The Questions of Developmental Biology

The gradient of Dpp concentration from dorsal to ventral is created by the opposing
actions of Tolloid (increasing Dpp) and Sog (decreasing it) (Figure 22.14; Marqués et al. 1997).
In Xenopus and zebrafish, the homologues of Tolloid Xolloid and BMP1, respectively have
the same function. They degrade chordin. The gradient of BMP4 from ventral to dorsal is
established by the antagonistic interactions of Xolloid or BMP1 (increasing BMP4) and chordin
(decreasing BMP4) (Blader et al. 1997; Piccolo et al. 1997). Thus, it appears that nature may have
figured out how to make a nervous system only once. The protostome and deuterostome nervous
systems, despite their obvious differences, seem to be formed by the same set of instructions.
Limb formation
It is also possible that nature has only one set of instructions for forming limbs (Shubin et
al. 1997). Nothing could be a better example of analogy than vertebrate and insect legs. Fly limbs
and vertebrate limbs have little in common except their function. They have no structural
similarities. Insect legs are made of chitin and have no inner skeleton. They are formed by the
telescoping out of ectodermal imaginal discs (see Chapter 18). Vertebrate limbs, on the other
hand (no pun intended), have no chitin, but possess a bony endoskeleton. These limbs are created
by the interaction of ectoderm and mesoderm (see Chapter 16). However, the genetic instructions
to form these two distinctly different types of limbs are extremely similar.
As we have seen, Sonic hedgehog is usually expressed in the posterior part of the
vertebrate limb bud. If it is expressed in the anterior part of the bud, mirror-image duplications
arise (see Figure 16.19; Riddle et al. 1993). In the Drosophila wing or leg imaginal disc, the
Hedgehog protein is expressed in the posterior portion of the disc. If it is expressed anteriorly,
mirror-image duplications of the wing will form (Figure 22.15; Basler and Struhl 1993; Ingham
1994). Furthermore, certain genes regulated by the Hedgehog proteins have been conserved as
well (Marigo et al. 1996). Thus, the anterior-posterior axis appears to be specified in the same
way in vertebrate and in insect limbs. The dorsal-ventral axis also appears to be specified
similarly. The ventral limb compartments of both insects and vertebrates appear to be specified
by the expression of the engrailed gene (Davis et al. 1991; Loomis et al. 1996), while the dorsal
compartment is defined by apterous (in insects) or its relative Lmx1 (in vertebrates) (Figure
22.16).