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A “rays-as-appendages” model for the origin of pentamerism in echinoderms
Published online by Cambridge University Press: 20 May 2016
Abstract
A new hypothesis concerning the evolutionary origin of pentameral symmetry in echinoderms is presented. The BA-A-BA pattern of Lovén's law in echinoids and the 2-1-2 symmetry of the edrioasteroid Stromatocystites appear to be morphogenetically related, and this pattern appears to be locked into development. This pattern may have originated from a unirayed 0-1-0, —A—, ancestor. I propose that a duplication of the uniray occurred and resulted in the addition of a pair of rays that followed Bateson's rules of symmetry to form a three-rayed 0-1-2, —A-BA, construction. This change occurred on the side that corresponds to the left side of the organism. This event made the individual asymmetric with respect to its anterior-posterior axis. Therefore I propose that morphogenetic regulation of bilaterality of the organism then led to homeotic expression of a mirror-image pair of rays on the opposite side. These two morphogenetic steps achieved the 2-1-2, BA-A-BA pattern. “Appendage status” of rays is assumed necessary to invoke the Batesonian mirror-image duplications of the model.
Three robust morphological characters emerge from the “rays-as-appendages” model: (1) 2-1-2, BAo-A-BA, organization; (2) “locked-in” pentamerism; and (3) a 2-3 pattern of right and left rays. Results based on ray homology research are presented for echinoids, asteroids, ophiuroids, edrioasteroids, ophiocistioids, and holothurians. I speculate that helicoplacoids may have 0-1-2 triradiate construction, and that solutes may have 0-1-0 uniray construction. The model has limits; it does not explain 1-1, or 2-1, or 2-2 or 1-1-1 organization of ambulacra.
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