622
Chlamydomonas
Fig. 1
Scanning electron micrograph of two
C. reinhardtii
gametes of opposite
mating-type during the initial stage of mating (Courtesy of U. Goodenough).
give rise to the fertilization tube of the
mating-type
+
cell. This series of complex
reactions ultimately leads to the fusion of
gametes and, subsequently, of their nuclei
and chloroplasts. The resulting zygote ma-
tures into a thick-walled zygospore, which
can be induced to undergo meiosis and
produce a tetrad consisting of four hap-
loid spores. Vegetative diploids can also
be selected from matings or poly(ethylene
glycol)-mediated cell fusions. They divide
mitotically and allow one to determine
whether a mutation is dominant or reces-
sive. Vegetative diploids have also proven
to be useful for isolating chloroplast and
mitochondrial mutations affecting photo-
synthesis and respiration respectively. The
most frequent mutations of this type are
nuclear and recessive and, therefore, are
not expressed in diploid cells, whereas
chloroplast and mitochondrial mutations
will segregate to yield mutant isolates.
Recently established methods of cyto-
duction allow the transfer of chloroplast or
mitochondrial markers into a deFned nu-
clear genetic background in
C. reinhardtii
.
In this process, fusion of cytoplasms of
the two parents occurs in the absence
of nuclear fusion. This leads to the ap-
pearance of cytoductants containing the
nuclear genome of one parent and cy-
toplasmic components of both parents.
During further divisions, segregation oc-
curs and the cytoplasmic marker of one
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