Timing of nucleolar DNA replication in Amoeba proteus.

Light- and electron-microscope autoradiography have been used to follow the incorporation of [3H]thymidine at different stages during the interphase of synchronously growing populations of Amoeba proteus. Two main patterns were found for tritiated thymidine incorporation, i.e. DNA synthesis. The major incorporation was in the central region of the nucleus, but a lesser degree of incorporation occurred in the nucleolar region. The bulk of this nucleolar DNA was found to be late replicating, i.e. it replicated during the G2 phase.     

The Contractile Vacuole of Amoeba proteus. III. Vacuolar Response to Phagocytosis1

The reaction of the contractile vacuole of Amoeba proteus to single and multiple phagocytosis under controlled conditions has been studied. Fluid intake into the cytoplasm from the phagosomes induces secretion by the contractile vacuole of equivalent excess volumes:. Vacuolar response is rapid (200 sec) and may be initiated by increases of protoplasmic hydration of as little as 1%. Cytoplasmic uptake of fluid from the phagosome can occur against an osmotic gradient; thus some form of active transport is implied.     

The Contractile Vacuole in Amoeba proteus: Temperature Effects

The influence of temperature on the various aspects of the contractile vacuole cycle of Amoeba proteus has been established. In the upper temperature range (20, 25 and 30 C) an increase in temperature results in shorter vacuolar cycles with greater systolic (final) volumes. The systole is rapid and always complete. At 35 C the vacuole shows the effect of heat stress, cycles are irregular in volume and duration with only partial systoles. In the lower temperature range (15, 10 and 5 C), a new phenomenon has been observed, the plateau. Instead of undergoing systole, after reaching a certain critical volume the vacuole abruptly ceases to grow in size and remains in a state of pause for a well defined period of time, ending at a comparatively slow but complete systole. The duration of this plateau as well as its inception and termination seem quite precisely controlled. Its effect, a decrease in the fluid output by the vacuole, is such as to adjust vacuolar output to near constant Q₁₀ kinetics over our temperature range. This is correlated with a single straight line fit in an Arrhenius plot. Available data do not permit a complete explanation of the nature of the plateau. It could represent a steady state between 2 opposing phenomena: active fluid influx into the vacuole and osmotic losses from the vacuole into the relatively hypertonic cytoplasm.     

The Chromatin Extrusion Phenomenon in Amoeba proteus Cell Cycle

Amoeba proteus is possibly the best known of all unicellular eukaryotes. At the same time, several quintessential issues of its biology, including some aspects of the cell cycle, remain unsolved. Here, we show that this obligate agamic amoebae and related species have a special type of cyclic polyploidy. Their nucleus has an euploid status only for a small fraction of the cell cycle, during metaphase and telophase. The rest of the time it has an aneuploid status, which is a consequence of polyploidization. Extrusion of “excess” chromatin from the nucleus in late interphase and during prophase results in depolyploidization. Such a strategy of life cycle in unicellular eukaryotes is thought to be the main mechanism of “resetting” the Muller's ratchet and is a satisfactory alternative to the meiotic recombination for agamic protists.     

The role of nucleus and cytoplasm in the inheritance of multiplication rates in Amoeba proteus cultured at different temperatures.

In Amoeba proteus the inheritance of the multiplication rate is controlled exclusively by the nucleus in successive cell generations. This result has been obtained with four different types of nucleo-cytoplasmic ‘hybrids’ at three different temperatures, 10 °, 17 °, and 25 °C. No contribution of the cytoplasm towards the inheritance of this character has been found.