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Abstract:
The dioecious and isogamous brown alga Scytosiphon lomentaria (Scytosiphonaceae, Ectocarpales) is widely dis- tributed in temperate and cold coasts around the world. S. lomentaria found in Japan consists of five cryptic species: species I–V. In species I, both sexual and asexual popula- tions have been reported. In this study, the sex of game- tophytes from asexual populations was checked using sex markers and by crossing experiments with female and male gametophytes from sexual populations. All gametophytes from the asexual populations were revealed to be female. In culture, gametes released by gametophytes from the asexual populations were compared with gametes released by gametophytes from the sexual populations. Gametes from the asexual populations had a significantly larger cell size and rapid parthenogenetic development. Although ga- metophytes of the asexual populations were suggested to be “female,” the cultures where their gametes were released had no detectable smell of their sexual pheromones. Fur- thermore, sporophytic discoid thalli, which are considered to be developed by parthenogenesis, were found at an area where an asexual population exists. These results suggest that the asexual populations consist of “females” that have a high reproductive ability in parthenogenesis, and that, an alternation of generations between macrothallic ga- metophytes and microthallic discoid sporophytes occurs through parthenogenesis of female gametes without sexual reproduction in the asexual population.
