Historical Constraints and the Evolution of Development 1127
developing brain.) In the most satisfying result (see Fig. 10-23), ectopic expression of
squid Pax- 6 also induced supernumerary eyes in Drosophila. Tomarev et al. write
(1997, p. 2424): "Squid Pax- 6 is able to induce ectopic Drosophila eyes on wings,
antennae, and legs, as was previously demonstrated for Drosophila eyeless and
mouse Pax-6. All Drosophila eye-specific structures including cornea, pigment cells,
cone cells and photoreceptors with rhabdomeres were formed in the ectopic eyes
induced by squid Pax- 6 DNA."
THEORETICAL ISSUES. TWO related questions have dominated the emerging
discussion of these Pax- 6 homologies: the putative status of this gene as a "master
control" for eyes, and the impact of Pax- 6 upon the claim for independent evolution
of eyes by convergence. Gehring (1996, 1998) bases his terminology of "master
control" upon three properties of Pax-6: its status as upstream regulator of a
substantial cascade of more specific eye-forming gene products; its interchangeability
among phyla, while always acting as a trigger to the downstream production of the
"right" eyes for any given animal; and its general ability to trigger the formation of
supernumerary eyes in odd places.
We may legitimately quibble, as Jacobs et al. (1998) and many others have
done, that upstream position in a cascade should not be equated with either causal or
temporal primacy, for novel regulatory elements can be introduced by evolution into
any position of a developing sequence. Nonetheless, I would not begrudge a
researcher the right to bestow an incisive name upon such an important discovery.
Pax- 6 may be no more important than hundreds of other genes in the sense that
usable eyes will not form, absent its normal operation. But the designation, as "master
control," of such early and such general action (including the key property of
interchangeability among phyla) does no violence to ordinary linguistic usage.
However, this very generality raises the crucial issue (see forthcoming pages for
a fuller discussion) of whether the action of Pax- 6 must then be regarded as too broad
and too universal to sustain any argument for meaningful constraint upon the
evolution of eyes in disparate phyla. After all, if activating Pax- 6 represents little
more than flicking on a master switch at the power plant (with animal development
than analogized to the operation of any electrical device thereafter), then its
admittedly necessary action fails to identify any channel of development specific
enough to warrant designation as a dedicated impetus for the evolution of one
adaptive solution over other attainable possibilities. However, in this case (but not in
others, as I shall argue on pp. 1034-1042), the actions of Pax- 6 are sufficiently
specific and precise to set a definite channel among conceivable alternatives, and not
just to open a floodgate through which subsequent cascades might flow in any
direction (see discussion of this point and listing of criteria for specificity by Jacobs
et al., 1998, p. 334, who conclude that this "documentation of eye homology was
quite a coup").
This acknowledgment of sufficient specificity for Pax- 6 then raises a final
question about the extent of revision thus required in evolutionary concepts of
convergence vs. constraint. Some enthusiasts have claimed that genetic