The Questions of Developmental Biology

(or inactivating) specific genes. The determination of the blastomeres and the activation of certain
genes are controlled by the spatial localization of the morphogenetic determinants within the egg
cytoplasm.
Conditional specification, however, also plays an important role in tunicate development.
One example of this process involves the development of neural cells. The nerve-producing cells
are generated from both the animal and the vegetal anterior cells, yet neither the anterior or
posterior cell of each half can produce them alone. When these anterior pairs are reunited, though,
the brain and palp tissues arise. Ortolani (1959) has shown that this region of ectoderm is not
determined for "neuralness" until the 64-cell stage, right before gastrulation. Similarly, while
most muscles form autonomously from the yellow crescent material of the B4.1 blastomere, the
most posterior muscle cells form through conditional specification by cell interactions with the
descendants of the A4.1 and b4.2 blastomeres (Nishida 1987, 1992a,b). Thus, although most
tissues are determined autonomously by segregation of the egg cytoplasm, certain tissues in
tunicate embryos have a conditional determination by cell-cell interaction.
Specification of the embryonic axes
The axes of the tunicate larva are among its earliest committments. Indeed, all of its
embryonic axes are determined by the cytoplasm of the zygote prior to first cleavage. The first
axis to be determined is the dorsal-ventral axis, which is defined by the cap of cytoplasm at the
vegetal pole. This cap defines the future dorsal side of the larva and the site where gastrulation is
initiated (Bates and Jeffery 1987). When small regions of vegetal pole cytoplasm were removed
from the zygote (between the first and second waves of zygote cytoplasmicmovement), the eggs
neither gastrulated nor formed a dorsal-ventral axis.
The second axis to appear is the anterior-posterior axis, which is also determined during
the migration of the oocyte cytoplasm. The yellow crescent forms in the region of the egg that
will become the posterior side of the larva. When roughly 10% of the cytoplasm from this
posterior vegetal region of the egg was removed after the second wave of cytoplasmic movement,
most of the embryos failed to form an anterior-posterior axis. Rather, these embryos developed
into radially symmetrical larvae with anterior fates (Figure 8.40). This posterior vegetal
cytoplasm (PVC) was "dominant" to other cytoplasms in that when it was transplanted into the
anterior vegetal region of zygotes that had had their own PVC removed, the anterior of the cell
became the new posterior, and the axis was reversed (Nishida 1994). The left-right axis is
specified as a consequence of these first two axes, and the first cleavage divides the embryo into
its future right and left sides.
Gastrulation in Tunicates