Chlamydomonas
641
basal bodies. Mutants with a variable num-
ber of flagella and basal bodies have also
been isolated. One of them,
vfl2
,hasasin-
gle amino acid change in centrin, a 20 kDa
calcium binding protein involved in the
formation of Fbers that show calcium-
dependent contraction. The
vfl2
mutant
has structural defects in the Fbrous struc-
tures in which centrin is present: the
nucleobasal body connectors, the distal
striated Fbers that link the two basal bodies
together, and the flagellar transition region
between the basal bodies and their flagella.
This mutant is also defective in basal body
localization and/or segregation.
While most of the nuclear mutations
affecting flagellar function and assembly
map on various chromosomes, several
are linked together on a linkage group
called the
uni
linkage group. The name
is derived from the
uni
mutation, which
affects the assembly of one basal body
and, consequently, leads to the appearance
of uniflagellated cells. The high degree
of clustering of functionally related genes
on the
uni
linkage group is unusual for
eukaryotic chromosomes.
±ragments of the
uni
linkage group have
been cloned by taking advantage of the re-
striction fragment length polymorphisms
that exist between
C. reinhardtii
and
C.
smithii
. Two genetically marked strains of
these two species were crossed, and the
progeny containing the
uni
linkage mark-
ers from the
C. smithii
parent and the other
nuclear markers from the
C. reinhardtii
parent were selected and backcrossed with
the
C. reinhardtii
parent. By repeating these
backcrosses several times, it has been pos-
sible to construct a strain with a
uni
linkage
group derived from
C. smithii
in a nuclear
genetic background that derives mostly
from
C. reinhardtii
. A repetitive element
present in both parental strains was then
used to detect R±LPs linked to the
uni
link-
age group and to clone DNA fragments of
the
uni
linkage group. These fragments
were used as probes to determine that the
uni
chromosome is a linear DNA molecule
consisting of 6 to 9 megabase pairs.
5
Perspectives
Chlamydomonas
has emerged as an at-
tractive model system for studies of the
molecular and cellular biology of eukary-
otic photosynthetic cells. This alga can be
manipulated with relative ease at the bio-
chemical, molecular, and genetic levels.
It is, at present, the only organism in
which nuclear, chloroplast, and mitochon-
drial transformations are feasible. The
ability to introduce genetically modiFed
genes into
C. reinhardtii
mutant strains
in which the endogenous gene is defec-
tive can provide important insights into
the structure–function relations of the
corresponding proteins. In this respect,
this alga will remain valuable for stud-
ies on flagellar function and assembly,
especially for understanding the complex
functional interrelationships between the
various flagellar subcomplexes
in vivo
.An
important point is that the use of
C.
reinhardtii
cana
lsobeextendedtotheun
-
derstanding of human diseases associated
with flagellar or ciliary dysfunction.
Because the photosynthetic apparatus of
C. reinhardtii
is very similar to its ho-
mologue in higher plants, this alga is
also an excellent system for studying the
biogenesis and function of the photosyn-
thetic complexes, and for investigating
more generally the genetic interactions
between the nuclear, chloroplast, and mi-
tochondrial compartments. Other areas
of research for which
Chlamydomonas
is
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