Breeding performance of Tetrahymena thermophila following storage for 5 to 6 years in liquid nitrogen

Although the ciliated protozoan, T. thermophila, approaches immortality while growing vegetatively, most stocks sooner or later lose the ability to produce viable progeny after conjugation. Liquid nitrogen storage eliminates, or drastically slows, this aging process.Multiple stocks of 11 inbred strains have been thawed after 1,2, and 5 to 6 years in storage and mated to a tester strain or among themselves. Significantly more viable progeny resulted from crosses of frozen-thawed cultures than from crosses of unfrozen controls. The performance of the inbred strains varied as did that of replicate samples from rapidly aging strains. Crosses made with samples in which relatively few cells had survived storage and thawing varied more than did those made with samples containing many survivors.These results (a) show that good breeding strains retain this ability during storage and (b) support the hypothesis that the mechanism of aging in Tetrahymena results from the accumulation of random defects rather than an intrinsic program. It appears, however, that no results from the present experiments provide any new insight into the mechanism of aging.