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Microtubules and control of macronuclear 'amitosis' in Paramecium
Abstract:
The 'amitotic' division of the macronucleus during binary fission in P. tetraurelia includes a detailed sequence of shape changes that are temporally coordinated with the adoption of a series of well-defined positions and orientations inside the cell. The deployment of nucleoplasmic microtubules that is spatially correlated with the shaping ritual is more complex and precise than has been reported previously. Macronuclear division is not amitotic. It is not a simple constriction into two halves. As a dividing macronucleus starts to elongate it becomes dorsoventrally flattened against the dorsal cortex of the organism and assumes an elliptical shape. Concurrently, an elliptical marginal band of intranuclear microtubules assembles that has the same spatial relationship to nuclear shape as the marginal microtubules assembles that has the same spatial relationship to nuclear shape as the marginal microtubule bands of certain elliptical vertebrate blood cells have to cell shape. The band breaks down as further elongation occurs and the nucleus adopts the shape of a straight and slender sausage. Most of the intranuclear microtubules assemble as elongation starts and break down shortly after elongation is completed; the majority are oriented parallel to the longitudinal axis of the nucleus throughout elongation. Some of them are attached to nucleoli and are coated with granules which are almost certainly derived from the cortices of nucleoli. The peripheral concentration, interconnexion, orientation, and overlapping arrangement of microtubules, and the reduction in microtubule number per nuclear cross-section as elongation proceeds at a rate of about 40 micrometers min-1, are all compatible with the provision of a microtubule sliding mechanism as the main skeletal basis for elongation. There are indications that this mechanism is augmented by anchorage and/or active propulsion of nucleoli that may perhaps facilitate fairly equitable segregation of chromosomal material to daughter nuclei.
Insights
The macronucleus of P. tetraurelia does not divide amitotically but elongates via a microtubule sliding mechanism. This process involves precise microtubule assembly and positioning for nuclear division.
Area of Science:
- Cell Biology
- Molecular Biology
- Microscopy
Background:
- The division of the macronucleus in Paramecium tetraurelia was previously thought to be amitotic.
- The precise mechanisms governing macronuclear shape changes during cell division were not fully understood.
Purpose of the Study:
- To investigate the detailed process of macronuclear division in P. tetraurelia.
- To elucidate the role of intranuclear microtubules in shaping the macronucleus during binary fission.
Main Methods:
- High-resolution light microscopy to observe macronuclear shape changes.
- Analysis of microtubule organization and dynamics within the dividing macronucleus.
- Correlation of microtubule deployment with nuclear morphology.
Main Results:
- Macronuclear division is not amitotic but involves significant elongation and shape transformation.
- Intranuclear microtubules assemble into a marginal band and subsequently align longitudinally, facilitating elongation.
- Microtubule sliding mechanism, potentially augmented by nucleolar propulsion, drives nuclear elongation at approximately 40 micrometers/min.
Conclusions:
- Macronuclear division in P. tetraurelia is a complex, microtubule-dependent process, refuting the amitotic model.
- The precise spatial and temporal organization of microtubules is crucial for accurate nuclear shape changes and elongation.
- The findings provide new insights into the mechanics of nuclear division in eukaryotes.