ANOXIA INDUCES RAPID COLD HARDENING IN THE HOUSEFLY MUSCA-DOMESTICA (DIPTERA, MUSCIDAE)

The housefly Musca domestica shows the recently described phenomenon of rapid cold hardening in which tolerance to a formerly lethal cold shock can be induced by a brief exposure to an acclimating temperature such as 30 min at 0-degrees-C. This paper describes a similar rapid increase in cold tolerance after a period of "acclimation" in an atmosphere of oxygen-free nitrogen. Direct transfer of 4-day-old pupae (pharate adults) of M. domestica from a culture temperature of +27 to -7-degrees-C for 2 h is lethal, whereas exposure to anoxic conditions at +27-degrees-C for 40 min confers a greatly enhanced tolerance to 2 h at -7-degrees-C, such that the subsequent emergence of adult flies is not significantly different from the untreated controls. Whilst the nature of the response to low temperature and anoxia is essentially similar, in comparison to cold induced rapid cold hardening the increased cold tolerance produced by anoxia is developed more rapidly, but to a lesser extent. For instance, 21% of pupae acclimated at 0-degrees-C for 3 h survived 10 h at -7-degrees-C, whereas, after a 3 h anoxia acclimation only 1.7% of pupae survived 6 h at -7-degrees-C. It is suggested that this rapid but temporary increase in cold hardiness is part of a generalized response to a range of factors which suppress metabolism, such as low temperature and anoxia.