1120 THE STRUCTURE OF EVOLUTIONARY THEORY
the University of California, San Diego. See also the general commentary of Hogan,
1995; De Robertis, 1997; and Gould, 1997c.)
The chordin (chd) gene of Xenopus codes for a protein that operates in
patterning the dorsal side of the developing embryo, and also plays an important role
in formation of the dorsal nerve cord. But sog, the homolog of chd in Drosophila, is
expressed on the ventral side of the developing larva, where it acts to induce the
formation of ventral nerve cords. Thus, the same gene by evolutionary ancestry acts
in the development of both the dorsal nerve tube in vertebrates and the ventral nerve
cords in Drosophila—in conformity with Geoffroy's old claim that the two phyla can
be brought into structural correspondence by inversion.
Two further discoveries then promoted this intriguing hint into a strong case.
First, major gene acting in development and specification of the dorsal surface in flies
(decapentaplegic, or dpp) has a vertebrate homolog (Bmp-4) that patterns the ventral
side of Xenopus. Moreover, the entire system seems to operate in a similar manner—
but inverted—in the two phyla. That is, dpp, diffusing from the top to the bottom, can
antagonize sog and suppress the formation of the ventral nerve cords in Drosophila—
while Bmp- 4 (the homolog of dpp) diffusing from the bottom to the top, can
antagonize chordin (the homologue of sog) and suppress the formation of the dorsal
nerve cord in vertebrates (see Fig. 10-20).
Second, the fly genes work in vertebrates, and vice versa. Vertebrate chordin
can induce the formation of central nerve tissue in flies, while fly sog can induce
dorsal nerve tissue in vertebrates. These three discoveries, taken