ORGANISM SURVIVAL AND CELL INTEGRITY IN A FREEZE-TOLERANT INSECT

Larvae and isolated fat body cells of the freeze-tolerant Heleomyza bo realis (Diptera, Heleomyzidae) were cooled to sub-zero temperatures be tween -5 and -40 degrees C at 0.1 degrees C min(-1) and then rewarmed at the same rate. Survival of the whole organism, cells cooled in vitr o and cells cooled in vivo were assessed using two fluorescent viabili ty stains. The whole animal supercooling point distribution was bimoda l, the two groups having means of -2.3 and -9.7 degrees C, suggesting a deficiency of potent ice-nucleating agents in the latter. Whole orga nism survival declined rapidly below -30 degrees C. Survival of cells cooled within the organism remained in excess of 95% above -25 degrees C and approached 80% at -35 degrees C, whereas the survival of cells cooled in vitro (isolated cells) declined rapidly at -20, reaching 45% at -40 degrees C. The reduced survival of isolated cells is attribute d to protective factors within the extra-cellular fluid or tissue stru cture of the larvae. The pattern of cell survival suggests membrane ru pture arising from shrinkage as the cause of cell death, whereas the p attern of whole organism survival is indicative of intra-cellular ice formation or failure of chill injury protective mechanisms at a specif ic temperature causing death of the organism.