Bilaterian animals have dorso-ventral and antero-posterior
axes, as well as left and right sides. The mechanisms for
establishment of both the dorso-ventral and antero-posterior
axes vary considerably among animals [2] but the
subsequent patterning of these axes appears to be more
conserved. Dorso-ventral patterning in Xenopus and
Drosophila is similar [3–5], effected in both by interaction of
signaling molecules from the neural side (Decapentaplegic
[DPP] and BMP2/4, respectively) and the anti-neural side
(Chordin and Short gastrulation, respectively). Thus, in
deuterostome evolution, there was apparently an inversion
of the dorso-ventral axis with respect to the substratum [6].
Commonalities in antero-posterior patterning in the
Bilateria are striking in later embryos but less well understood
for early embryos For example, Hox genes are
expressed in a colinear pattern along the antero-posterior
axes of both deuterostomes and protostomes. One problem
is that early antero-posterior patterning roughly coincides
with gastrulation, and gastrulation varies considerably
among different phylogenetic groups. Nevertheless, some
common themes in the genetic basis of early antero-posterior
patterning are emerging, and are the focus of my
review. Coverage of the vertebrates is here largely limited
to amphibians. Left–right asymmetry [7] is characteristic
of relatively few bilaterians and will not be reviewed here.