Temperature influences on dimorphism of microthallus in a brown alga Cladosiphon okamuranus

The life cycle of edible brown alga Cladosiphon okamuranus is composed of two generations, sporophyte and gametophyte, with parthenogenesis of asexual zoids and gametes. Regardless of the origin of a motile cell, almost all microthalli, except for a prostrate filament, experience prostrate disks with assimilatory filaments as their developmental steps. The disks from different origins are morphologically undistinguishable resulting in confusion of ‘seed’ cultures in factual cultivation scenes. In this study, we therefore attempted to characterize the prostrate disks morphologically by using haploid and diploid strains of C. okamuranus, which were maintained by parthenogenesis. Two types of prostrate disks, a disk with long assimilatory filament (long type) and a disk with short assimilatory filament (short type), were identified in the diploid culture and their appearance ratios were influenced by temperature, although the haploid strain possessed only short type. The long type showed a similar morphology to the disks on culture nets harvested from an actual cultivation site. Our results imply that this morphological differentiation (dimorphism) in the diploid strain may be a divergent step between macrothallus bearing unilocular sporangia or microthallus bearing only plurilocular sporangia, and water temperature may be one of the key regulatory factors for the morphogenesis in dimorphism.