Planta (1992)188:551 558 Planta Springer-Verlag1992 Zygote formation in the homothallic green alga Chlamydomonas monoica Strehlow H. van den Ende*, M.L. van den Briel, R. Lingeman, P. van der Gulik, and T. Munuik Department of Molecular Cell Biology,Department of Fundamental and Applied Ecology,University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, The Netherlands Received 19 February; accepted 17 July 1992 Abstract. 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 ceils 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 (sib- lings). Key words: Autogamy - Chlamydomonas- Gametogene- sis - Mating - Sexual interaction Introduction Among the species of the unicellular, biflagellate, green alga Chlamydomonas, two mating systems can be distin- guished: heterothallism and homothallism. In hetero- thallic species, clonal cells have one and the same mating type, either mating type plus or mating type minus (mt § or mt-). Mating type is genetically stable and inherits as one mendelian locus (Matagne 1987). In homothallic species, a single haploid cell can give rise to oppo- * To whomcorrespondenceshouldbe addressed; FAX: 31 (20) 525 7715 site mating types during vegetative growth and-or gametogenesis (VanWinkle-Swift and Hahn 1986). In the heterothallic C. reinhardtii, zygote formation is the result of sexual interaction between cells which have been ex- posed to nitrogen stress and have differentiated into gametes (Martin and Goodenough 1975; Treier et al. 1989). Gametes of opposite mating type adhere specifi- cally to each other by their flagella. Sexual adhesion induces intracellular signals that evoke responses neces- sary for fusion. The most conspicuous responses are the release of the cell wall and the activation of a mating structure, a special zone of the protoplast surface be- tween the flagella, by which each gamete fuses with its partner (Snell 1990). When the conditions are optimal, zygote yields approaching 100 % can be obtained. At high cell densities, flagellar adhesion results in mass agglutina- tion of the cells, involving hundreds of cells of different mating type clumping together. Apparently, it is a very effective strategy to stimulate the frequency and intensity of physical contact between potential partners. Similar sexual interactions are observed in the heterothallic C. eugametos. Particularly in this species it has been ob- served that cells are mating competent in the early part of the G1 phase of the cell cycle, up to the commitment point for cell division (Tomson et al. 1985; Zachleder et al. 1991). The homothallic C. monoica has been investigated genetically to large extent (VanWinkle-Swift and Thuer- auf 1991), but the sexual process has not been subjected to extensive analysis. In this species, zygote formation is triggered by nitrogen deprival, but mass agglutination of gametes does not occur and maximal zygote yields are generally in the order of magnitude of 25 % of the total cell population (VanWinkle-Swift and Aubert 1982). We have set out to analyze zygote production in this species in order to assess to what extent the mating process differs from that of heterothallic species. Eventually, we hope to learn more about mating-type differentiation, in order to be able to determine whether it is strictly con- trolled as in yeasts (Klar 1987; Herskowitz 1988), or is subject to environmental variability.