced their hatching proportion considerably. We suspect that the length variation of a refractory period (initial phase of developmental arrest when resting forms remain insensitive to hatching stimuli) followed by a reactivation period may constitute the simplest two-step physiological mechanism allowing staggering revival of the dormant forms in subsequent generations that maximise chances for survival in unpredictably changing habitats.
Highlights
Most living creatures on Earth that occupy temporarily deteriorating habitats suspend development during inhospitable periods
Some ephippial eggs formed in autumn and stored wet, were ready to hatch without temporal storage a few days after egg formation (6.5% for PK and 3.5% for PU)
The results of our study: (1) confirmed the existence of a refractory period occurring during the suspended development of D. magna ephippial eggs originating from temporary waters, (2) challenged existing reports of shorter refractory periods of dried than wet ephippial. eggs of D. magna, and (3) documented considerable variability in the length of the refractory period within cohorts of the resting eggs from given location and season
Summary
Most living creatures on Earth that occupy temporarily deteriorating habitats suspend development during inhospitable periods. The higher the risk of unsuccessful reproduction during the growing season, the higher the incidence of postponed development of dormant forms until the seasons In their simulation studies, Menu et al.[9] and Ślusarczyk et al.[10] considered longer diapause periods than the earlier models and revealed that short living univoltine organisms inhabiting unpredictably changing habitats may use a diversified length of developmental arrest to promote long term survival of their genetic lines. The simplest mechanism of diapause termination might be an inspection of environmental conditions by the dormant form and the subsequent resumption of development with the return of favourable environmental conditions[1] Such a mechanism may be supported by a conditional process determining the length of a refractory period[1] called a maintenance phase in animals[11] or physiological dormancy in plants[12], i.e. a temporal phase in which the dormant forms remain insensitive to stimuli initiating their development. We suspect that the length diversification of the refractory period may be the simplest mechanism allowing gradual revival of the dormant forms in subsequent generations and diversifying the risk of faulty choice of activity in unpredictably changing habitats
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