Observations of chromosomal behaviour in living meiotic zygotes of Chlamydomonas reinhardtii (Chlorophyceae)

Abstract

Clarification of chromosome number and behaviour during meiosis is important for genetic investigations of the unicellular alga Chlamydomonas reinhardtii. However, the chromosome number and their morphology remain uncertain in this alga. We stained living zygotes of C. reinhardtii with the highly sensitive DNA fluorochrome SYBR Green I and observed chromosomal behaviour during meiosis. Zygotes with two and four cells, resulting from the first and second meiotic divisions, appeared approximately 16 and 20 h, respectively, after mature zygotes were exposed to light under experimental conditions. In prophase I, chromosomes in the nucleus changed from long, thread-like structures to bivalents. At metaphase I and anaphase I, chromosomes assumed raft-like configurations but chromosomal configurations were less distinct at metaphase II and anaphase II. The nuclei were always localized near the zygotic surface. By observing bivalents at diakinesis we determined the total number of chromosomes to be 18. Chromosome lengths ranged from 0.5 to 2 μm. Using approximate fluorescence intensities, we estimated DNA content of the largest chromosome to be 3-6 times greater than that of the smallest. © 2008 British Phycological Society.