is recovered in slk19Δ and cdc14 mutant cells
when replacing Sli15 with its constitutively de-phosphorylated form, Sli15-6A (13). Expression
of SLI15-6A in slk19Δ cells restored chromosome compaction and segregation to near wild-type levels. Thus, Ipl1 must be on the midzone to
adapt condensation of endogenous long chromosomes (Fig. 4, A and B). Because SLI15-6A did
not rescue the localization of separase in slk19Δ
cells (fig. S9), Ipl1 regulated condensation independently of separase localization.
To examine the effects of midzone-bound Ipl1
on the segregation of long chromosomes, we visualized the distal region of LC(XII:IV)cen4Δ,
marked by the TRP1 locus (2.8 Mb from CEN12),
in slk19Δ mutants. Separation of the distal TRP1
locus, but not of a centromere-proximal one, was
delayed in slk19Δ mutants (P < 0.001) (Fig. 4, B
to D). As a consequence of this delay, spindle
breakdown preceded TRP1 segregation in 27%
of LC(XII:IV)cen4Δ slk19Δ mutant cells (Fig. 4,
C and E). Expression of SLI15-6A largely suppressed these defects (Fig. 4E). Thus, Slk19 affected segregation mainly through targeting of
Ipl1/Aurora-B to the spindle midzone, which
was especially important for the segregation of
Together, our results indicate that yeast cells
adjust the compaction of chromosomes to secure their segregation by the spindle. One key
component of the underlying “ruler” may be the
anaphase spindle, acting through the kinase aurora B at the midzone. Because long chromosome arms are exposed longer to the midzone
than short ones, this model (fig. S11) accounts
for their increased compaction and explains why
compaction is also greater in the daughter cell.
This simple model could also explain how small
cells, with short spindles, still segregate their
chromosomes at mitosis. Indeed, small cells such
as whi3Δ mutants (16) hypercondensed their native chromosome IV (fig. S12), indicating that
natural chromosomes adapt their compaction to
anaphase spindle length.
Large variations in cell size and spindle length
are common within species, and hyperlong chromosomes are well tolerated, at least in
Drosophila (17). Similarly, chromosome rearrangements
can increase chromosome size without diminishing cellular proliferation during cancer (18) or
during size variations of rDNA loci (19). Perhaps spindle length and the level of chromosome
condensation are not predetermined but are mutually coordinated through feedback regulatory
loops. The mechanism described here is likely
to be only one of such coupling systems. These
probably play important roles not only during
cell size changes but also in allowing chromosome rearrangements during speciation (20)
and the survival of chromosomally unstable cancers (21).
References and Notes
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Acknowledgments: We thank P. Meraldi, Z. Shcheprova,
C. Weirich, and S. Buvelot for helpful comments and
critical reading of the manuscript; D. Clarke
(University of Minnesota) for reagents; C. Iannone
and R. Tejedor for help with qPCR; F. Campelo
for help with statistical analysis; the ETH Light
Microscopy Center; the CRG Advanced Light
Microscopy Unit; and the CRG Ultrasequencing Unit.
This project was supported by grants from La Caixa
to G.N., the Spanish Ministry of Science to T.G.
(BFU09-09168) and M.M. (BFU09-08213), and the
Swiss National Science Foundation to Y.B.
Supporting Online Material
Materials and Methods
Figs. S1 to S12
13 December 2010; accepted 28 February 2011
Published online 10 March 2011;
DNA Synthesis Generates Terminal
Duplications That Seal End-to-End
Mia Rochelle Lowden,1,2 Stephane Flibotte,3 Donald G. Moerman,3 Shawn Ahmed1,2*
End-to-end chromosome fusions that occur in the context of telomerase deficiency can trigger
genomic duplications. For more than 70 years, these duplications have been attributed
solely to breakage-fusion-bridge cycles. To test this hypothesis, we examined end-to-end fusions
isolated from Caenorhabditis elegans telomere replication mutants. Genome-level
rearrangements revealed fused chromosome ends having interrupted terminal duplications
accompanied by template-switching events. These features are very similar to disease-associated
duplications of interstitial segments of the human genome. A model termed Fork Stalling
and Template Switching has been proposed previously to explain such duplications, where
promiscuous replication of large, noncontiguous segments of the genome occurs. Thus, a
DNA synthesis–based process may create duplications that seal end-to-end fusions, in the
absence of breakage-fusion-bridge cycles.
commonly occur in developing tumors (1). In
many organisms, the instability of dicentric
chromosomes impedes elucidation of the initial structures of fusions events and, therefore,
a mechanistic understanding of their genesis
(Fig. 1A). Because Caenorhabditis elegans has
holocentric chromosomes, end-to-end fusions
derived from telomerase-deficient backgrounds
can be transmitted stably during mitosis and meiosis (Fig. 1A). In other organisms, subtelomeric
duplications that occur at critically shortened
telomeres have been attributed to chromosome
fusion followed by breakage during mitosis:
the breakage-fusion-bridge (BFB) model (2, 3).
To test the hypothesis, we genetically isolated
C. elegans end-to-end chromosome fusions on
the basis of the meiotic nondisjunction phenotype that they cause when heterozygous (4–6).
Most human somatic cells are deficient for telomerase and experience pro- gressive loss of telomeric DNA at chromosome ends. Critically shortened telomeres
can elicit high levels of end-to-end chromosome
fusion, resulting in genome rearrangements that
1Department of Genetics, University of North Carolina, Chapel
Hill, NC 27599, USA. 2Department of Biology, University of
North Carolina, Chapel Hill, NC 27514, USA. 3Department of
Zoology, University of British Columbia, Vancouver, British
Columbia V6T 1Z4, Canada.
*To whom correspondence should be addressed. E-mail: