Occurrence of somatic diploidization in the life history of Coilodesme japonica (Dictyosiphonales, Phaeophyta)

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Abstract
Diploid macroscopic sporophytes of Coilodesme japonica Yamada grow epiphytically on Cystoseira hakodatensis (Yendo) Fensholt. Meiosis occurs in the unilocular sporangium initial cell. Unispores germinate into haploid filamentous microthalli which bear plurilocular sporangia. Plurispores may germinate asexually into filamentous microthalli at all the culture conditions tested. Sexual differentiation (as evidenced by a pheromone-like odour from female strains) occurs after transfer of the strains to a 5°C (12: 12 LD) condition. Fertilization, however, was not observed. When the strains are transferred to higher temperatures (10 or 14°C long/short day) some plurispores of certain strains become round and vacuolated (morphologically similar to the medullary cells of macrothallus), while others produce a very long germination tube and others again develop into filamentous microthalli. Both the vacuolated cells and the germlings carrying a long germination tube develop into the macrothallus. Comparison of microspectrofiuorometric data of DAPI-stained nuclei (macrothallus from the field; microthallus and macrothallus from culture) reveals that the macrothallus nuclei of both field and culture material show 2C and 4C levels, whereas those from the microthallus remain at 1C and 2C levels. The microspectrofiuorometric studies also demonstrate that diploidization occurs in the early stages, i.e. at the one-cell stage after induction of the parthenogenetic macrothallus. The chromosome number of C. japonica is n = 12–13, and 2n =24–26.

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