Culture studies on reproduction of Sphaerotrichia divaricata (Chordariales, Phaeophyceae)

The life history of isolates of Sphaerotrichia divaricata from three sites in the Atlantic Ocean and three in the Pacific Ocean was studied in culture. Of the Pacific plants, two sporophytes were heterozygous, with offspring consisting of dioecious microscopic gametophytes, whereas microthallic progeny of a third sporophyte were asexual. Microthallic progeny of sporophytes from the Atlantic were either dioecious gametophytes (1 case), female only (2 cases), or asexual (2 cases). Fusion of isogametes which were formed between 0° and 15°C in long days occurred between compatible microthallus clones of all sexual isolates. In higher temperatures or short days microthallic progeny of all isolates formed only asexual zoids which reproduced the microscopic generation. In the sexual isolates zygotes and unfused gametes developed into diploid or haploid macrothalli, respectively. In the asexual isolates, macrothalli were formed by apomictic germination of zoids or as vegetative outgrowths from the microthalli. During early development of the macrothallus, the central filament showed diffuse growth. Subsequently, differentiation of the characteristic spherical apical cell resulted in a subapical meristem. The chromosome number of S. divaricata is n = 14–18.