Historical Constraints and the Evolution of Development 1095
broader role of SEP genes in "providing flower-specific activity" (p. 528) in
combination with genes of the ABC series.)
Other studies provide additional confirmation (in modern genetic form) for
Goethe's original formalist notion of leaves as a ground state. A "meristem identity
factor" LEAFY (LFY) potentiates APETALA1 (API), which, in turn, activates the ABC
floral genes. Wagner et al. (1999, p. 582) demonstrate that this sequence of LEAFY to
API is "necessary and sufficient for this transition" (p. 582). Standard techniques for
documenting the effects of both loss and gain-of-function mutants confirm this
cascade. In the lfy- 6 mutant, suppressing the action of LFY, "most flowers are
replaced by leaves and second-order shoots"; while over expression of either LFY or
API "results in formation of flowers or leaves and flowers in positions normally
occupied by leaves" (Wagner et al., 1999, p. 582. See further confirmations in Busch
et al., 1999).
Extending the model to other angiosperm clades, Hofer et al. (1997) studied
PEAFLO, the pea homolog of LFY. They performed several experiments to extend
Goethe's formalist concept of morphological serial homology, now abetted by new
data on genetic and developmental homology, between leaves and flower parts. They
state: "A striking comparison can be made between the similar developmental units
of compound leaves and flowers: both arise laterally from primordia derived from the
shoot apical meristem; both produce lateral, leaf-like organs; and both are
determinate." Hofer et al. (1997) then affirmed and extended the evidence for
developmental homology by (1) identifying pleiotropic mutants that affect both leaf
and floral development in similar ways, and (2) by studying homeotic mutations that
"result in the conversion of floral organs to leaf-like structures" (p. 581). Their
concluding remark, reinforced by a later observation of Theissen and Saedler (2001,
p. 469), might have caused Faust to lose his bet with Mephistopheles—by inducing
such delight that this restless, archetypal romantic might finally have savored a
present moment with sufficient gusto to blurt out the fateful phrase that would seal
his doom: "verweile doch, du bist so schon" (stay awhile, thou art so beautiful). Hofer
et al. write (1997, p. 586): "Compound leaves and flowers can thus be considered to
be derivatives of the same ancestral structure." Theissen and Saedler simply
conclude: "Goethe was right when he proposed that flowers are modified leaves."
HOXOLOGY AND GEOFFROY'S FIRST ARCHETYPAL THEORY OF
SEGMENTAL HOMOLOGY
AN EPITOME AND CAPSULE HISTORY OF HOXOLOGY. These Goethian
confirmations extend, at least for now, little beyond the serial homology of
apparently disparate parts on the same plant. But archetypal claims for homology
across distantly related phyla raise far more serious theoretical problems. No
Shockwaves attended the discovery of common genetic and developmental pathways
for the serial array of arthropod appendages, despite their functional differentiation as
antennae, mouthparts, legs, genital claspers, etc.