Abstract
Great progress has recently been made in cryobiology. One field, however, has been neglected: the temporal sequence of the effects of photoperiod and temperature, and their relative importance in cold hardening. This is relevant to the question of impor- tance of diapause in cold-hardiness. Denlinger (1991) outlined the categories of such relations and stressed a great need for further detailed research. A survey of studies done over the past decade revealed many gaps in the evidence and the ambiguous nature of the data on the photoperiodic regulation of cold-hardiness. We hope that this review will stimulate further research in this field. Among several directions where research is most needed we have stressed (1) simultaneous recording of changes in survival and dynamics of suspected cryoprotectants (stressed also by Danks, 1996), (2) checking the regulation of different phases of cold hardening, and (3) discrimination between direct and indirect (mediated via neuroendocrine system) effects of environmental cues on cold harden- ing.
Highlights
There was a lengthy debate about the relation between diapause and cold-hardiness
Crucial evidence for a direct link between diapause and cold-hardiness is an increase in cold-hardiness without any decrease in temperature, i.e. without cold acclimation. Such a link has been proved in P. apterus, where the diapause itself is associated with a certain level of cold-hardiness that is later, in the second phase, increased by low temperatures
In addition to P. apterus, evidence for a direct link between diapause and cold-hardiness was reported in the aphid parasitoid Aphidius ervi, reared at 15°C, when survival was recorded at –10°C (Langer & Hance, 2000) and in Euseius finlandicus, where the survival at –4°C was higher in diapause mites, in spite of high rearing temperature of 20°C (Broufas & Koveos, 2001) (Table 2)
Summary
There was a lengthy debate about the relation between diapause and cold-hardiness. Contrasting views were expressed, ranging from “diapause as a prerequisite for cold-hardiness” to “both phenomena as independent events”. The termination of adult diapause by JHA in A. nigripennis is associated with a decrease in both myo-inositol level and survival at low temperatures (Watanabe & Tanaka, 1998b, 1999a, 2000). Diapause is a prerequisite for a further increase in supercooling capacity by cold acclimation, further decreasing the SCP by about 5–6°C (Fig. 1, Hodkova & Hodek, 1994, 1997).
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