Abstract
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.
Highlights
Ever since the modern synthesis, efforts to sort out the source(s) of unique phylogenic characteristics start with the assumption of de novo novelty: “mutation did it.” Symbiosis offers an alternative source of cladogenic innovation albeit requiring compounding hypotheses with horizontal gene transfer
The origins of cnidocysts in Cnidaria offers a unique opportunity for examining a hypothetical role for symbiosis in metazoan evolution
Microsporidians are not sufficiently homologous to Cnidaria to account for cnidocysts, and Myxozoa bearing cnidocytsts in the form of polar capsules arose from Cnidaria and could not have given rise to cnidarian cnidocyts
Summary
Ever since the modern synthesis, efforts to sort out the source(s) of unique phylogenic characteristics start with the assumption of de novo novelty: “mutation did it.” Symbiosis offers an alternative source of cladogenic innovation albeit requiring compounding hypotheses with horizontal gene transfer. Morphological similarities had long been recognized between cnidarian cnidocysts, and microbial “cnidocysts” [4,5], the “peduncle,” “rhizoid,” and “perforator” of dinoflagellates [6,7,8], the trichocysts of trypanosomes [9], zooflagellates [10] and mastigophorans [11], the “apicoplasts” (apical complexes) of “Sporozoa,” the “polaroplast,” of microsporidian [12], and the “polar capsules” of myxosporidians [13,14,15,16,17,18,19]. These hypotheses, whether invoking sporozoans, microsporidians, or protists as the source of cnidocytes had fatal weaknesses, and alternatives were necessary to overcome these weaknesses
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