TITLE:
Symmetry of echinoderms: From initial bilaterally-asymmetric metamerism to pentaradiality
AUTHORS:
Sergey V. Rozhnov
KEYWORDS:
Echinodermata; Symmetry; Asymmetry; Metamerism; Pentaradiality; Body Plan; Hox Genes; Colinearity
JOURNAL NAME:
Natural Science,
Vol.6 No.4,
February
27,
2014
ABSTRACT: Echinoderm radial
symmetry has first appeared in the ambulacral system, when the ambulacral
channel assumed the shape of a closed ring or a horseshoe with approximated
ends, and then spread onto other organ systems. Its origin was a natural
consequence of a steady increase of the original asymmetry of bilaterally-asymmetric
three-segmented ancestors of echinoderms, culminated by closing of the
ancestral linear metamerism into radiality. The evolutionary transformation
from a simple pouchlike hydrocoel with one side channel to an elongated
hydrocoel (located under the oesophagus)
with two side channels, and finally to a nearly closed horseshoeshaped
hydrocoel with three radial channels can be reconstructed based on the theca
structure and the number of ambulacra in the row SolutaCincta-Helicoplacoidea.
After the hydrocoel with three outbound ambulacral channels circled around the
oesophagus as a horseshoe, it either closed
into a ring, or its ends became closely approximated. This has
determined the primary triradiate symmetry, which quickly transformed into
pentaradial symmetry of the 2-1-2 type as a result of branching of two of the
three primary radial channels. This occurred no earlier than the Late
Vendian, when the first bilaterians appeared to have begun to acquire body
appendages, and no later than the Early Cambrian, when the first skeletal
remains of echinoderms entered the fossil record (in the Atdabanian). The 2-1-2
pentaradial symmetry evolved into the true pentaradiality as a result of
shifting the timing of tentacle branching to earlier stages of ontogenesis
and even spreading of the five tentacle primordia over the ambulacral ring.
This occurred during the Ordovician.