Genetics of Apoptosis

2.4

Programmed cell death in Porifera

Although species of the Porifera (sponges) are clearly multicellular, they lack the
structural features of the Eumetazoa. Here our knowledge regarding apoptosis is still
rather fragmentary. Caspases have not yet been discovered in sponges. However, there
are reports of apoptotic cells in sponges (Wiens et al., 1999), and two genes encoding
bcl-2 and death domain-motifs have been reported in the sponges Geodia cydonium
and Suberites domuncula (Wiens et al., 2000). Unfortunately, no functional data
regarding these genes in sponges are yet available.
The above results demonstrate clearly that the three basic functions for which
apoptosis is used in higher eukaryotes—cell number regulation, tissue sculpting
during embryogenesis (metamorphosis), and ooplasm formation—are also found in
the cnidarians Hydra and Hydractinia. There is no evidence yet for a role of apoptosis
in defense against infections. This may, however, simply be due to the lack of an
appropriate model system for infections in cnidarians. The results cited above also
demonstrate that caspases are present in Hydra and Hydractinia, and that they are
activated when apoptosis in initiated. In contrast to this rich yield of evidence for
apoptosis and caspases in cnidarians, there is little evidence in sponges and single-
celled eukaryotes (see below) for either. The lack of evidence of caspases is particularly
telling since complete or almost complete genome sequences are available for yeast,
Dictyostelium, and Trypanosoma. In none of these genome sequences has evidence
been found of caspases. We conclude—tentatively—from these facts that caspase-
dependent apoptosis arose coincident with the evolution of true multicellular animals
—the Eumetazoa.

3.

Cell death in single-celled eukaryotes

The idea that cellular suicide makes sense only if the dying cell sacrifices itself forthe
sake of an organism overlooks the fact that single-celled animals also
performdifferentiation programs and thus create more than one form of themselves.
Thesurvival of such organisms could depend on a sacrifice of cells in one
developmental stage to allow survival of another stage that propagates the genome.
It is therefore not surprising to find programmed cell death in single-celled eukaryotes.
Wewill review here the data available about programmed cell death in the slime
moldDictyostelium discoideum, the ciliate Tetrahymena thermophila, and the
kinetoplastid parasites Trypanosoma and Leishmania.

3.1

Differentiation and cell death in D. discoideum

Considerable progress has been made in recent years in understanding programmed
cell death in the slime mold D.discoideum. Dictyostelium cells grow as single cells on

156 GENETICS OF APOPTOSIS