Mq. Martindale et Jq. Henry, THE DEVELOPMENT OF RADIAL AND BIRADIAL SYMMETRY - THE EVOLUTION OF BILATERALITY, American zoologist, 38(4), 1998, pp. 672-684
Understanding the evolutionary origin of novel metazoan body plans con
tinues to be one of the most sought after answers in biology. Perhaps
the most profound change that may have occurred in the Metazoa is the
appearance of bilaterally symmetrical forms from a presumably radially
symmetrical ancestor. The symmetry properties of bilaterally symmetri
cal larval and adult metazoans are generally set up during the cleavag
e period while most ''radially'' symmetrical cnidarians do not display
a stereotyped cleavage program. Ctenophores display biradial symmetry
and may represent one intermediate form in the transition to bilatera
l symmetry, The early development of cnidarians and ctenophores is com
pared with respect to the timing and mechanisms of axial determination
. The origin of the dorsal-ventral axis, and indeed the relationships
of the major longitudinal axes, in cnidarians, ctenophores, and bilate
rian animals are far from certain. The realization that many of the mo
lecular mechanisms of axial determination are conserved throughout the
Bilateria allows one to formulate a set of predictions as to their po
ssible role in the origins of bilaterian ancestors.