Historical Constraints and the Evolution of Development 1133
along potential pathways of realizable form, but basic taxonomic order reflects the
limits and preferred channels of internal potential as much, or more, than the
happenstances of immediate selective advantage.)
To cite just one impressive case of extensive parallelism in the taxonomic order
of a substantial clade within a phylum, Averof and Patel (1997) have studied the
action of the Hox genes Ubx and abdA in specifying the form of gnathal and thoracic
appendages in Crustacea. In most arthropods, the gnathal appendages (maxillae) are
specialized for feeding, and the larger thoracic appendages for locomotion. But in
numerous and phyletically varied crustacean taxa, appendages of the anterior thoracic
segments have been reduced in size and specialized for primary utility in feeding.
These thoracic feeding limbs are called maxillipeds, and phyletic analysis clearly
illustrates their multiple independent evolution within the Crustacea.
As a general, and presumably ancestral, pattern in Crustacea, the transition
between gnathal and thoracic segments marks the anterior expression boundary for
Ubx and abdA, and these genes do not operate in the gnathal region, where smaller
feeding appendages (maxillae) develop. The branchiopods, for example, follow this
basic scheme: Ubx and abdA are expressed throughout the thorax, and no maxillipeds
form (Averof and Akam, 1995). These genes do not operate in the anterior gnathal
segments, which develop the smaller maxillae. In other groups, the generation of
maxillipeds on anterior thoracic segments correlates precisely with the suppression of
Ubx and abdA in these segments alone, and these specialized appendages then grow
to resemble the smaller maxillae of the adjacent anterior (gnathal) region of the AP
axis, where Ubx and abdA are not expressed in normal development.
The precision of this correlation is impressive, and presumably causal. Among
the malacostracans, for example, the leptostracans also develop no maxillipeds, and
Ubx and abdA are expressed throughout the thorax. But in peracarids, the first, and
sometimes the second, of eight thoracic appendages develop as maxillipeds. Averof
and Patel (1997) document the suppression of both Ubx and abdA in these anterior
thoracic segments with maxillipeds. In Mysidium colombiae, for example, Tl
generates a maxilliped, whereas the appendage of T2 remains primarily a swimming
organ, but develops gnathal features at its distal end. Averof and Patel found that Ubx
and abdA are entirely repressed in Tl, but expressed in the proximal portion of the T2
endopod, while being excluded from the distal portion that acquires the gnathal
features of a maxilliped.
The familiar decapods (lobsters, crabs, shrimp) generally bear eight thoracic
segments, the anterior three with maxillipeds and the posterior five with walking legs
(hence the name of the group, meaning 10-footed). This situation correlates perfectly
with the suppression of Ubx and abdA in the first three thoracic segments by
backward shifting of their joint anterior expression boundary. The finer scale
variations within the clade also follow the same developmental rule. For example,
although adult lobsters of the most familiar (and edible) Homarus americanus bear
the usual five pairs of large thoracic limbs and three pairs of anterior maxillipeds,
only Tl and T2 show