CELL STRUCTURE AND FUNCTION 26: 609–617 (2001) REVIEW
© 2001 by Japan Society for Cell Biology
Drosophila Male Meiosis as a Model System for the Study of Cytokinesis in Animal
Maria Grazia Giansanti1,2, Silvia Bonaccorsi1,2, Elisabetta Bucciarelli1, and Maurizio Gatti1,2
1Istituto Pasteur-Fondazione Cenci Bolognetti, and 2Centro di Genetica Evoluzionistica del CNR, Dipartimento
di Genetica e Biologia Molecolare, Universita` di Roma “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy
system allows easy and unambiguos identification of mutants defective in cytokinesis through the examination of
spermatid morphology. Moreover, cytokinesis defects and protein immunostaining can be analyzed with exquisite
cytological resolution because of the large size of meiotic spindles. In the past few years several mutations have
been isolated that disrupt meiotic cytokinesis in Drosophila males. These mutations specify genes required for the
assembly, proper constriction or disassembly of the contractile ring. Molecular characterization of these genes has
identified essential components of the cytokinetic machinery, highlighting the role of the central spindle during cy-
tokinesis. This structure appears to be both necessary and sufficient for signaling cytokinesis. In addition, many
data indicate that the central spindle microtubules cooperatively interact with elements of the actomyosin contrac-
tile ring, so that impairment of either of these structures prevents the formation of the other.
Drosophila male meiosis offers unique opportunities for mutational dissection of cytokinesis. This
cytokinesis/central spindle/contractile ring/meiosis/Drosophila
Cytokinesis is the complex process by which two cells
separate at the end of cell division. While in plant cells this
process is mediated by the deposition of cell wall material
between the two daughter cells, in animal cells cytokinesis
is accomplished by the contraction of a ring-shaped cellular
structure containing actin and myosin II filaments. This
structure is anchored to the plasma membrane at the equator
of the dividing cell and constricts in purse string fashion,
leading to the separation of the two daughter cells. The
cytokinetic process can be subdivided into at least four
subprocesses that must be tightly coordinated to ensure the
fidelity of chromosome segregation (reviewed by Glotzer,
1997; Straight and Field, 2000). First, interactions between
the spindle and the cortex determine the site of cleavage fur-
row formation. Second, an actomyosin-based contractile
ring assembles at this cortical site. Third, the actomyosin
ring constricts, leading to furrow ingression. Fourth, during
both furrow ingression and the completion of cytokinesis
new membrane is added to allow separation of the daughter
Although genetic, biochemical and cell biological ap-
proaches have provided some insight into the cellular struc-
tures that orchestrate the final step of cell division, still the
mechanisms underlying the assembly and functioning of the
cytokinetic apparatus remain largely unknown. A powerful
approach for molecular dissection of cytokinesis is the iden-
tification and molecular characterization of genes that con-
trol this process. This approach has been exploited in both
budding and fission yeast, leading to the discovery of sever-
al proteins involved in cytokinesis (reviewed by Goldberg et
al., 1998, Field et al., 1999, Robinson and Spudich, 2000).
More recently, a genetic approach has also been employed
in Drosophila melanogaster and Caenorhabditis elegans,
and many additional gene products required for cytokinesis
have been identified and characterized (reviewed by Field et
al., 1999; Gatti et al., 2000; Robinson and Spudich, 2000).
The advantages offered by Drosophila for genetic analy-
sis are well known. Moreover, this organism offers unique
opportunities for cytological examination of mutant pheno-
types and immunolocalization of gene products involved in
cytokinesis (reviewed by Goldberg et al. 1998; Gatti et al.,
2000). Drosophila cytokinesis can be examined in different
cell types, including cells undergoing embryonic divisions
after cellularization (Adams et al., 1998; Prokopenko et al.,
1999), larval neuroblasts (Gatti and Baker, 1989; Karess et
al., 1991; Castrillon and Wasserman 1994; Gunsalus et al.,
*To whom correspondence should be addressed: Maria Grazia Giansanti
Dipartimento di Genetica e Biologia Molecolare, Universita` di Roma “La
Sapienza, Ple. Aldo Moro5, 00185 Roma, Italy.
Tel: +39–06–49912842, Fax: +39–06–4456866
M.G. Giansanti et al.
1995; Giansanti et al., 2001; Wakefield et al., 2001), and
male meiotic cells (Gunsalus et al., 1995; Williams et al.,
1995; Hime et al., 1996; Basu et al., 1998; Bonaccorsi et
al., 1998; Carmena et al., 1998; Giansanti et al., 1998; Her-
rmann et al., 1998; Giansanti et al., 1999; Brill et al., 2000;
Wakefield et al., 2001). Thus, phenotypic analysis of muta-
tions affecting this process is likely to provide information
on whether different cells utilize different mechanisms to
Here we focus on cytokinesis in Drosophila male mei-
osis, describing the advantages of this systems for mutation-
al and phenotypical analysis of cytokinesis. In addition, we
describe the main results obtained from the study of meiotic
cytokinesis of Drosophila males, highlighting the role of
central spindle during this process.
Male meiosis as a model system for the study of
Male meiosis occurs in the context of a complex develop-
mental process, called spermatogenesis, that leads to the
formation of 64 spermatozoa, starting from a single gonial
cell generated by the asymmetric division of a germ line
stem cell. Concomitantly with the formation of this founder
cell, the asymmetric division of two cyst progenitor cells
generates two cyst cells which engulf the primary gonium
and its progeny till completion of spermatogenesis. The pri-
mary gonial cell undergoes four rounds of mitotic divisions,
giving rise to 16 primary spermatocytes. These cells, after a
dramatic growth phase, that results in a 25-fold increase in
nuclear volume, enter meiotic division, producing in turn 32
secondary spermatocytes and 64 spermatids (reviewed by
Lindsley and Tokuyasu, 1980; Fuller, 1993; Cenci et al.,
1994; Maines and Wasserman, 1998).
In both gonial and meiotic divisions the execution of
cytokinesis does not lead to complete separation of the
daughter cells, which remain connected by cytoplasmic
bridges, called the ring canals (Lindsley and Tokuyasu,
1980; Hime et al., 1996). These ring canals, whose function
during spermatogenesis is still poorly understood, develop
from arrested contractile rings and contain components of
the cytokinetic apparatus, such as anillin, the Peanut septin
and myosin II (Hime et al., 1996, Giansanti et al., 1999; our
Both meiotic divisions occur within a double nuclear
membrane which is invaginated and possibly fenestrated at
positions underlying the asters, but does not otherwise ap-
pear to disassemble (Tates, 1971; Church and Lin, 1982). In
addition, spermatocytes undergoing meiotic divisions are
surrounded by three to five layers of parafusorial mem-
branes which lie parallel to the spindle axis (Tates, 1971;
Fuller, 1993). During meiosis I and meiosis II, mitochondria
line up along the parafusorial membranes and are equally
partitioned between the two daughter cells at each division.
At the end of meiotic division the mitochondria inherited by
each cell fuse to form a complex interlaced structure, called
the Nebenkern. Thus, immediately following meiosis, at the
so-called onion-stage, a cyst is composed of 64 spermatids,
each containing a single nucleus associated with a single
This peculiar structure of Drosophila spermatids provides
one of the main advantages of male meiosis for the study of
cytokinesis. Mutants affecting cytokinesis can be easily and
unambiguously identified by examining the onion-stage
spermatids. In wild type, each onion-stage spermatid con-
sists of a round, phase-light nucleus associated with a single
phase-dark Nebenkern of similar size (reviewed by Fuller,
1993). Failures in cytokinesis abrogate proper mitochondri-
al partition between the daughter cells and result in aberrant
spermatids composed of an abnormally large Nebenkern as-
sociated with multiple nuclei (Fuller, 1993). Thus, sperma-
tids composed of a large Nebenkern associated with two or
four nuclei of regular size, are diagnostic of failures in mei-
otic cytokinesis (Fuller, 1993; Castrillon and Wasserman,
1994; Gunsalus et al., 1995; Williams et al., 1995; Giansan-
ti et al., 1998). On the other hand, since the volume of an
onion-stage nucleus is proportional to its chromatin content
(Gonzalez et al., 1989), spermatids containing a large
Nebenkern associated with nuclei of different dimensions
are indicative of errors in both chromosome segregation and
cytokinesis (Herrmann et al., 1998; Carmena et al., 1998;
Wakefield et al., 2001). In this context, it should be noted
that meiosis occurs in testes of third instar larvae, thus also
allowing examination of the meiotic phenotype of mutants
that die at the larval-pupal boundary (see, for example,
Gunsalus et al., 1995, Wakefield et al., 2001).
A further advantage of male meiosis for the phenotypical
analysis of cytokinesis is provided by the weak spindle
checkpoint that characterizes spermatocyte divisions. While
in larval brain cells the presence of disorganized spindles
activates the spindle integrity checkpoint, precluding the
observation of cell division subsequent to arrested
metaphases (Carmena et al., 1998; Avides and Glover,
1999; Wakefield et al., 2001), in spermatocytes the spindle
checkpoint is not stringent and causes only a small delay in
progression through meiosis (Basu et al., 1999, Rebollo and
Gonzalez, 2000). For example, mutations in the polo and
abnormal spindle (asp) genes, which cause metaphase ar-
rest in larval brain cells, do not block meiotic division in
males but produce frequent failures in cytokinesis (Her-
rmann et al., 1998; Carmena et al., 1998; Wakefield et al,
2001). Thus, by analyzing meiotic division, it is possible to
ask whether genes involved in spindle formation also play a
role during later stages of cell division, including cyto-
A third feature that makes male meiosis a highly suitable
system for the analysis of cytokinesis is the large size of the
meiotic spindles. For example, in telophases I and telo-
phases II the pole-to-pole distances are respectively 45 and
30 ?m (Cenci et al., 1994). In addition, telophases of both
Cytokinesis in Drosophila Male Meiosis
meiotic divisions display a prominent central spindle–the
bundle of antiparallel, interdigitating microtubules between
the segregating daughter nuclei–which is pinched in the
middle during cytokinesis (Cenci et al., 1994). The favour-
able cytological features of Drosophila spermatocytes have
allowed precise localization of many components of the cy-
tokinetic apparatus. These components include contractile
ring-associated proteins, such as actin, myosin II, anillin
and septins; proteins that regulate F-actin polymerization,
like profilin; microtubule-associated proteins, such as
KLP3A, Pavarotti (Pav) and Abnormal spindle (Asp); and
the Polo and Aurora B kinases. Each of these components
exhibits a specific temporal and spatial pattern of accumula-
tion in the cleavage region, which is summarized in Table I
and Fig. 1.
Finally it should be noted that recent studies have shown
that meiotic cytokinesis can be successfully examined in
living testes, allowing visualization of of the dynamic be-
havior of GFP-labelled proteins involved in cytokinesis
(Rebollo and Gonzalez, 2000; Sampaio et al., 2001).
The central spindle is the only source of signals
that stimulate meiotic cytokinesis in Drosophila
An open question about cell cleavage in animal systems
is the source of signals that stimulate contractile ring forma-
tion and cytokinesis. It has been suggested that these signals
may be provided either by the metaphase chromosomes
(Earnshaw et al., 1991), or the asters (Rappaport, 1961,
Hiramoto, 1971, Rappaport, 1986) or the central spindle
(Rappaport and Rappaport, 1974, Cao and Wang, 1996,
Fishkind et al., 1996). To address this question we have ge-
netically micromanipulated Drosophila male meiosis by
PROTEINS INVOLVED IN MEIOTIC CYTOKINESIS OF Drosophila MALES
IN MALE MEIOSIS
REQUIRED FOR CYTOKINESIS
CYTOLOGICAL DEFECTS IN
MALE MEIOTIC CELLS
Abnormal spindle Abnormal spindle
Disorganization of both CS and CR
Aurora B (b)
Absence of both CS and CR
6, 7, 8
Four wheel drive
Absence of both CS and CR
Absence of both CS and CR
Failure of CR constriction
Myosin II (b)
Equatorial cortex, CR
Absence of both CS and CR
Absence of both CS and CR
Absence of both CS and CR
19, 20, 21
22, 23, 24
25, 26, 27
TwinstarND ND YesYesFailure of CR disassembly3, 5
(a) embryonic cells of cycles 14-16; (b) mutants in genes encoding these proteins are either not available, or have not been examined for defects in cyto-
kinesis. CS: central spindle; CR: contractile ring; ND not determined. References: 1 Saunders et al., 1997. 2 Wakefield et al., 2001. 3 Gunsalus et al., 1995.
4 Field and Alberts, 1995. 5 Giansanti et al., 1999. 6 Adams et al., 2001. 7 Giet and Glover, 2001. 8 Our unpublished results: using an antibody kindly pro-
vided by D. Glover, we have found that Aurora B concentrates in the CS midzone of Drosophila spermatocytes. 9 Cooley et al., 1992. 10 Giansanti et al.,
1998. 11 Castrillon and Wasserman, 1994. 12 Basu et al., 1998. 13 Brill et al., 2000. 14 Williams et al., 1995. 15 Molina et al., 1997. 16 Ohkura et al., 1997.
17 Our unpublished results: using an antibody kindly provided by C. Field, we have found that Myosin II concentrates in the cleavage furrow of Drosophila
spermatocytes. 18 Adams et al., 1998. 19 Carmena et al., 1998. 20 Herrmann et al., 1998. 21 Sampaio et al., 2001. 22 Neufeld and Rubin 1994. 23 Hime et
al., 1996. 24 Our unpublished experiments : we have observed that in testes of larvae homozygous for peanut null mutations meiotic cyotokinesis is normal.
25 Karess et al., 1991. 26 R. Karess, personal communication: recent experiments have shown that Spaghetti squash-GFP accumulates in the cleavage fur-
row. 27 Our unpublished results: spermatocytes of spaghetti squash mutants are defective in both the CS and the CR.
M.G. Giansanti et al.
cytes. Cells were immunostained for tubulin (green) and anillin (orange); chromatin (blue) was detected by Hoechst 33258 staining. Anillin is a conserved
protein that contains an actin binding domain and a pleckstrin homology (PH) domain (Field and Alberts, 1995; Oegema et al., 2000). Anillin concentrates
in the equatorial cortex of Drosophila spermatocytes during anaphase (cell at the top left) prior to the assembly of the actin-enriched contractile ring. How-
ever, after contractile ring assembly, anillin precisely co-localizes with this structure throughout cytokinesis. In mutants devoid of both the central spindle
and the contractile ring (see below) the anillin band forms but fails to contrict. Based on these observations, it has been suggested that anillin may link the
contractile ring to the plasma membrane, mediating the interactions between these structures (Giansanti et al., 1999). Arrows point to two ring canals. Bar,
10 ?m. B. Schematic localization of proteins involved in spermatocyte cytokinesis. During late anaphase, when the central spindle has just begun to assem-
ble, anillin and myosin II concentrate in a narrow circumferential band at the cell equator, while KLP3A (yellow) and Pav (dark green) start to accumulate
in the central spindle midzone. During early- and mid-telophase, when a prominent and dense central spindle has formed, F- actin, myosin II, anillin and the
septins co-localize in the contractile ring. At this stage profilin (light blue) starts to accumulate at the equatorial cell cortex; KLP3A and Pav concentrate in
the central spindle midzone, while Asp (pink) accumulates at the minus ends of central spindle microtubules. During late telophase all these proteins display
the same subcellular localizations seen in mid-telophases. For the roles of these proteins in cytokinesis see Table I and text below.
Spatial and temporal localization of proteins involved in meiotic cytokinesis of Drosophila males. A. Anillin localization in primary spermato-
Cytokinesis in Drosophila Male Meiosis
means of mutations that cause the formation of meiotic cells
devoid of either the asters or the chromosomes.
We have identified a gene we call asterless (asl) that is
required for aster formation during male meiosis (Bonac-
corsi et al., 1998). asl mutants have morphologically normal
centrioles but fail to accumulate centrosomal material
around these organelles. Despite the absence of asters, mei-
otic cells of asl mutants develop poorly focused anastral
spindles which mediate chromosome segregation, although
in a highly irregular way. Remarkably, asl spermatocytes
eventually form a morphologically normal ana-telophase
central spindle and assemble a regular actomyosin ring
which undergoes normal contraction (Bonaccorsi et al.,
1998). Thus, the central spindles of asl mutants appear to
have full ability to induce cytokinesis. These findings
strongly suggest that in Drosophila male meiosis the central
spindle can recruit and accumulate the cytokinetic signals in
the absence of both functional centrosomes and asters.
To elucidate the role of chromosomes in signaling cyto-
kinesis we made use of fusolo (fsl), a recently isolated male
sterile mutation that disrupts chromosome segregation dur-
ing both meiotic divisions of Drosophila males (E. Buccia-
relli, M. G. Giansanti, S. Bonaccorsi and M. Gatti, unpub-
lished results). During the first meiotic division, in about
half of fsl spermatocytes all chromosomes segregate to one
pole only. However, in the aberrant telophases I of fsl
mutants cytokinesis is normal, leading to the formation of
secondary spermatocytes that are completely devoid of
chromosomes. In these secondary spermatocytes, centro-
somes nucleate astral arrays of microtubules that move to
the opposite cell poles, giving rise to bipolar spindles. These
spindles, despite the absence of chromosomes, assemble
morphologically regular central spindles and elongate to
form telophase figures that are undistinguishable from their
wild type counterparts. Moreover, fsl secondary spermato-
cytes assemble a regular contractile apparatus and undergo
cytokinesis, even in the absence of chromosomes (E.
Bucciarelli, M. G. Giansanti, S. Bonaccorsi and M. Gatti,
Taken together, the results on asl and fsl mutants indicate
that neither the astral microtubules, nor the chromosomes
are required for signaling cytokinesis in Drosophila male
meiosis. Thus, at least in this system, the central spindle is
both necessary and sufficient to stimulate the cytokinetic
The analysis of mutations that disrupt both the
central spindle and the contractile ring reveals
interactions between these cytokinetic structures
Several Drosophila mutants have been identified that dis-
rupt meiotic cytokinesis in males and affect specific cellular
structures involved in this process. These mutants can be
subdivided into two broad categories: those displaying both
a central spindle and a contractile ring and those severely
defective or devoid of both these structures (Table I).
The mutations that disrupt the assembly of both the cen-
tral spindle and the contractile ring identify a variety of
genes with diverse functions. The simultaneous absence of
both these cytokinetic structures has been observed in mu-
tants in the chickadee (chic), diaphanous (dia), spaghetti
squash (sqh), KLP3A and polo genes, and has been pheno-
copied by treatment with cytochalasin B (Williams et al.,
1995; Carmena et al., 1998; Giansanti et al., 1998; Her-
rmann et al., 1998; M. G. Giansanti, S. Bonaccorsi and M.
Gatti, unpublished results).
The KLP3A gene encodes a kinesin-like protein that ac-
cumulates in the central spindle midzone of spermatocytes
(Williams et al., 1995). Because kinesins act as micro-
tubule-based motors and have microtubule-binding activity,
KLP3A mutations are likely to affect primarily the forma-
tion of the central spindle, and secondarily the assembly of
the contractile ring. A simultaneous absence of the central
spindle and the contractile ring has been also observed in
embryonic cells of mutants in the pavarotti (pav) locus,
which encodes a kinesin-like protein related to the mamma-
lian CHO1/MKLP1and to the C. elegans ZEN-4 (Adams et
al., 1998; Powers et al., 1998; Raich et al., 1998). The role
of Pav in spermatocyte cytokinesis could not be determined,
as the extant mutants in this locus die during embryogene-
sis, preventing the cytological analysis of male meiosis.
However, several findings indicate that Pav is required for
central spindle formation both in embryonic and meiotic
cells. Pav forms a complex with Polo kinase, and both Pav
and Polo accumulate in the central spindle midzone of sper-
matocytes. In addition, polo mutant spermatocytes are se-
verely defective in both central spindle and actomyosin
ring, and display failures in cytokinesis (Adams et al., 1998;
Carmena et al., 1998; Herrmann et al., 1998). It is thus
likely that the Pav-Polo complex is primarily required for
spermatocyte central spindle formation and secondarily for
contractile ring assembly.
The phenotypes of chic and sqh, on the other hand, would
suggest the opposite: that a primary defect in contractile
ring assembly can secondarily disrupt central spindle for-
mation. chic encodes a Drosophila profilin, a small actin
binding protein that promotes actin polymerization (Cooley
et al., 1992; Giansanti et al., 1998). sqh encodes a regulato-
ry light chain of myosin II that is also likely to be involved
in the assembly of the actomyosin contractile ring (Karess
et al., 1991). Thus, the results on chic, sqh and KLP3A sug-
gest the existence of a cooperative interaction between ele-
ments of the actin-based contractile ring and the central
spindle microtubules: when either of these structures is
perturbed, the proper assembly of the other is disrupted
(Giansanti et al., 1996, 1998).
The finding that dia mutants lack both the central spindle
and the contractile ring is consistent with the idea that these
cytokinetic structures are interdepedent, and suggests the
hypothesis that Dia may mediate the underlying micro-
M.G. Giansanti et al.
tubule-F actin interactions. Dia is a member of the FH
(formin homology) protein family. These proteins are high-
ly conserved and play an essential role in cytokinesis also in
fungi, worms and mammals. In all these organisms FH pro-
teins interact with both Rho GTPases and profilin, and thus
appear to be involved in the organization of the actin cy-
toskeleton (reviewed by Wasserman, 1998). However, re-
cent studies have identified mDia as a downstream Rho ef-
fector that associates with mouse fibrobast microtubules,
promoting their stabilization (Palazzo et al., 2001). Thus, it
is conceivable that in Drosophila spermatocytes Dia con-
tributes both to the stabilization of central spindle microtu-
bules and to the assembly of the contractile ring.
Additional insight into the relationships between the cen-
tral spindle and the contractile ring is provided by the cyto-
logical analysis of a number of newly isolated mutants de-
fective in spermatocyte cytokinesis. By screening a large
collection of male sterile mutants isolated by B. Wakimoto,
D. Lindsley, E. Koundakjian and C. Zuker (unpublished
work), we have isolated mutants in 19 genes required for
cytokinesis. The cytological analysis of representative mu-
tant alleles in 18 loci revealed that 7 of them are severely
defective in both the central spindle and the contractile ring,
while 11 of them allow the assembly of both these structures
but are defective either in ring constriction or disassembly
(Giansanti et al., 1999b). These results establish a strong
correlation between the presence of the central spindle and
the contractile ring, giving further support to the hypothesis
that these structures are interdependent.
Although the results on Drosophila male meiosis strongly
suggest that the central spindle and the contractile ring are
mutually dependent, this is not true in all animal cells. Stud-
ies on mammalian cells have shown that central spindle
plays an essential role during cytokinesis but have provided
limited information on whether perturbations in the acto-
myosin ring assembly disrupt the central spindle (Cao and
Wang, 1996; Wheatley and Wang, 1996; Eckley et al.,
1997; reviewed by Gatti et al., 2000). In contrast, studies on
C. elegans have clearly shown that, at least in the early stag-
es of embryonic cytokinesis, the actomyosin ring and the
central spindle can assemble independently (Powers et al.,
1998; Raich et al., 1998; Jantsch-Plunger et al., 2000).
Why do Drosophila spermatocytes, and possibly mam-
malian cells, differ from C. elegans embryos in the interac-
tions between the central spindle and the contractile ring?
To answer to this question we should bear in mind that in
both Drosophila and mammalian cells the central spindle
assembles in the proximity of the equatorial cortex, while in
the large C. elegans embryonic cells the central spindle as-
sembles in the center of the cell at considerable distance
from the cortex. In these embryonic cells the central spindle
and the actomyosin ring come into contact only after
substantial furrow ingression. We thus speculate that in C.
elegans embryos cytokinesis consists of two steps: an early
step, where the central spindle and the contractile ring as-
semble independently in distant cellular regions, and a late
step that begins when the central spindle and the contractile
ring have come into contact. It is conceivable that during
this late step the central spindle and the contractile ring in-
teract cooperatively to complete cytokinesis successfully.
The role of Abnormal spindle (Asp) in central
spindle assembly and cytokinesis
Asp is a microtubule binding protein of 220kD that pos-
sesses cdc2 kinase and MAP kinase phosphorylation sites,
as well as putative calmodulin and actin binding sites
(Saunders et al., 1997). Asp localizes to the polar region of
the Drosophila mitotic and meiotic spindles and is required
for spindle pole formation (Saunders et al., 1997; Avides
and Glover, 1999; Wakefield et al., 2001). Besides its func-
tion at the spindle poles, Asp appears to perform an addi-
tional function during central spindle assembly. Asp exhib-
its a striking enrichment at the minus ends of central spindle
microtubules in both mitotic cells and spermatocytes
(Wakefield et al., 2001). In asp mutants, a large fraction of
spermatocyte telophases display severe defects in central
spindle morphology, fail to organize a regular actomyosin
ring and do not complete cytokinesis. The abnormalities
seen in the central spindle of asp spermatocytes suggest that
the Asp protein may help to cross-link the minus ends of
central spindle microtubules, preventing them from sliding
and splaying apart (Wakefield et al., 2001). Taken together,
these observations strongly suggest that central spindle for-
mation not only depends on microtubule plus end-associat-
ed proteins such as KLP3A and Pav, but also on minus end-
associated proteins such as Asp. These proteins are likely to
work in concert to ensure proper orientation, alignment and
stabilization of central spindle microtubules, allowing the
correct formation of the acto-myosin contractile apparatus
required for cytokinesis.
Genes required for actomyosin ring constriction
Mutations in four genes allow the formation of both the
central spindle and the actomyosin ring but affect either
ring contriction or diassembly. In four weel drive (fwd) and
giotto (gio) mutants the central spindle remains normal
throughout meiotic division but the cytokinetic ring fails to
constrict properly, thus impairing completion of cytokinesis
(Brill et al., 2000; our unpublished observations). fwd
encodes a phosphatydilinositol 4-kinase (PI 4-kinase), that
belongs to a family of proteins involved in the synthesis of
the membrane phosphilipid PIP2 (Brill et al., 2000). The
function specified by the gio mutation has not yet been
identified as the molecular characterization of gio is still
under way. Although fwd encodes a product involved in
membrane trafficking, this gene appears to be required for
contraction of the actomyosin ring. This finding suggests
Cytokinesis in Drosophila Male Meiosis
that proper membrane behavior is an essential requirement
for actomyosin ring constriction. A precedent for this con-
clusion is provided by studies on C. elegans embryos de-
pleted of Syntaxin-4, a cytokinesis specific t-SNARE in-
volved in membrane-vesicle fusion processes. These
embryos exhibit frequent failures in cleavage furrow ingres-
sion, suggesting an underlying defect in the contractile ring
machinery (Jantsch-Plunger and Glotzer, 1999).
In twinstar (tsr) and l(3)7m62 mutant spermatocytes the
central spindle has a normal appearance throughout meiotic
division. The contractile ring assembles normally too and
undergoes a normal contraction. However, at the end of
both meiotic divisions the contractile rings of these mutants
fail to disassemble, overgrow and form large and persistent
F-actin aggregates that interfere with completion of cyto-
kinesis (Gatti and Baker, 1989; Gunsalus et al., 1995; Gian-
santi et al., 1999; S. Bonaccorsi, M. G. Giansanti and M.
Gatti, unpublished results). These results indicate that tsr
and l(3)7m62 are primarily involved in the disassembly of
the contractile ring. Consistent with this conclusion, the mo-
lecular characterization of tsr showed that this gene encodes
a polypetide homologous to cofilins (Gunsalus et al., 1995),
a family of low molecular mass actin-binding proteins that
can sever and depolymerize actin filaments in vitro (Moon
and Drubin, 1995). l(3)7m62 has not been characterized at
the molecular level.
The genetic control of meiotic cytokinesis
The mutations affecting Drosophila cytokinesis have
been identified either by screening collections of male
sterile mutants for failures in meiotic cytokinesis, or by
examining late lethal mutants for defects in neuroblast cyto-
kinesis (Table I and references therein). Interestingly, while
some of the genes required for meiotic cytokinesis are also
needed for neuroblast cytokinesis, others appear to be spe-
cifically involved in meiotic cytokinesis. Conversely, there
are genes that control neuroblast cytokinesis which seem to
have little or no role during meiotic cytokinesis (Table I).
Null mutations in KLP3A, a gene encoding a kinesin-like
protein expressed both in testes and in somatic tissues, are
viable, have no effects on neuroblast mitosis but disrupt
meiotic cytokinesis in males (Williams et al., 1995). Simi-
larly, null mutants in fwd, that are fully viable but male
sterile, specifically affect spermatocyte cytokinesis (Brill et
al., 2000). Another gene which seems to be involved in
meiotic but not in neuroblast cytokinesis is chic. This gene,
which encodes a Drosophila profilin, is identified by via-
ble leaky alleles which cause sterility in both sexes, as well
as by null alleles causing lethality during embryogenesis
(Cooley et al., 1992; Verheyen and Cooley, 1994).
Heteroallelic chic combinations resulting in lethality at the
larval-pupal boundary exhibit severe disruptions in meiotic
cytokinesis but do not affect neuroblast cell division,
suggesting that the chic function is either unnecessary or
has only a minor role during neuroblast cytokinesis (M. G.
Giansanti, S. Bonaccorsi, and M. Gatti, unpublished re-
On the other hand, the peanut (pnut) gene, which encodes
a Drosophila septin, is required for neuroblast cytokinesis
but appears to be dispensable for spermatocyte cytokinesis.
The Pnut protein co-localizes with the contractile ring in
several Drosophila cell types, including embryonic, imagi-
nal disk, neuroblast and male meiotic cells (Neufeld and
Rubin, 1994; Hime et al., 1996; our unpublished results).
Null mutants in the pnut gene die at at the larval-pupal tran-
sition and display polyploid cells in their brains, consistent
with a defect in cytokinesis (Neufeld and Rubin, 1994).
However, in testes of larvae homozygous for null pnut mu-
tations, spermatocytes display regular central spindles and
contractile rings and do not exhibit defects in cytokinesis (S.
Bonaccorsi M. G. Giansanti, and M. Gatti, unpublished re-
sults). Thus, although Pnut is associated with the contractile
ring, it does not appear to have a crucial role in meiotic
cytokinesis of Drosophila males. In addition, the analysis of
germline clones homozygous for pnut has shown that this
gene is not required for cytokinesis during the cystobast
divisions in females (Adam et al., 2000).
Taken together these results indicate that the mechanisms
underlying neuroblast and spermatocyte cytokinesis are dif-
ferent, at least in part. This conclusion is consistent with
previous studies on diverse organisms that have led to the
suggestion that cytokinetic mechanisms may vary in differ-
ent cell types (reviewed by Satterwhite and Pollard, 1992;
Goldberg et al., 1998). The identification and characteriza-
tion of additional genes involved in Drosophila cytokinesis
is thus likely to provide information not only on the gene
products underlying cytokinesis, but also how various
combinations of these products are used by cells employing
different strategies to accomplish cytokinesis.
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(Received for publication, November 29, 2001
and accepted, November 29, 2001)