Stanley Shostak
A complex hypothesis is offered for the origins of cnidarian cnidocysts through symbiogeny. The two-part hypothetical pathway links the origins of tissues through an early amalgamation of amoebic and epithelial cells to and the later introduction of an extrusion apparatus from bacterial parasites. The first part of the hypothesis is based on evidence for morphological, molecular, and developmental similarities of cnidarians and myxozoans indicative of common ancestry. Support is drawn from Ediacaran fossils suggesting that stem-metazoans consisted of symbiogenic pairs of epithelial-like shells enclosing amoeba-like cells. The amoeba-like cells would have evolved into germ cells and cells differentiating as or inducing nerve, muscle, and gland cells. The second part of the hypothesis proposes that cnidocysts evolved in a cnidarian/myxozoan branch of the metazoan tree through the horizontal transfer of bacterial genes encoding an extrusion apparatus to proto-cnidarian amoebic cells and consequently to the Cnidarian germ line. Evidence for bacterial genes in Cnidaria and transposable elements are cited in support of this part of the hypothesis.