1108 THE STRUCTURE OF EVOLUTIONARY THEORY
to renewed respect for such "trivial" data of gross anatomy, the anterior expression
boundaries of several Hox genes map consistently to specific rhombomeres.*
The striking similarity between the action of vertebrate Hox in rhombomeres
and insect Hox in metameres generates strong suspicions of homology. For example,
some vertebrate Hox sequences follow the common insect pattern that the anterior
expression boundary of each successive 5' gene "skips" a segment, appearing two
segments towards the animal's posterior. In mice, Hoxb- 2 turns on in the third
rhombomere, Hoxb- 3 in the fifth, and Hoxb- 4 in the seventh. Moreover, cell
populations of the rhombomeres seem to follow the same "compartment" rules of
insect parasegments—i.e., cells originating before the formation of rhombomere
boundaries may place progeny in several rhombomeres, but the clones of all cells
formed after the development of a rhombomere boundary do not transgress into
adjacent rhombomeres.
These observations may lead a skeptic to admit that some segmental homology
exists, but only between the bulk of an arthropod's body and a relatively insignificant
portion of a vertebrate's anterior end (and not even to the crucial face or forebrain).
At this point, however, a key paleontological fact should convert skepticism into
strong interest. The rhombomeres of the embryonic hindbrain correlate directly with
the pharyngeal arches developing just alongside (Fig. 10-16). In fact, each pharyngeal
arch corresponds with two rhombomeres (Raff, 1996, p. 343). As we should
remember from our elementary courses, all early vertebrate embryos develop
pharyngeal arches, or gill slits. Tetrapods lose these structures in later embryology,
but their positions determine important aspects of embryological topology (including
migratory paths of neural crest cells and the subsequent locations of cranial nerves, as
mentioned above), while some of their parts transform into important organs of
gnathostome vertebrates. (Most famously, the jaw arises from the first gill arch, while
an element of the second arch becomes, in jawed fishes, the hyomandibula
(suspending the upper jaw to the braincase) and later, in tetrapods, the stapes, or
hearing bone.)
But, more importantly for acknowledging a meaningful segmental homology
between arthropods and vertebrates, the rhombomeres and their underlying Hox
codes do not only generate some important features of later tetrapod anatomy.
- "Whole animal biologists," including the author of this book, can only experience
enormous hope and gratification when colleagues trained in molecular and experimental
traditions recognize the utility of data so often ignored and disparaged as antiquarian or
superannuated. In fact, such a pattern has often been repeated in the history of science, as
when the initial recognition of Mendelian mutations led early geneticists to reexhume old
data long dismissed as mere description of phenomenological oddity—for example, the lit-
erature (dating to the earliest days of scientific publishing) on developmental anomalies.
When molecular biologists value such classical data more highly (and utilize them more
fruitfully) than practitioners in the classical fields manage to do themselves, then we may
truly hope for an integrated biology based on the prospect, so often expressed but so little
realized until recently, that molecular and organismic biology might finally consummate a
union on the common field of development.