The effects of inhibitors of RNA and protein synthesis on cytological development during meiosis

Caught in the Act: Live-Cell Imaging of Plant Meiosis

Live-cell imaging is a powerful method to obtain insights into cellular processes, particularly with respect to their dynamics. This is especially true for meiosis, where chromosomes and other cellular components such as the cytoskeleton follow an elaborate choreography over a relatively short period of time. Making these dynamics visible expands understanding of the regulation of meiosis and its underlying molecular forces. However, the analysis of meiosis by live-cell imaging is challenging; specifically in plants, a temporally resolved understanding of chromosome segregation and recombination events is lacking. Recent advances in live-cell imaging now allow the analysis of meiotic events in plants in real time. These new microscopy methods rely on the generation of reporter lines for meiotic regulators and on the establishment of ex vivo culture and imaging conditions, which stabilize the specimen and keep it alive for several hours or even days. In this review, we combine an overview of the technical aspects of live-cell imaging in plants with a summary of outstanding questions that can now be addressed to promote live-cell imaging in Arabidopsis and other plant species and stimulate ideas on the topics that can be addressed in the context of plant meiotic recombination.

The duration of meiosis

A survey of work on meiotic duration in diploid plants shows that the duration is positively correlated with the DNA content per nucleus and with the mitotic cycle time. However, meiotic duration is not solely determined by the DNA content per nucleus but is also affected by chromosomal organization, DNA structure and the developmental pattern of the organism. Thus, in three polyploid plant species meiosis is much shorter and in three animal species it is much longer than would be expected in diploid plant species having corresponding DNA contents. Differences in meiotic duration in plant species are usually the result of proportional differences in all the stages of meiosis. Factors affecting the initiation, control and duration of meiosis are discussed. The consequences of changes in nuclear DNA content on developmental processes and the life cycle in plants are considered. It is suggested that DNA influences development in two ways, first directly through its informational content, and second indirectly by the physical mechanical effects of its mass independent of its informational content.

Interaction of daunomycin with meiotic chromosomes in Vicia faba

Daunomycin interferes with the meiotic cycle and structural integrity of chromosmes if administered to microsporocytes of Vicia faba during the meiotic prophase substages prior to diplotene. The principal cytological consequences of daunomycin treatment are as follows: induced achromatic lesions, chromosome fragmentation, terminal and interstitial deletions, loose or non-pairing regions, multivalent associations, reduced chromosome condensation, chromosome stickiness, dicentric and side-arm bridges and chromatid loops. A correlation between inhibition of DNA or RNA and the cytological manifestation of structural anomalies or unfolding of meiotic events is discussed.

THE EFFECT OF TEMPERATURE ON MEIOSIS AND POLLEN DEVELOPMENT IN WHEAT AND RYE

The durations of meiosis and pollen development were estimated in Petkus Spring rye and Chinese Spring wheat grown at 15, 20 and 25 °C. Over the range 15-25 °C, meiotic duration and pollen maturation time decreased with increasing temperature and the ratio between the durations of these stages at 25 °C and at 15 °C (Q10) indicated that the temperature responses of meiotic and pollen development were similar in both species. Both within and between species meiosis and pollen maturation had constant relative durations at each temperature. The effect of low temperature on the duration of meiosis and pollen development was similar in Holdfast, an extreme type of winter wheat, as in Chinese Spring. The relevance of these results to hybrid cereal breeding are mentioned. In plants grown at 25 °C, abnormal meiotic and pollen development resulting in male sterility was seen in some anthers. The cause of male sterility appeared to be thickening of tapetal cell walls concurrent with abnormal late meiotic development in PMC's.

Nuclear DNA content and minimum generation time in herbaceous plants

Many components of cell and nuclear size and mass are correlated with nuclear DNA content in plants, as also are the durations and rates of such developmental processes as mitosis and meiosis. It is suggested that the multiple effects of the mass of nuclear DNA which affect all cells and apply throughout the life of the plant can together determine the minimum generation time for each species. The durations of mitosis and of meiosis are both positively correlated with nuclear DNA content and, therefore, species with a short minimum generation time might be expected to have a shorter mean cell cycle time and mean meiotic duration, and a lower mean nuclear DNA content, than species with a long mean minimum generation time. In tests of this hypothesis, using data collated from the literature, it is shown that the mean cell cycle time and the mean meiotic duration in annual species is significantly shorter than in perennial species. Furthermore, the mean nuclear DNA content of annual species is significantly lower than for perennial species both in dicotyledons and monocotyledons. Ephemeral species have a significantly lower mean nuclear DNA content than annual species. Among perennial monocotyledons the mean nuclear DNA content of species which can complete a life cycle within one year (facultative perennials) is significantly lower than the mean nuclear DNA content of those which cannot (obligate perennials). However, the mean nuclear DNA content of facultative perennials does not differ significantly from the mean for annual species. It is suggested that the effects of nuclear DNA content on the duration of developmental processes are most obvious during its determinant stages, and that the largest effects of nuclear DNA mass are expressed at times when development is slowest, for instance, during meiosis or at low temperature. It has been suggested that DNA influences development in two ways, directly through its informational content, and indirectly by the physical-mechanical effects of its mass. The term 'nucleotype' is used to describe those conditions of the nucleus which effect the phenotype independently of the informational content of the DNA. It is suggested that cell cycle time, meiotic duration, and minimum generation time are determined by the nucleotype. In addition, it may be that satellite DNA is significant in its nucleotypic effects on developmental processes.