Culture studies on the life history ofScytothamnus australisandScytothamnus fasciculatus(Phaeophyta) with electron microscope observations on sporogenesis and gametogenesis

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Abstract
Scytothamnus australis and S. fasciculatus have a southern circumpolar distribution. Wild macrothalli are diploid sporophytes bearing unilocular meiosporangia. Meiospores develop into filamentous microscopic gametophytes which, grown under short-day conditions, form plurilocular gametangia. The gametophytes are dioecious and produce isogametes. Zygotes and unfused gametes develop into diploid and haploid sporophytes, respectively. In all culture conditions haploid sporophytes also grow directly from gametophytes. The erect sporophytes are initiated on the gametophytic filaments in close association with phaeophycean hairs. Chromosome counts gave 24–28 for the diploid phase and 8–14 for the haploid phase. One wild tetraploid population was found. Ultrastructural details of meiosporogenesis and gametogenesis are described. During meiosporogenesis the chloroplast temporarily loses its stellate form and the central pyrenoid occupies a lateral position. The resemblance between the life history of Scytothamnus spp. and those of species belonging to the orders Chordariales and Dictyosiphonales is discussed in relation to the problematical taxonomy of the “lower” brown algae as a whole.