AHA! Found it in a study of cellular clonal aging
" . . . a cell cannot undergo unlimited reproduction . . . and undergoes senescence.
. . . Paramecium has a limit of vegetative reproduction without sexual reproduction but Tetrahymena does not always have a limited lifespan.
“Comparing the two species would increase our knowledge of the mechanism of cellular clonal aging.”
Pretty good memory I have, considering I read about this around forty years ago.