Historical Constraints and the Evolution of Development 1131
the highest possible degree. Yet reason tells me, that if numerous gradations
from a perfect and complex eye to one very imperfect and simple, each grade
being useful to its possessor, can be shown to exist... then the difficulty of
believing that a perfect and complex eye could be formed by natural selection,
though insuperable by our imagination, can hardly be considered real (1859,
pp. 186-187).Expanding his discussion in later editions, Darwin specifies a potential starting
point of maximal simplicity in structure and function: "The simplest organ which can
be called an eye consists of an optic nerve, surrounded by pigment-cells and covered
by translucent skin, but without any lens or other refractive body. We may, however,
... descend even a step lower and find aggregates of pigment-cells, apparently serving
as organs of vision, without any nerves, and resting merely on sarcodic tissue. Eyes
of the above simple nature are not capable of distinct vision, and serve only to
distinguish light from darkness" (1872b, p. 135).
Therefore, following this theme, if we wish to develop a complete evolutionary
explanation for the role of Pax- 6 homologies in regulating the formation of complex
structures in the lens-eyes of several phyla, we also need to understand its ancestral
role in species with much simpler organs of vision, or without eyes at all. Why, in
short, did Pax-6, rather than some other molecule, become the homologous "master
control gene" of such complex structures, especially if the common ancestor of
modern phyla with lens-eyes had only evolved eyes of much simpler form and
function?
Fortunately, even at our current embryonic stage of research, some intriguing
hints exist for a resolution, thus completing the intellectual structure of an
evolutionary argument for important parallelism in the evolution of eyes, as regulated
by the positive channel of Pax- 6 homologies. Pax- 6 homologs have been cloned from
three cnidarian genera—from a jellyfish and a hydra (Sun et al., 1997, though
questioned by Catmull et al., 1998). Catmull et al. speculate (1998, p. 355) that "the
capture of a homeobox by an ancestral Pax gene probably permitted a transition from
functions in cell-fate specification to roles in anterior patterning," and later to still
more specialized roles in the development of the central nervous system and finally in
the specification of eyes. Because Acropora lacks eyes, despite showing sensitivity to
light, Catmull et al. suspect that the Pax- 6 homolog of this cnidarian may regulate
anterior (and distal) patterning of the nervous system. (They also conjecture, on the
same grounds, that the Pax- 6 homolog in the blind nematode C. elegans may operate
as a plesiomorphic regulator of the head region, rather than as a sign of heritage from
an eyed ancestor.)
But Sheng et al. (1997), in an intriguing discovery that might link Pax- 6 to an
ancestral function tied more closely to vision, found that Drosophila Pax- 6 directly
regulates the expression of the visual pigment rhodopsin in photoreceptor cells.
Sheng et al. (1997, p. 1122) therefore propose that "the evolutionarily ancient role of
Pax- 6 was to regulate structural genes (e.g. rhodopsin) in primitive photoreceptors,
and only later did it expand its function