Modern Interpretation and Modification of the Placula Hypothesis of Metazoan Origin
Here, a nonsymmetric and axis-lacking bauplan (placula) transforms into a typical symmetric metazoan bauplan with a defined oral–aboral or anterior– posterior body axis. In the placula transformation, a primitive disk consisting of an upper and a lower epithelium (lower row), which can be derived from a flattened multicellular protist, forms an external feeding cavity between its lower epithelium and the substrate (second row from bottom). The latter is achieved by the placula lifting up the center of its body, as this is naturally seen in feeding Trichoplax (i.e., the two Trichoplax images derive from a nonfeeding (first row) and feeding (second row) individual. If this process is continued, the external feeding cavity increases (cross section, third row) while at the same time the outer body shape changes from irregular to more circular (see oral views). Eventually, the process results in a bauplan in which the formerly upper epithelium of the placula remains outside (and forms the ectoderm) and the formerly lower epithelium becomes ‘‘inside’’ (and forms the entoderm; upper row). This is the basic bauplan of Cnidaria and Porifera. Three of the four transformation stages have living counterparts in the form of resting Trichoplax, feeding Trichoplax, and cnidarian polyps and medusae (right column).
The above-outlined transformation of a placula into a cnidarian bauplan involves the development of a main body axis and a head region, which allows the invention of new structures and organs for feeding. From a developmental genetics point of view, a single regulatory gene would be required to control separation between the lower and upper epithelium (three lower rows). If the above scenario were correct, the following empirical data would be congruent with it. In the form of the putative ProtoHox/ParaHox gene, Trox-2, in Trichoplax, we find a single regulatory gene, marks the differentiation of an as yet undescribed cell type at the lower–upper epithelium boundary in Trichoplax. More than one regulatory gene would be required to organize new head structures originating from the ectoderm–entoderm boundary of the oral pole (upper row). Quite noteworthy, two putative descendents of the Trox-2 gene, Cnox-1 and Cnox-3, show these hypothesized expression patterns (Diplox expression upper row; for simplicity, only the ring for Cnox-1 expression is shown). Cnox-1 and Cnox-3 expression both mark the ectoderm-entoderm boundary at the oral pole in the hydrozoan Eleutheria dichotoma. Both genes are expressed in parallel in a ring-shaped manner at the tip of the manubrium, with Cnox-3 being expressed more ectodermally and Cnox-1 being expressed more entodermally (unpublished data).