The Questions of Developmental Biology

But within the crustacean lineages, there are interesting variations on this theme.
Averof and Patel (1997) have shown that if a thoracic segment does not express Ubx and abdA, it
converts its anterior locomotor limb into a feeding appendage called a maxilliped. Thus, brine
shrimp such as Artemia have a uniform expression of Ubx and abdA in their thoracic segments,
and they lack maxillipeds. Lobsters such as Homaris lack Ubx and abdA expression in their first
and second thoracic segments, and these segments have paired maxillipeds (Figure 22.8).
The fossil record suggests that the earliest crustaceans lacked maxillipeds and had uniform
thoracic segments. This would mean that the presence of maxillipeds is a derived characteristic
that evolved in several crustacean lineages.
Why snakes don't have legs
As shown in Chapter 11, the expression patterns of Hox genes in vertebrates determines
the type of vertebral structure formed. Thoracic vertebrae, for instance, have ribs, while cervical
(neck) vertebrae and lumbar vertebrae do not. The type of vertebra produced is specified by the
Hox genes expressed in the somite.
One of the most radical alterations of the vertebrate body plan is seen in the snakes.
Snakes evolved from lizards, and they appear to have lost their legs in a two-step process.
Both paleontological and embryological evidence supports the view that snakes first lost their
forelimbs and later lost their hindlimbs (Caldwell and Lee 1997; Graham and McGonnell 1999).
Fossil snakes with hindlimbs, but no forelimbs, have been found. Moreover, while the most
derived snakes (such as vipers) are completely limbless, more primitive snakes (such as boas and
pythons) have pelvic girdles and rudimentary femurs. The missing forelimbs can be explained by
the Hox expression pattern in the anterior portion of the snake. In most vertebrates, the forelimb
forms just anterior to the most anterior expression domain of Hoxc-6 (Gaunt 1994; Burke et al. 1995).
Caudal to that point, Hoxc-6, in combination with Hoxc-8, helps specify vertebrae to be thoracic.