In order to understand the mechanistic bases for evolution, it
is necessary to understand how development evolves (Raff,
1996). We have studied two congeneric species of sea urchins
that diverged only about 10 million years ago (Smith et al.,
1990; McMillan et al., 1993), but have very different modes of
early development: Heliocidaris tuberculata has a typical
indirect-developing feeding larva, whereas its congener H.
erythrogramma rapidly develops directly to the juvenile adult.
The radical changes in early development that these two
species have undergone over a relatively short evolutionary
time span both emphasizes the unexpected plasticity of
embryological programs, and offers an experimental system in
which we can begin to dissect the molecular and cellular
mechanisms that underlie such changes.
H. tuberculata has a 90 Ám diameter egg, and produces
a bilaterally symmetric pluteus larva (Fig. 1A) with
characteristic features including a functioning gut, arms with
skeletal rods, and a complete ciliary band arrayed along the
arms, surrounding the oral territory and separating oral from