The Questions of Developmental Biology

In Chlamydomonas, recognition occurs first on
the flagellar membranes (Figure 2.10; Bergman et al.
1975; Goodenough and Weiss 1975). The flagella of two
individuals twist around each other, enabling specific
regions of the cell membranes to come together. These
specialized regions contain mating type-specific
components that enable the cytoplasms to fuse.
Following flagellar agglutination, the plus individuals
initiate fusion by extending a fertilization tube. This tube
contacts and fuses with a specific site on the minus
individual. Interestingly, the mechanism used to extend
this tube the polymerization of the protein actin is
also used to extend processes of sea urchin eggs and
sperm. In Chapter 7, we will see that the recognition and
fusion of sperm and egg occur in an amazingly similar
manner.
Unicellular eukaryotes appear to possess the basic
elements of the developmental processes that
characterize the more complex organisms:
protein sinthesis is
controlled such that
certain proteins are
made only at certain
times and in certain
places; the structures of
individual genes and
chromosomes are as
they will be throughout
eukaryotic evolution;
mitosis and meiosis
have been perfected;
and sexual reproduction
exists, involving
cooperation between
individual cells. Such
intercellular cooperation
even more important with the
evolution of multicellular organisms.
*After a recent formal name change,
this species is now called
Acetabularia acetabulum. For the
sake of simplicity, however, we will
use Hämmerling's designations here.