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Micromanipulated bivalents can trigger mini-spindle formation in Drosophila melanogaster spermatocyte cytoplasm
Abstract:
Single (individual) bivalents in cultured Drosophila melanogaster primary spermatocytes were detached from the spindle with a micromanipulation needle and placed in the cytoplasm. Such bivalents are prevented from rejoining the spindle by a natural membrane barrier that surrounds the spindle, but they quickly orient as if on a spindle of their own and the half-bivalents separate in anaphase. Serial section electron microscopy shows that a mini-spindle forms around the cytoplasmic bivalent, i.e., the microtubule density in the vicinity of the bivalent is much greater than in other cytoplasmic regions. This microtubule population cannot be accounted for solely by kinetochore nucleation and/or capture of microtubules. Furthermore, the mini-spindles frequently form at odd angles to the main spindle, so that at least one pole has no relationship to the poles of the main spindle. We conclude that a bivalent, or factors that become associated with the bivalent as a result of the manipulation, can either stabilize microtubules or promote their assembly. The bivalent activates latent microtubule organizing centers, or alternatively, polar organizing material has been passively transported from the main spindle to the cytoplasm by the micromanipulation procedure.
Insights
Isolated bivalents in Drosophila spermatocytes spontaneously formed mini-spindles in the cytoplasm. This suggests bivalents can organize microtubules independently of the main spindle apparatus.
Area of Science:
- Cell Biology
- Genetics
- Microscopy
Background:
- During cell division, chromosomes attach to the spindle apparatus for proper segregation.
- The spindle ensures accurate chromosome separation into daughter cells.
Purpose of the Study:
- To investigate the self-organizing capacity of bivalents outside the main spindle.
- To determine if bivalents can nucleate or stabilize microtubules independently.
Main Methods:
- Micromanipulation of individual bivalents in cultured Drosophila melanogaster primary spermatocytes.
- Serial section electron microscopy to analyze microtubule organization around isolated bivalents.
Main Results:
- Detached bivalents formed distinct mini-spindles in the cytoplasm, independent of the main spindle.
- Increased microtubule density around cytoplasmic bivalents indicated de novo spindle formation.
- Mini-spindles often oriented at angles unrelated to the main spindle, suggesting autonomous organization.
Conclusions:
- Bivalents possess the intrinsic ability to organize microtubules, forming functional mini-spindles.
- This organization may involve activating latent microtubule organizing centers or recruiting spindle material.
- Findings challenge the sole reliance on the main spindle for chromosome-associated microtubule organization.