Historical Constraints and the Evolution of Development 1139
and meaning of similarities in development between the appendages of arthropods
and vertebrates (Tickle, 1992; Tabin, Carroll and Panganiban, 1999; Panganiban et
al., 1997; Shubin, Tabin and Carroll, 1997; Arthur, Jewett and Panchen, 1999;
Minelli, 2000; for example). I will state my own tentative reading of these
preliminary data up front: most documented homologies are too brick-like to impart a
sufficiently strong and specific constraint for validating either the actual homology of
limbs themselves, or even a claim for predominant parallelism in the evolution of
homoplastic appendages. This situation may be contrasted with the highly and
specifically channeled developmental homologies underlying the establishment and
differentiation of major body axes, several aspects of segmentation itself, and the
evolution of important homoplastic organs at several levels, including eyes among
phyla and maxillipeds among crustacean taxa. These homologies are more than
sufficiently column-like to validate channels of internal constraint as primary
determinants of specific final products. However, some attributes of homoplastic
features in arthropod and vertebrate appendages do offer intriguing hints that, even
here, developmental homologies may be sufficiently column-like in some cases to
implicate constraints of internal generating channels as major causes of similarity in
adult structures.
As a prime example of a brick-like developmental homology, now regarded as
too broad and loosely constraining to specify important details of final products as
outcomes shaped by internal channels, but often regarded as more column-like in the
first excitement of discovery, the Drosophila distal-less gene (Dll) is expressed at the
distal tip of developing appendages and seems important in regulating their
outgrowth from the body axis (Cohen et al., 1989). In the mid 1990's, researchers
found a mammalian homolog (called Dlx) that seems to operate in virtually the same
way, with expression along the distal edge of the chick wing bud (Carroll et al., 1994;
Panganiban et al., 1995).
But as studies proceeded, an embarras de richesses soon became apparent, as
distal-less homologs were found at the terminal regions of almost any structure that
grows out from a central mass or body axis in all three great groups of bilaterians—
including annelid parapodia, onychophoran lobopodia, tunicate ampullae and
echinoderm tube feet (Panganiban et al., 1997). Lee and Jacobs (1999) then pointed
out that not only does distal-less seem to regulate the proximodistal axis of any
outgrowth, but it also tends to show preferred action in early embryos (including
maternal transcripts in several cases), in animal poles and anterior regions of
developing embryos, and in ectodermal germ layers. Thus, distal-less may not only
display the broad function of regulating outgrowths at their distal tips; it may also
operate in the service of even more basic distinctions that can only be designated as
early, anterior and top. Distal-less, in this sense, must be regarded as a quintessential
Pharaonic brick of protean character, or just about as non-specifically unconstraining
as an internal developmental element can be. If anyone wanted to argue that insect
and vertebrate appendages should be deemed homologous because both are regulated
by distal-less homologs, then