"Is there programmed cell death among single-celled organisms? The answer gets caught up in a question of semantics between a cell choosing to die and being forced to die. But some forms of programmed death are found in unicellular organisms, including bacteria. The death of the mother cell during sporulation, the process in which spores are created, could be considered a kind of programmed cell death. Certain parasites, such as trypanosomes (which cause malaria), change form to elude the immune response from their host; the laggards who o fail to undergo the change will die off in a kind of cellular altruism.
"Another parallel example occurs among slime molds, such as Dictyostelium discoideum, a species at the interface between unicellular and multicellular organisms. These animals spend most of their lives as unicellular amoebae. But, when starved, the cells aggregate and meld into a single 'slug' that migrates and eventually forms a funguslike structure, consisting of a stalk topped by a ball of spores. The spores disperse in search of a more hospitable environment. The stalk cells do not reproduce, so in a sense they sacrifice themselves. In most single-celled organisms (particularly bacteria), it is not clear whether cell death follows the same pattern or biomolecular mechanisms as the apoptosis that occurs in higher organisms.
"One highly unusual form of cell death occurs in cells infected by certain plasmid viruses that instruct the host cell to create two chemicals: a long-lived toxin and an unstable antidote. During replication, about 1 percent of the cells lose the parasitic plasmid in their DNA; the daughter cells still contain the toxin but can no longer manufacture the antidote, so they die. This is a rare instance of cellular murder rather than suicide.