Zygote formation in the homothallic green alga Chlamydomonas monoica Strehlow.

Mating between cells of opposite mating type within a clonal population of Chlamydomonas monoica results in thick-walled zygotes. Zygote formation was studied in cells from continuous cultures fed with culture medium containing nitrate concentrations sufficient or limiting for growth. The factors that were considered were cell density and nitrate content of the medium. The following results were obtained: (i) Zygotes were only formed by cells that had experienced a relatively low nitrogen level that did not limit cell division. (ii) Cells were competent to mate only during a limited period of time after their release from the mother cell wall. (iii) There was a correlation between zygote yields and the number of low-nitrogen cells that were able to execute a cell division under the conditions being tested. (iv) The zygote yield per cell division was independent of the cell density. These findings indicate that the strategy used by C. monoica cells to find a mate is not dependent on random encounters. A possible explanation is that at least a large proportion of zygotes is formed by matings between cells originating from the same mother cell (siblings).