THE LIFE CYCLE OF PRASIOLA MERIDIONALIS SETCHELL AND GARDNER

Abstract

The life cycle of Prasiola meridionalis is diplohaplontic, consisting of an alternation between morphologically dissimilar sporophytic and gametophytic generations. Mature diploid cells at the apex of the thallus divide meiotically, each producing four haploid cells. Eight pairs of chromosomes were counted at first meiotic prophase. The haploid cells divide mitotically, forming polystromatic, gametophytic tissue which becomes a continuation of the monostromatic, sporophytic, or somatic tissue within the same thallus. Patches of dark green cells, containing potential macrogametes, alternate with patches of very light green color which produce microgametes, forming a mosaic pattern in the gametophytic tissue at the apex of the thallus. Oogamy exists in this species, the spherical macrogamete possessing no flagella. Two or more smaller biflagellate microgametes may approach one macrogamete, but only one unites with it to form the zygote. P. meridionalis reproduces asexually by aplanospores which are formed within the diploid somatic tissue. The new thalli resulting from the germination of aplanospores are morphologically similar to those produced from the zygotes.Cultures of P. meridionalis thalli grow well in modified Provasoli's medium, when maintained at temperatures of 5–8 °C and provided with alternate light and dark periods. Gametes are liberated only when fruiting thalli are first illuminated for 2 hours with fluorescent tubes and then kept in the dark for several hours at temperatures of 3–5 °C.