UC San Diego

Planarians, free-living flatworms, are able to regenerate complete individuals from miniscule fragments. However, little is known about the effects of regeneration on aging, or senescence, in planarians. Using an asexual strain of the planarian, Schmidtea mediterranea, we studied long- term differences in aging between the two fragments of fission, the anterior (head) and posterior (tail), by tracking fecundity and viability. Fecundity was quantified by measuring patterns in the time interval between fissions. Viability was tracked as survival rates over time. We created separate lines of successive heads and successive tails for observation. We present the first demonstration that the head and tail products of fission are not identical with respect to aging. Lines of successive heads showed aging through reduced fecundity over time; and all lines ultimately died out. Conversely, lines of successive tails showed no aging based on uniform fecundity and no recorded deaths. Furthermore, we found that all tails were equally rejuvenated regardless of the age of the original planarian at the time of fission. Thus, asexual fission can act as a mechanism for rejuvenation in tails at the cost of aging in heads