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J. Cell Biol.,
Volume 141, Number 6, June 15, 1998 1383-1391
Department of Biology, University of California, Santa Cruz, California 95064
In Sciara, unfertilized embryos initiate parthenogenetic development without centrosomes. By
comparing these embryos with normal fertilized embryos, spindle assembly and other microtubule-based events can be examined in the presence and absence of
centrosomes. In both cases, functional mitotic spindles
are formed that successfully proceed through anaphase
and telophase, forming two daughter nuclei separated
by a midbody. The spindles assembled without centrosomes are anastral, and it is likely that their microtubules are nucleated at or near the chromosomes. These
spindles undergo anaphase B and successfully segregate sister chromosomes. However, without centrosomes the distance between the daughter nuclei in
the next interphase is greatly reduced. This suggests
that centrosomes are required to maintain nuclear
spacing during the telophase to interphase transition.
As in Drosophila, the initial embryonic divisions of Sciara are synchronous and syncytial. The nuclei in fertilized centrosome-bearing embryos maintain an even
distribution as they divide and migrate to the cortex. In
contrast, as division proceeds in embryos lacking centrosomes, nuclei collide and form large irregularly
shaped nuclear clusters. These nuclei are not evenly
distributed and never successfully migrate to the cortex. This phenotype is probably a direct result of a failure to
form astral microtubules in parthenogenetic embryos
lacking centrosomes. These results indicate that the primary function of centrosomes is to provide astral microtubules for proper nuclear spacing and migration
during the syncytial divisions. Fertilized Sciara embryos produce a large population of centrosomes not associated with nuclei. These free centrosomes do not form
spindles or migrate to the cortex and replicate at a significantly reduced rate. This suggests that the centrosome must maintain a proper association with the
nucleus for migration and normal replication to occur.
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