Structural Constraints, Spandrels, and Exaptation 1245
provide enzymatic protection as well as visual refraction (Wistow, 1993, p. 301).
As an example of exaptation associated with duplication, two genes produce delta
crystallin in chickens and ducks, with the deltai gene specialized for lens expression,
and the delta2 gene producing the same enzyme, arginino-succinate lyase, in non-lens
tissue, but also generating some lens crystallin as well. Interestingly, both genes are
equally active in the duck lens, which thus includes ASL enzyme activity (through
the enhanced action of the delta2 gene) at a 1500 fold higher level than in chicken
lenses (for no understood function as yet).
A presumably much older duplication occurred in the alpha crystallins present in
most vertebrates, with the alphaA and alphaB genes now residing on different
chromosomes. The alphaA gene has specialized for production of its lens crystallin,
but maintains some activity in other organs of some species. However, the alphaB
gene has retained more of its original function in generating a heat shock protein,
while also coding for lens crystallin.
Interestingly, and beginning to unite crucial themes of the last two chapters, the
alphaA crystallin gene of chickens is regulated by at least 5 control sites (Cvekl et al.,
1994). Sites C and E bind Pax- 6 (the famously homologous "master regulator" of eye
development in squids, arthropods, and vertebrates—see pp. 1123-1132) in the lens
to stimulate alphaA crystallin promotor activity, thus controlling high expression of
this gene in the lens and repression in fibroblasts (Cvekl et al., 1994, p. 7363). These
authors also report that Pax- 6 binds to the lens-specific regulatory enhancer of the
delta! crystallin gene of chickens, and to the lens-specific regulatory sequence of the
zeta crystallin gene of guinea pigs.
Finally, some lens crystallin genes undergo more extensive duplication, usually
followed by a further specialization of some copies for lens functions, as expected.
"For the beta and gamma crystallins, multiple gene duplications have led to gene
families with six or more members that seem to be specialized for lens" (Piatigorsky
and Wistow, 1991, p. 1079). In the most extensive example of duplication, the squid
genome includes at least 10 S-crystallin genes, all derived from the gene that
produces the glutathione S-transferase (GST) enzyme. These S-crystallin genes are
expressed only in the lens and cornea and now lack enzymatic function, with one
exception of "very slight GST activity" in a single S-crystallin (Piatigorsky et al.,
1994, p. 243): "The S-crystallin genes encoding the inactive enzyme derivatives have
acquired an additional exon which probably contributes to the loss of enzyme activity
of the crystallin."
To close this long section, and these details of a developing classic, with a
lovely corroborative tale in the venerable tradition of natural history, the squid
Euprymna scolopes collects phosphorescent bacterial symbionts in a "light organ"
located in the center of its mantle cavity. "The squid uses the light emitted by the
symbiotic bacteria in its behavior, presumably in anti-predatory displays and/or
intraspecific communication" (Montgomery and McFall-Ngai, 1992, p. 21000). The
light organ also includes a lens formed as