Nuclear phase alternation in the life cycle of Derbesia (Chlorophyta)

Abstract

Life cycle changes in nuclear DNA content were investigated in the marine multinucleate green alga Derbesia tenuissima, using fluorescence microscopy and microspectrophotometry. Relative nuclear DNA content varied from 1C to 2C levels in the gametophytes and from 2C to 4C levels in the sporophytes. The process of zoosporogenesis could be divided into five stages on the basis of nuclear behaviour in the developing sporangia. During the initial stages (1 and 2) of zoosporogenesis, nuclear DNA content varied from 2C to 4C levels; 1C nuclei were first observed in stage 3 sporangia that were forming a plug complex between the sporangial base and the main axis of sporophyte. In stage 4 sporangia, the DNA content of the mitotic nuclei of pro- and metaphase was 2C. The nuclear DNA content in stage 5 sporangium varied from 1C to 2C. The haploid chromosome number (n = 6) was observed in stage 4 sporangia. Hence, in the normal life cycle of Derbesia, the haploid ‘Halicystis’ generation alternates with a diploid ‘Derbesia’ generation, and meiosis occurs in stage 2 sporangia before plug formation.