Chlamydomonas
621
been used for studies on mating and chloroplast inheritance. However, since in
contrast to
C. reinhardtii
both these species are obligate photoautotrophs, their use
is more limited.
Chlamydomonas monoica
is particularly suited for the analysis of the
genetic control of sexuality, and several mutants defective in mating-type or zygote
formation have been isolated and characterized.
Major technical advances have been achieved recently with
C. reinhardtii.
They
include the development of reliable transformation methods for the nuclear,
chloroplast, and mitochondrial compartments. Gene tagging and nuclear gene
rescue of mutants with genomic libraries are also feasible.
Chlamydomonas
genomics
projects have created large sets of ESTs that have greatly facilitated the cloning of
nuclear genes of known function, and the
Chlamydomonas
nuclear genome sequence
has been determined recently.
With these new tools,
C. reinhardtii
has become a powerful model system in which
it is possible to perform an extensive molecular genetic dissection of fundamental
biological processes, in particular photosynthesis, organellar biogenesis, and flagellar
function and assembly.
1
The Organism
The characteristic features of
C. reinhardtii
are its unique cup-shaped chloroplast,
which occupies nearly half of the cell
volume, and its two flagella (Fig. 1). In
this organism, photosynthetic function is
dispensable, provided a reduced carbon
source such as acetate is included in the
growth medium. The alga can, therefore,
be grown under three different regimes:
phototrophic growth with CO
2
assimilated
through photosynthesis as the unique
carbon source, heterotrophic growth in the
dark with acetate, and mixotrophic growth
in the light with acetate. These growth
properties have been used extensively to
isolate and maintain numerous mutants
de±cient in photosynthetic activity. The
cell division cycle of this alga can also
be synchronized by subjecting cells to
alternate light and dark cycles.
Haploid vegetative cells of
C. reinhardtii
exist as mating-type
+
or mating-type
,
determined by alternative alleles of a
nuclear gene, and can be propagated indef-
initely through mitotic divisions (Fig. 2).
Gametogenesis is usually induced when
vegetative cells are starved for nitro-
gen. Gametes formed from synchronously
grown
C. reinhardtii
display several char-
acteristic features such as loss of ribo-
somes, alterations of chloroplast morphol-
ogy, starch accumulation, and reduced
photosynthetic activity. The mixing of ga-
metes of opposite mating type is followed
by a rapid adhesion of their flagella. Since
an individual gamete can adhere to more
than one cell of opposite mating type,
clumps of gametes are formed. This re-
sponse appears to be mediated through
gamete-speci±c glycoproteins called
ag-
glutinins
, which are associated with the
flagellar membrane and can be released
as vesicles from the flagellar tips. This
agglutination triggers flagellar-tip activa-
tion, loss of cell walls, and activation of
mating structures, in which specialized
regions differentiated at the cell anterior
previous page 1295 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 1297 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off