Reductionism in Biology 365eye-spot development in the buckeye butterfly.
Once these details were elucidated inDrosophilait became possible to determine
the expression of homologous genes in other species, in particular inPrecis coenia.
To begin with, nucleic acid sequencing showed that genes with substantially the
same sequences were to be found in both species. In the butterfly these homologous
genes were shown to also organize and regulated the development of the wing,
though in some different ways. For instance, in the fruit flywingless organizes
the pattern of wing margins between dorsal and ventral surfaces, restricts the
expression ofapterous to dorsal surfaces and partly controls the proximo-distal
access wheredistal-lessis expressed. In the butterfly, winglessis expressed in
all the peripheral cells in the imaginal disk which will not become parts of the
wing, where it programs their death. [Nijhout, 1994, p. 45]Apterouscontrols the
development of ventral wing surfaces in both fruit flies and butterflies, but the
cells in which it is expressed in theDrosophila imaginal disk are opposite those
in which the gene is expressed inPrecisimaginal disks. As Nijhout describes the
experimental results:
The most interesting patterns of expression are those ofDistal-less.
InDrosophila Distal-lessmarks the embryonic premordium of imagi-
nal disks and is also expressed in the portions of the larval disk that
will form the most apical [wing-tip] structures...InPrecislarval disks,
Distal-lessmarks the center of a presumptive eyespot in the wing color
pattern. The cells at this center act as inducers or organizers for devel-
opment of the eyespot: if these cells are killed, no eyespot develops. If
they are excised, and transplanted elsewhere on the wing, they induce
an eyespot to develop at an ectopic location around the site of implan-
tation... the pattern ofDistal-lessexpression inPrecisdisks changes
dramatically in the course of the last larval instar [stage of develop-
ment]. It begins as broad wedge shaped patters centered between wing
veins. These wedges gradually narrow to lines, and a small circular
pattern of expression develops at the apex of each line...
What remains to be explained is why only a single circle ofDistal-less
expression eventually stabilizes on the larval wing disks. [Nijhout, p.
45]In effect, the research program in developmental molecular biology is to iden-
tify genes expressed in development, and then to undertake experiments — par-
ticularly ectopic gene-expression experiments–which explain the long established
observational “regularities” reported in traditional developmental biology. The
explanantiauncovered are always “singular” boundary conditions insofar as the
explananda are spatiotemporally limited patterns, to which there are always ex-
ceptions of many different kinds. The reductionistic program in developmental
molecular biology is to first explain the wider patterns, and then explain the ex-
ceptions — “defects of development” (if they are not already understood from