Historical Constraints and the Evolution of Development 1169
regionalized and individualized, leading to suppression by different elements in
various parts of the body: "The evolution of Scr-responsive elements led to the
modification or elimination of prothoracic wings and the evolution of abdA and Ubx -
responsive elements led to the elimination of abdominal wings and, in the Diptera, to
the reduction of metathoracic wings."
For the most general question of specialization of appendages for a wide variety
of forms and functions from their uniform state on all segments posterior to the head
of a homonomous ancestor, several lines of evidence identify walking appendages
(either uniramous, or biramous with an upper gill branch) as the ancestral state for a
homonomous ancestor, and as a continuing "ground state" for modern more
differentiated forms as well. First, the most homonomous modern groups—the
Myriapoda among the arthropods, and the Onychophora as a sister group to the entire
arthropod phylum—bear leg-like structures on each segment. Second, numerous
Cambrian arthropods that cannot be placed into modern groups share the common
property of nearly identical biramous appendages on all postoral segments, and only a
pair or two of antennae on any preoral segments—as in Marrella, the most common
fossil in the Burgess Shale (see Fig. 10-29 and Gould, 1989c). Third, as discussed
previously (pp. 1132-1134), the extensive suite of thoracic segments that bear
identical leg-like appendages in many modern Crustacea also show extensive and
complete overlap of expression for several Hox genes.
Proceeding down the AP axis of complexified arthropods, we first note that
antennae develop in the most anterior segments where no Hox expression occurs.
This situation probably marks retention of the ancestral condition. Even the most
homonomous forms, including myriapods and onychophores, exhibit some
specialization in the head segments at the anterior end, and only grow identical
appendages on subsequent postoral segments. Thus, the original Hox complex
probably never regulated development at the extreme anterior end around the mouth,
and antennae probably represent the plesiomorphic condition for segments with no
Hox action. Interestingly, and in confirmation, the suppression of all Hox activity in
Tribolium yields the lethal