1162 THE STRUCTURE OF EVOLUTIONARY THEORY
on both major branches). Did echinoderms delete their ancestral determinants to
evolve such an aberrant morphology or did they acquire entirely new regulatory
genes and developmental rules?
Few data now exist to address this important issue, but preliminary results
suggest that echinoderms have retained their genetic homologies with other bilaterian
phyla, while coopting several of these genes (with stable function in other phyla) for
different roles in their own unique development. In a pioneering study, Lowe and
Wray (1997) documented the expression in echinoderms of orthologs of three
important regulatory genes that encode transcription factors with a homeodomain,
and that generally function in the same broad way in both vertebrates and arthropods
(and must therefore be plesiomorphic to any derived condition in echinoderms):
distal-less for proximo-distal patterning in outgrowth of limbs, engrailed for
neurogenesis along the axis of the CNS, and orthodenticle for the differentiation of
anterior structures.
Lowe and Wray documented a full spectrum of results, ranging from retention to
cooption for markedly different echinoderms roles. At an extreme of retention, the
brittle star Amphipholis squamata expresses engrailed in neuronal cell bodies along
the five radial nerves. Lowe and Wray note (1997, pp. 719-720): "This expression is
superficially similar to that in bilaterial animals, in which engrailed is expressed
within a serially repeated subset of ganglionic neurons along the antero-posterior
axis. It is possible that a neurogenic role for engrailed is widely conserved among
triploblastic animals."
In the intermediary state of a retained general role transferred to novel organs,
sea urchins express distal-less at the distal ends of the five primary podia (tube feet)
soon after their formation—thus preserving the standard function of regulating
outgrowths from a body axis by expression at their distal tips, but now applied to an
autapomorphic outgrowth with no homolog in any other bilaterian phylum! Finally,
at the extreme of full cooptation (for new functions in new organs), brittle stars
express orthodenticle in ectoderm overlying the terminal ossicles at the ends of the
arms—a position with only tenuous and hypothetical connection to the anterior end
of the AP axis in bilaterian phyla. Moreover, at least for engrailed and orthodenticle
(the copy-number of distal-less remains undetermined in echinoderms), only one
ortholog exists in any echinoderm studied so far—so new functions cannot be
ascribed to the cooptation of duplicated copies.
Lowe and Wray's final statement (1997, p. 721) emphasizes the important
conclusion that genetic and developmental homologies of triploblast animals still
permit enormous flexibility in evolutionary diversification—primarily by the
principle of cooptation: "The highly derived body architecture of echinoderms
evolved at least in part through extensive modifications in the roles and expression
domains of regulatory genes inherited from their bilaterial ancestors. Even the limited
number of genes and species we examined demonstrates a remarkable evolutionary
flexibility in genes that have previously been considered interesting mainly for their
conserved roles in arthropods and chordates."