1124 THE STRUCTURE OF EVOLUTIONARY THEORY
cord, several parts of the brain, and especially in the morphogenesis of vertebrate
eyes, "first in the optic sulcus, then in the optic vesicle, the pigmented and the neural
retina, the iris, in the lens and finally in the cornea" (Gehring, 1996, p. 12).
Although the existence of a Drosophila homolog could certainly have been
anticipated—the Pax genes, after all, were found with fly probes—few researchers
expected that a Drosophila version would also function in the same basic way. But
the Drosophila Pax- 6 homolog mapped to the eyeless (ey) locus (Quiring, et al.,
1994), named for a mutation discovered early in the 20th century, and producing, in
homozygous state, flies with strongly reduced eyes, or lacking eyes entirely.
Moreover, the conservation between mammalian and insect Pax- 6 sequences is
impressively high, with 94 percent amino acid identity in the paired box and 90
percent in the homeobox (Gehring, 1996).
The similar function of these Pax- 6 homologs in different phyla was then
dramatically affirmed by expressing the mouse gene in Drosophila (Haider et al.,
1995), and finding that the mammalian version could still induce the formation of
normal fly eyes. Noting that Pax- 6 acts as an upstream regulator of a large set of
more specific genes, Gehring (1996, p. 14) makes the obvious, but important, point:
"Of course, the eyes that are induced by the mouse gene are Drosophila compound
eyes, since the mouse gene is only the switch gene and another 2500 genes from
Drosophila are required to assemble an eye."
In the boldest of all experiments, leading to results that attracted substantial and
well-deserved public attention, Gehring and colleagues then found that ectopic
expression of either the murine or Drosophila version of Pax- 6 could induce
supernumerary eyes on the antennae, legs and wings of flies (Fig. 10-21), thus
supporting Gehring's designation of Pax- 6 as a "master control gene" for the
development of eyes. Gehring (1996, p. 13) wrote that these "ectopic eyes are
morphologically normal with normal photoreceptors,
10 - 21. From Gehring, 1966. One of the most remarkable discoveries from the early days of
evo-devo. An ectopic eye (smaller and to the left of the normal eye in A; enlarged in B) can be
induced in Drosophila by targeted expression of the mouse homolog of Pax- 6 in flies.