text: The single-cell eukaryote Paramecium tetraurelia can undergo both asexual and sexual reproduction.  Asexual or clonal reproduction occurs by binary fission.  Binary fission involves mitosis-like behavior of the chromosomes similar to that of cells in higher organisms.  The sexual forms of reproduction are autogamy, a kind of self-fertilization, and conjugation, a kind of sexual interation between different cells.  Clonal asexual reproduction can be initiated after completion of autogamy or conjugation.  P. tetraurelia is able to replicate asexually for many generations but the dividing cells gradually age and after about 200 cell divisions, if the cells fail to undergo another autogamy or conjugation, they lose vitality and expire.  This process is referred to as clonal aging.  Experiments by Smith-Sonneborn, Holmes and Holmes and Gilley and Blackburn showed that accumulation of DNA damage is the likely cause of clonal aging in P. tetraurelia. This aging process has similarities to the aging process in multicellular eukaryotes (See DNA damage theory of aging).