In the asexual fission phase of growth, during which cell divisions occur by mitosis rather than meiosis, clonal aging occurs leading to a gradual loss of vitality. In some species, such as the well studied "Paramecium tetraurelia", the asexual line of clonally aging paramecia loses vitality and expires after about 200 fissions if the cells fail to undergo meiosis followed by either autogamy (self-fertilization) or conjugation (outcrossing) (see aging in "Paramecium"). DNA damage increases dramatically during successive clonal cell divisions and is a likely cause of clonal aging in "P. tetraurelia".