Specialization of B-type cyclins for mitosis or meiosis in S. cerevisiae.

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Copyright 0 1995 by the Genetics Society of America

Specialization

of

B-Type Cyclins

for

Mitosis

or

Meiosis

in S.

c e r e a e

Christian

Dahmann’

and

Bruce Futcher

Cold Spring Harbor Laborato?, Cold Spring Harbor, New York 1 1 7 2 4

Manuscript received January 19, 1995 Accepted for publication April 13, 1995

ABSTRACT

The C L B l , CLB2, and CLB3 genes encode Btype cyclins important for mitosis in Saccharomyces cerevisiae,

while a fourth Btype cyclin gene, CLB4, has no clear role. The effects of homozygous clb mutations on

meiosis were examined. Mutants homozygous for clbl clb3, or for clbl clb4, gave high levels of sporulation,

but produced mainly two-spored asci instead of four-spored asci. The cells had completed meiosis I but not meiosis 11, producing viable diploid ascospores. C L B l and CLB4 seem to be much more important

for meiosis than for mitosis and may play some special role in meiosis 11. In contrast, CLB2 is important

for mitosis but not meiosis. The level of Cdc28-Clb activity may be important in determining whether meiosis I1 will occur.

M

OST eukaryotic species have three types of nuclear division: mitosis, meiosis I, and meiosis 11. These modes of division have many common features. In mitosis and in meiosis 11, the sister centromeres mi- grate to opposite poles. In contrast, in meiosis I, duplicated homologous chromosomes are paired, and the homologs separate from one another, while the sister centromeres remain together. At the molecular level, little is known about the distinctions between these three modes of division.

All three modes of division probably require the activity of a protein kinase called Cdc28 in Saccharomyces cerevisiae and called cdc2 in other organisms. Initiation of mitosis depends on this kinase activity, at least in part because formation of the spindle depends on the kinase activity (FITCH et al. 1992). The active kinase is a complex composed not only of the catalytic subunit, Cdc28, but also of an activating regulatory subunit called a cyclin (EVANS et al. 1983; reviewed by DRAETTA

1993). The catalytic subunit is present throughout the cell cycle, but has little or no activity as a monomer. In G2 and M phases “mitotic” cyclins, which are usually of the B-type structural class, accumulate. These form a complex with the catalytic subunit, and, subject to appropriate phosphorylation reactions, the complex ac- quires a protein kinase activity. This is necessary but not sufficient for mitosis (reviewed by NURSE 1990). At some point after anaphase, an unknown cellular signal causes the cyclins to be ubiquitinated and destroyed, causing dissolution of the spindle and exit from mitosis

(MURRAY et al. 1989; S U R ” et al. 1993). Less is known about the role of cyclin-Cdc28 complexes in meiosis,

Curresponding author: Bruce Futcher, P.O. Box 100, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724.

E-mail: futcher@cshl.org

Bohr-Gasse 7, A-1030 Vienna, Austria.

Present address: Reserach Institute of Molecular Pathology, Dr.

even though some of the earliest studies of cyclins involved meiotic cells (e.g., PICARD et al. 1985; STANDART et al. 1987; WESTENDORF et al. 1989).

The yeast S. cerevisiae has >10 known cyclin homologs. Three of these-Clnl, Cln2, and Cln3-seem

pri-

marily involved in Start, the Gl/S transition. Two more-Clb5 and Clb6-are E-type cyclins primarily involved in Sphase. Four more-Clbl, Clb2, Clb3, and Clb4”are B-type cyclins known to be involved in mitosis (reviewed by NASMVTH 1993). These latter four are the subject of this work.

Clbl and Clb2 are a closely related pair of B-type cyclins

(76%

identical through the cyclin-homology re- gion)

(SURANA

et al. 1991). They become expressed sometime late in Sphase until a time late in mitosis. Clb3 and Clb4 are a second closely related pair of E- type cyclins (FITCH et d . 1992; RICHARDSON et al. 1992). They become expressed near the beginning of Sphase

( i e . , earlier than Clbl and Clb2), but also stay on until late in mitosis. Mutational analysis has revealed that each cyclin gene is individually dispensible. The clb2

mutant has a slight phenotype suggesting a delayed mitosis, while the other three single mutants have no de- tectable phenotype. The clbl clb2, the c a l clb2 clb3 and the clbl 6162 clb3 6164 mutants are inviable (FITCH et al.

1992; RICHARDSON et al. 1992). Although we and others (FITCH et al. 1992; RICHARDSON et al. 1992) have previously found the clb2 clb3 mutant to be inviable, during the course of the experiments below we obtained viable though unhealthy 6162 6163 spore clones. Growth tem- peratures slightly below 30” may have allowed viability. Notably, the clb3 6164 double mutant and the clbl clb3 clb4 triple mutant are viable and healthy. Analysis of

clbl clb3 clb4 triple deletion mutants with a conditional

clb2 allele shows that Clb-deficient cells arrest in G2 with replicated DNA and duplicated but unseparated spindle pole bodies. There is no mitotic spindle. It has

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been proposed that Clb3 and Clb4, which are expressed early, normally lead to formation of the short mitotic spindle. Clbl and Clb2, which are expressed later, may be involved in elongation of the mitotic spindle. When Clb3 and Clb4 are absent, Clbl and Clb2 are presumably capable of substituting for them

(FITCH

et al. 1992). It is not clear why the cell has so many cyclins. The mystery is made more acute by the fact that the clbl clb? clb4 triple deletion mutant has only a slight phenotype. One possibility is that some of the cyclins have specialized roles in meiosis I or meiosis 11. To address this, we constructed diploids homozygous for many of the possible combinations of clb deletions, and examined their meiotic phenotypes. Some similar mutants have previously been analyzed by GRANDIN and REED

(1993); in general, our results are consistent with the previous biochemical results, but we present new as-

pects of the mutant phenotypes.

MATERIALS AND METHODS

Yeast strains: Strains are shown in Table 1. Strains W303 MATa and W303 MATa were obtained from R. ROTHSTEIN (THOMAS and ROTHSTEIN 1989). The alleles of clbl, clb2, c1b3

and clb4 were those described by RTCH et al. (1992). Media: Standard yeast media were used for growth and maintenance of strains (SHERMAN 1991). Strains dependent on GAL-CLB2 were maintained on YEP

+

1% raf

+

1 % gal (1% yeast extract, 2% peptone, 1% raffinose, 1% galactose, with the sugars filter-sterilized). Presporulation medium was GNA (1% yeast extract, 3% Difco nutrient broth, and 5%

glucose). Sporulation medium 1 was 1% potassium acetate, 0.1% glucose, 0.125% yeast extract. Sporulation medium 2 was 3% potassium acetate, 0.1% raffinose.

Measurement of sporulation frequencies: Cells were grown to early stationary phase in liquid GNA (Figure 1) or liquid YEP

+

1% raf

+

1% gal (Table 2). Cell were washed, and then resuspended at a density of 1 to 2 X

lo7

cells per milliliter in liquid sporulation medium 1 (for Figure 1) or liquid sporulation medium 2 (for Table 2). Cells were shaken gently in a vessel with a high surface area to liquid volume ratio for 4 days at room temperature (about 23"). Cells were then examined by phase microscopy, and vegetative cells, lysed cells, monads, dyads, triads, and tetrads were counted.

Other methods: Cell volumes were measured using a Coulter Chanelyzer. FACS analysis for DNA content was as described by NASH et al. (1988).

RESULTS

General effects of clb mutations on sporulation: The various viable homozygous clb mutants were sporulated, and the proportion of vegetative cells, lysed cells, tetrads, triads, dyads and monads were counted. Vegeta- tive cells (i.e., cells that failed to sporulate) and monads

(i.e., cells that constructed an ascus containing a single spore) had distinctively different morphologies. In our hands, monads often had two DAPI-stainable nuclei, one encapsulated in a spore and one not, while triads often had four DAPI-stainable nuclei, three encapsulated and one not. Previous workers have shown that monads, dyads and triads can be generated when some

of the products of meiosis fail to be encapuslated in spores (PONTEFRACT and MILLER 1962; MOENS et al.

1974; DAVIDOW et al. 1980).

Many of the mutants had roughly the same overall percentage sporulation as the wild-type control (-30%). The main exception to this was the clbl cZb3 clb4 triple mutant, where sporulation efficiency was -5% (Figure 1). This culture had -25% lysed cells, which were not seen in most other cultures, so it may be that in this mutant initiation of meiosis often leads to lysis rather than sporulation.

clbl deletions, either alone or in combination with other mutations, caused a noticeable reduction in sporulation efficiency, and an increase in the proportion of asci containing dyads rather than tetrads (Figure 1). When strains with just one of the four CLBs intact were examined, the strain with CLBl alone (BF439, Table 2) sporulated quite well, with a high proportion of tetrads; the strains with CLB3 or CLB4 alone (BF437 and BF436, Table 2) sporulated less well, with a very high proportion of dyads among the asci; and the strain with CLB2

alone (CD140, Figure 1) sporulated very poorly, and 100% of the asci were dyads. In agreement with GRANDIN and REED (1993), these results suggest that in

meiosis, CLBl is the most important of the four cyclins, and CLBB is least important.

Curiously, many of the strains bearing a clb2 deletion

(e.g., CD133, BF434, CD138, Figure 1) sporulated somewhat more efficiently that the wild-type cells. Although we have no good explanation for this, a clb2 deletion does increase cell size slightly (data not shown), and this increased size could promote efficient sporulation

( CALVERT and DAWS 1984).

Because the clbl clb2 clb? mutant, the clbl clb2 clb4

mutant, the clb2 clb? clb4 mutant, and the clbl clb2 clb3 c1b4 mutant are inviable because they cannot complete mitosis, we could not examine their phenotypes in meiosis. Therefore, we constructed analogous strains where CLBB had been replaced by GAL-CLB2. These strains were maintained on YEP

+

raffinose

+

galactose plates, and then sporulated in the absence of galactose (Table 2). Replacement of CLB2 by GAL-CLB2

made little difference in the results, except that the sporulation efficiency decreased two- to sixfold, which could have been due to the different presporulation conditions (GNA us. YEP

+

galactose). Perhaps sur- prisingly, even the clbl GAL-CLB2 clb? clb4 quadruple mutant was able to make dyads; the sporulation frequency was

-2%

of that in the wild-type cells. These experiments are difficult to interpret because expres- sion of our GAL-CLB2 construct has not been directly assayed during meiosis. In meioisis, GAL1 and GAL10

are expressed at a low level even in the absence of galactose (KABAcK and FELDBERG 1985), and so our strains may have had biologically significant levels of

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Yeast Cyclins for Meiosis

TABLE 1 Yeast strains

959

Strains Genotypes

W303a

W303a

CD131

IFO1-2n CD123-28

CD132 CD133 CD134 CD 135 CD 136 CD137 CD138 CD139 CD 1 40 BF434 BF436 BF437 BF439 BF440

BF3867b BF416 CD 1 1 2 - 6 ~

BF4166a BF4168c BF41BlOb BF41612a BF41614b BF41617d BF450

BF45 1

BF452

BF453

MATa ade2-1 trpl-1 leu2-3, 112 his3-11, 15 ura3 canl-100 ssdl-d MATaade2-1 tq1-I leu2-3, 112 his3-11, 15 u r a 3 c a d - 1 0 0 ssdl-d M A T a / M A T a W303a X W303a diploid.

Isogenic to CD131, clb2::LEU2/+ clb3::TRPl/+

Isogenic to CD131, clbl::URA3/+ clb2::LEU2/clb2::LEU2 clb3::TRPl/+ clb4::HIS3/+

Isogenic to CD131, clbl::URA3/clbl::URA3

Isogenic to CDl31, clb2::LEU2/ clb2::LEU2

Isogenic to CD131, c1b3::TRPl/clb3::TRPl

Isogenic to CD131, clb4::HIS3/c164::HIS3

Isogenic to CD131, clbl::URA3/clbl::URA3 clb3::TRPl/clb3::TRPl

Isogenic to CD131, clbl::URA3/clbl::URA3 clb4::HIS3/cblb4::HIS3

Isogenic to CDl31, clb2::LEU2/ clb2::LEU2 clb4::HIS3/ clb4::HISjr

Isogenic to CD131, clb3::TRPl/clb3::TRPl clb4::HIS3/clb4::HIS3

Isogenic to CD131, clbl::URA3/clbl::URA3 clb3::TRPl/clb3::TRPl clb4::HIS3/clb4::HIS3

Isogenic to CD131, clb2/clb2 clb3/clb3

Isogenic to CDl31, clbl/ clbl clb2/ clb2 clb3/ clb3 GAL-CLB2/ GAL-CLB2/

Isogenic to CD131, clbl/clbl clb2/clb2 clb4/clb4 GAL-CLB2/GAL-CLB2

Isogenic to CD 13 1, clb2/ clb2 clb3/clb3 clb4/ clb4 GAL-CLB2/ GAL-CLB2

Isogenic to CD131, clbl/clbl clb2/clb2 clb3/clb3 clb4/clb4 GAL-CLB2/ GAL-CLB2

Isogenic to W303a, clbl::URA3 clb4::HIS3 M A T a adel met14 ura3 his3

M A T a / a clbl::URA3/+ clb4::HIS3/+ ura3/ura3 his3/his3 adel/+ ade2/+ leu2/+

MATa clbl::URA3 clb4::HIS3 ura3 his3 adel leu2 trpl M A T a clbI::URA3 clb4::HIS3 ura3 his3 met14 trpl MATa clbl::URA3 clb4::HIS3 ura3 his3 adel leu2 MATa clbl::URA3 clb4::HIS3 ura3 h i d adel leu2 M A T a clbl::URA3 clb4::HIS3 ura3 his3 met14 M A T a clbl::URA3 clb4::HIS3 ura3 his3 met14

MATa/a clbl/clbl clb4/clb4 ura3/ura3 his3/his3 adel/+ leu2/+ metl4/+ trpl/+

MATa/a clbl/clbl clb4/clb4 ura3/ura3 his3/his3 adel/+ leu2/+ metl4/+ trpl/+ URA3::GAL-CLB2/ URA3

metl4/+trpl/+ (CD112-6c X BF38B7b)

(=BF4166a X BF41614b)

(=BF4166a X BF41617d)

(=BF41610b X BF4168c)

(=BF41612a X BF41Mc)

IFO1-2n was constructed by crossing strain No. 245 (FITCH et al. 1992) with a clb2::LEU2 derivative of W303a. Strains BF434, 436, 437, 439, and 440 were constructed by crossing appropriate spore clones from the sporulation of IFO1-2n.

duced by the clbl GAL-CLB2 clb3 c1b4 quadruple mutant may have been due to cyclin function from some other cyclin, such as CLB5 or CLBG.

Some clb mutants produce dyads containing viable, diploid ascospores: One striking effect was that clbl clb4 double mutants, clbl clb3 clb4 triple mutants, and, to a lesser extent, clbl clb3 double mutants produced dyads rather than tetrads (Figures 1 and 2). Asci from various mutants were dissected. In all cases, including the clbl clb3 clb4 homozygous triple mutant, spore viability was high.

The homozygous clbl clb4 and clbl clb3 double mutants were analyzed in detail. Viability of the ascospores in the dyads was -90%. Viability of the ascospores in the rare tetrads was also -90% for both the clbl clb4

and the clbl clb3 mutants. Thus, even though it was rare for a meiosis to produce four spores in the clbl clb4

double mutant, a full meiosis was nevertheless carried out faithfully when it did occur.

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A

B

0

FIGURE I.-Sporulation in clb mutan&. (A) Efficiency of sporulation. The percentage sporulation of different homozygous

clb mutants is shown graphically. The percentage is calcrllatcd exactly as "Percent asci" in Table 2. (B) Dyads as a percentage of total asci. The percentage of asci that are dyads for different homozygous clb mutants is shown graphically. The percentage is calculated exactly as "Percent dyads/asci" in Table 2. Strains were isogenic with W303 and were homozygous for the mutations shown. GNA was the presporulation medium, except for the r1h2 dl)? cells (see below). In general, tetrads were much morc common than triads, and dyads were much more common than monads. About 1000 cells were assayed for each genotype. For the clbl clh3 clh4 mutant, 1094 cells were examined, and n o triads o r tetrads were seen. The rlb2 rll~? cells werc ass;~yrtl in a separate experiment from the other cells, and YEP

+

2% galactose was used as the presporulation medium instead o f GNA. This genotype has been described as inviable (FITCII F / nl. 1992; RI(:IIAKI)SON f / al. 1992), but although these cells were vc-?

sick, they nevertheless formed colonies.

(for the clbl clb3 strain) o r mainly dyads (for the I-//1l clb4 strain; data not shown). These asci were dissected, and again viability was high, with M A T a , M A T ? , and nonmaters among the progeny (data not shown). These results strongly suggested that the nonmaters arising from the original dyads were M A T a / a I-l/~1/I-/l~1 &4/

clb4 (or cZh3/clb3) diploids like the original diploid.

The homozygous clbl clb4 double mutants go through

meiosis I, but not meiosis 11: The diploidy of the ascospores in the dyads suggested that the parental cell had gone through only one of the two meiotic divisions. To see which division had been carried out, we crossed three centromere-linked markers into the dl11 1-1/14 dou-

ble mutant. The markers used were adel (chromosome

I ) , (chromosome

N ) ,

and mpf 14 (chromosome X [ ) .

The marked haploids arising from the crosses were no

TABLE 2

Results from sporulation of GAcCLB2 strains

Strain Relevant r b mutations

Percent Percent

asci dyads/ascus

CD131

cI>n

20 1.7

IFOl-2n & I / + clbZ/rlI)Z rll13/+ rl1)4/+ UM3::GAI,-CI,RZ/UM3 24 12

CD 123-28 clbZ/

+

dh3/

+

29 19

BF436 clbl clh2 r1h3 G A I A " 4 56

B437 rlbl rlb2 ~1114 GAI.-CIB2 7 90

BF439 cN)Z dl13 clh4 GAI,-CIJ32 14 23

AF440 rlbl r1b2 rlb3 rU14 GAIAY,RZ 0.5 100

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Yeast Cvclins for Meiosis 961

I

FI(;c'KF. 8.-Morphology of tetrads and dyatls. ( X ) Tctrads from the sporulation of CD131, Cl.B/Cl,li (R) Four dyads from the sponllation of CD137, cll)l/cll~I cl1~4/c11,4. The field also includes a monad (lower center), vegetative cclls, and lysed cells.

longer isogenic with each other, and so to partly control for the effects of other unidentified segregating genes we made four different marked diploids, each homozygous for clhl and c11,4, and heterozygous for n&I, m ~ t 1 4 ,

and tqll. After plating on sporulation medium, all three diploids gave >SO% sporulation, ant1 >9O% of the asci were dyads. Upon dissection, >90% of the ascospores were viable. As shown in Table 3, the centromere-linked markers almost always segregated in each dyad-that is, if one spore clone were Ade', then the other was Ade-. This is the segregation expected from a meiosis

I division. If a meiosis I1 division had occurred, then most of the dyads should have had two phenotypically Ade' spores, which would have been ADF;I/n&I hetero- zygotes.

The map distance from I J W 2 to the centromere [assayed as the proportion of (Leu', Leu') dyads] was "5 cM, which was not significantly different from wild

TABLE 3

Segregation of centromere-linked markers in dyads

Dvad type

Diploid Marker +,-

+,+

- _

RF4.50 CldP I I0 0 0

mrl I 4 10 0 0

BF45 1 rcdr I 17 I 0

w i d I 4 17 0 1

16 1 1

RF452 U d P l 20 1 2

m d I 4 21 1 1

BF453 U d P I 9 6 0

m114 15 0 0

14 1 0

Total U d P l 56 8 2

m t 1 4 63 1 1

I q l 10 0 0

17711 22 0 1

I?$ I 62 2 2

A

"+,-"

dyad is one where one spore clone is phenotypi-

cally wild type and the other is mutant for the indicated marker. A

"+,--"

dyad presumably arises from a meiosis I division: one pair of sister chromatids goes to one pole, giving a diploid homozygous for ( r . ~ . ) the wild type allele, while the other pair of sister chromatids goes to the other pole, giving a diploid homozygous for the mutant allele.

"+,+"

and "-,-" indicate that both spore clones in the dyad were phenotypically wild type, o r both were mutant, respectively.

"+,+"

dyads (presumably two heterozygous and therefore phenotypically wild type diploid spores) are expected from a meiosis I1 division, but also arise from a meiosis I division when there has been recombination between one marker and its centromere.

type. The map distance from MAT to the centromere was -25 cM, again similar to wild type. This suggest5 that recombination in the clbl/clbI cUA/cZh4 mutant was not greatly perturbed.

Exceptional dyads: Several exceptional dyads did not show segregation for one or more of the centromere markers (Tables 3 and 4). We were not surprised to see 11 (+,+) dyads, because this was approximately the number expected due to recombination between the three markers and their centromeres. However, there

TABLE 4

Examples of exceptional dyads

Spore

clone MAT Leu Ade Trp Met

RF451-la a

+

RF451-13a a / a

+

-

RF451-lb a -

+

-

RF451-13b a

+

-

RF452-5a a -

BF452-3b a / a

+

- -

-

- -

+

-

RF452-17a a/a - -

IZF452-17b ? -

+

- -

-

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was also a class of (-,-) dyads, and at least for diploids BF451 and 452, these were similar in frequency to the (+,+) dyads, suggesting the two classes of dyads may have been generated by a common mechanism. (-,-)

dyads cannot be generated by simple recombination; but require gene conversion, or chromosome loss, or some more complicated mechanism.

Exceptional events at one locus were correlated with exceptional events at another locus (Table 4). For in- stance, all four exceptional events involving trpl occurred in dyads that had other exceptional events; the probability of this occurring by chance is

-lop5.

The correlation suggests that some aberrant event is affect- ing several chromosomes at once.

DISCUSSION

Division of labor: The S. cereuisiae B-type cyclins seem to be somewhat specialized for mitosis or meiosis. CLB2

seems to be the most important B-type cyclin for mitosis and CLB4 the least important. In meiosis, the ranking is quite different; CLBl and CLB4 seem most important and CLB2 least important. Clb2 protein is expressed at very low levels (if at all) in meiosis (GRANDIN and REED

1993), and there is no clear evidence for any meiotic phenotype associated with clb2 deletions. In contrast, strains lacking CLBl have low sporulation efficiencies (Figure 1) (GRANDIN and REED 1993), and strains lacking both CLBl and CLB4 seldom undertake meiosis I1

(Figure 1). The comparison between the clbl clb4 mutant and the clb2 clb4 mutant is particularly striking; the former is almost entirely incapable of producing tetrads, while the latter is nearly wild type. However, on those rare occasions when a clbl clb4 mutant did undertake meiosis 11, the tetrads were viable and appar- ently normal. Thus, there is no absolute requirement for CLBl or CLB4.

Cdc28 activity and the control of meiosis:

An

early sign that meiosis I and meiosis 11 were differentially sensitive to the activity of Cdc28/cdc2+ was the discov- ery of the twsl mutation in Schizosaccaromyces pombe (NA-

KASEKO et al. 1984). This mutation proved to be an allele of cdc2 (NIWA and YANACIDA 1988; GRALLERT and SIPICZM 1990) and caused meiosis to yield two-spored asci. Like the spores in the present study, these spores were viable and diploid and arose from a meiosis in which meiosis I had occurred and meiosis I1 had not. Many other alleles of cdc2 also cause two-spored asci of the same type (HAYLES et al. 1986; GRALLERT and SIPIC- ZKI 1990), as do mutations in two genes whose wild- type alleles allow activation of the cdc2 protein kinase, namely cdc25 (a cdc2 tyrosine phosphatase) and cdcl3

(a B-type cyclin) (GRALLERT and SIPICZM 1991). Later, the same phenomenom was observed in S. cermisiae,

when it was found that many temperature-sensitive alleles of cdc28 caused two-spored asci during meiosis at a semipermissive or permissive temperature (SHUSTER

and BYERS 1989). Again, the spores were diploid and viable and arose from a meiosis I division. Electron microscopy revealed that the duplicated spindle pole bodies failed to separate in preparation for meiosis I1 (SHUSTER and BYERS 1989). Interestingly, failure of spindle pole body separation is a terminal phenotype of a clb- mutant (FITCH et al. 1992). It seems likely the

cdc2 mutations, the cdc28 mutations, and the clbl c1b4

double mutations all lower at least some types of cyclin/ cdk kinase activity in meiosis. In all three cases, the meiosis I1 spindle may fail to form and the spindle pole bodies may fail to separate. This could cause exit from meiosis after the first meiotic division. FURUNO et al.

(1994) have recently reported related results for meiosis in Xenopus oocytes: when Mos is ablated, cdc2 activity drops after meiosis I as usual, but the normal rapid reactivation fails to occur. These oocytes with low cdc2 activity do not then enter meiosis 11; instead, they enter interphase, and then replicate DNA. Thus, Xenopus as

well as yeast may have special mechanisms to restore high cdc2/Cdc28 activity immediately after meiosis I,

and this activity may be essential for preventing an im- mediate return to G1 and S, and may also be essential for promoting meiosis 11. These mechanisms may oper- ate only on some types of cyclins.

Some mutants produce dyads of a different kind. Mu- tants homozygous for spol2, or for spo13, go through a single meiotic division in which most of the chromosomes undertake a meiosis 11-like division (an equa- tional division) (KLAPHOLZ and ESPOSITO 1980), while other chromosomes undertake a meiosis I-like division (a reductional division) (HUGERAT and SIMCHEN 1993; reviewed by SIMCHEN and HUGERAT 1993). Recent work on $013 suggests that it may interact with or modulate Cdc28 activity (McCARROLL and ESPOSITO 1994). A particularly relevant result is that overexpression of SPO13

can largely suppress the tendency of a cdc28-1 mutant to give dyads (MCCARROLL and ESPOSITO 1994). Thus, meiosis I and meiosis I1 may be distinguished by different kinds of regulation of Cdc28 activity.

This work was supported by National Institute of General Medical Sciences grant GM-45410 to B.F.

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Yeast Cyclins for Meiosis 963

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