Historical Constraints and the Evolution of Development 1111
bands eventually form, no more than three are expressed at any one time, because
older (anterior) bands fade as new bands appear at the tail bud (see description in
Kimmel, 1996). Since the her1 bands form and fade before the appearance of
somites, Miiller et al. (1996) traced the fate of cells from the her1 bands in later
embryos. In a particularly gratifying result, cells of the first herl band formed somite
5, while cells of the second band generated somite 7, thus confirming homology of
action for pair-rule genes (expression in every other somite) as well as homology of
genetic sequence.
Pennisi (1997) then described the work of Pourquie and colleagues on chairy
(for chick hairy), the chick homolog of the same Drosophila pair-rule gene hairy.
This study added important data on the timing of gene action, again linking the
spatial order along a major body axis to a temporal sequence that can easily implicate
heterochrony, the classical rubric for elucidating relationships between ontogeny and
phylogeny, as a pathway (and preferred channel) for evolutionary change. The early
chick embryo grows an elongated region where about 50 somites will originate, one
at a time, starting at the anterior end, and taking about 90 minutes for each to form.
Pourquie and colleagues found that chairy first becomes active in the rear 70% or so
of the entire elongated region. The band of expression then narrows and shifts
forward towards the head, finally becoming concentrated in a thin stripe at the rear
edge of the next somite to form. After this stripe appears, the gene turns on again
over the same broad region, beginning the cycle anew and ending in a sharp stripe at
the next posterior segment in the developing array.
(2) A segment polarity gene in amphioxus. Although vertebrate homologs of
arthropod segment polarity genes do not seem to function in establishing
segmentation (for their expression begins only after somitic boundaries have formed),
AmphiEn, the only amphioxus homolog of the Drosophila segment polarity gene
engrailed, appears in stripes at the posterior border of the first eight somites to
develop (Holland et al., 1997). (Drosophila engrailed appears in a similar position at
the anterior borders of developing parasegments, which become the posterior borders
of adult segments, since each final segment forms from the junction of the posterior
half of one parasegment with the anterior half of the next parasegment in the A-P
array.) Holland et al. (1997, p. 1723) draw a strong inference about segmental
homology: "The segmental expression of AmphiEn in forming somites suggests that
the functions of engrailed homologs in establishing and maintaining a metameric
body plan may have arisen only once during animal evolution. If so, the protostomes
and deuterostomes probably shared a common segmented ancestor. "
(3) Does resegmentation occur in developing vertebrae, and could such a
process be homologous with the conversion of embryonic insect parasegments to
adult segments? As De Robertis (1997) reminds us, anatomical data known for more
than a century indicate that a subset of cells in each somite (called the sclerotome)
forms a vertebra. But each adult vertebra arises by