Live imaging of rapid chromosome movements in meiotic prophase I in maize

Abstract

The ability of chromosomes to move across the nuclear space is essential for the reorganization of the nucleus that takes place in early meiotic prophase. Chromosome dynamics of prophase I have been studied in budding and fission yeasts, but little is known about this process in higher eukaryotes, where genomes and chromosomes are much larger and meiosis takes a longer time to complete. This knowledge gap has been mainly caused by difficulties in culturing isolated live meiocytes of multicellular eukaryotes. To study the nuclear dynamics during meiotic prophase in maize, we established a system to observe live meiocytes inside intact anthers. We found that maize chromosomes exhibited extremely dynamic and complex motility in zygonema and pachynema. The movement patterns differed dramatically between the two stages. Chromosome movements included rotations of the entire chromatin and movements of individual chromosome segments, which were mostly telomere-led. Chromosome motility was coincident with dynamic deformations of the nuclear envelope. Both, chromosome and nuclear envelope motility depended on actin microfilaments as well as tubulin. The complexity of the nuclear movements implies that several different mechanisms affect chromosome motility in early meiotic prophase in maize. We propose that the vigorous nuclear motility provides a mechanism for homologous loci to find each other during zygonema.

Fig. 1.

Chromosome motility in zygonema. (A) Three chromosome movement classes observed in a group of five zygotene meiocytes: rotational movements of the entire chromatin (yellow lines marking chromatin mass edges), rapid, short-distance movements of small chromosome segments (blue and green), and slower-paced movements of chromosome segments inside the chromatin mass (red). (B and D) Cumulative tracks marking rotational movements of the entire chromatin in the two meiocytes traced in A after 145 s. (C and E) Starting and ending positions of the rotating chromatin in the nuclei from B and D. (F and G) Cumulative tracks of the small chromosome segments tracked in the two meiocytes in A after 145 s. (Scale bar, 5 μm.)

Fig. 2.

Large-magnitude rotational motions in zygonema. (A) A zygotene nucleus showing sliding rotation of the entire chromatin (red) along the NE. Green, cytoplasm stained with Rhodamine 123. (B) Cumulative tracks of two anonymous chromosome marks located at the nuclear periphery in the nucleus shown in A after 200 s. (C) A zygotene nucleus exhibiting rotational motion that displaces the nucleolus. (D) The cumulative tracks of the NE (yellow) and nucleolus (green) in the nucleus in B after 190 s. (Scale bar, 5 μm.)

Fig. 3.

Dynamic changes of the NE shape in zygonema. A zygotene nucleus was imaged every 60 s for 300 s. Overexposing the images enhanced the contrast between the nucleus and cytoplasm and allowed the delineation of the NE. Four chromosome marks (red, blue, purple, and turquoise) were also tracked for comparison. (A) The nucleus at 0 s. (B) The same nucleus at 300 s. (C) The NE and the chromosome marks at 0 s. (D) Cumulative tracks of the NE and chromosome marks after 300 s. (Scale bar, 5 μm.)

Fig. 4.

Chromosome movements in pachynema. (A) Rotational movements of the entire chromatin in a pachytene nucleus. Yellow lines mark chromatin mass edges. (B) Cumulative tracks from A after 570 s, but without chromatin shown. (C) A pachytene nucleus overlaid with trajectories of chromosome marks shown every 60 s for 240 s. Green and blue mark the chromosome end and an interstitial knob, respectively, of a chromosome arm, which exhibits long-distance sweeping movements. Red: a chromosome loop whose both ends are embedded in the chromatin mass; cyan: a stationary chromosome region on the periphery of the chromatin mass; magenta: a free, fast moving chromosome end. (D) Same as C, but without tracking overlay. (Scale bar, 5 μm.)

Fig. 5.

Comparison of chromosome movement patterns in maize meiocytes in zygonema and pachynema.