Historical Constraints and the Evolution of Development 1115
anatomical homology, because any similar action occurs within segments at an
arthropod's frontal end, but operates within the apparently unsegmented mid and
forebrain of developing vertebrates). In the chick midbrain, rostral to the anterior
limit of Hox action, a long-range signaling region, located at the isthmic constriction
between the posterior end of the midbrain and the rhombomeres behind, regulates AP
patterning within the unsegmented field of the developing midbrain. Signals from this
isthmus regulate the action of En- 1 and En-2, two engrailed genes homologous with
the prominent segment-polarity engrailed gene of Drosophila. In chicks, the
engrailed gradient (see Fig. 10-18) spreads from the isthmus in both directions,
decreasing anteriorly through the mesencephalic vesicle and also posteriorly through
the first rhombomere (Lumsden and Krumlauf, 1996, p. 1112). Moreover, these
authors add (p. 1112), "En expression is the earliest known marker for mesencephalic
polarity."
Finally, although this argument only applies to the relationship of vertebrates
with other chordates, and not to any protostome group, the forebrain, and even the
neural crest, may not be so confined to true vertebrates as previous views generally
assumed. In overt appearance, the anterior end of amphioxus does not include any
organs comparable with the vertebrate mid or forebrain. But Holland and Holland
(1998) report that amphioxus homologs of two genes with important action in the
vertebrate fore and midbrain also operate in generating the anteriormost cerebral
structures of amphioxus. (AmphiOtx, the homolog of the vertebrate Otx that operates
in both fore and midbrain, is expressed at the anterior end, and in the ventral and
lateral walls, of the cerebral vesicle in amphioxus. AmphiDll, the homolog of
vertebrate Dlx that operates in the forebrain, is expressed at the extreme anterior end
of the cerebral vesicle, and also in the dorsal wall.)
Holland and Holland conclude (1998, p. 651) "the expression patterns of these
amphioxus genes suggest that the cerebral vesicle is largely homologous to the
vertebrate forebrain, but cannot rule out a midbrain homo-log." Moreover, the
expression pattern of AmphiDll during neurulation implies "that the epidermal cells
bordering the neural plate may represent a
10 - 18. Possible homology in genetic action at the anterior end of both arthropods and
vertebrates. In chicks, a gradient of Engrailed expression spreads from the isthmus of the
developing brain in both directions, decreasing anteriorly through the mesencephalic vesicle,
and also decreasing posteriorly through the first rhombomere. From Lumsden and Krumlauf,
1996.