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The occurrence of mlcroscoplc algae that are endophytes and potential pathogens of kelps was quantified during 1994 in wild populations of Laminaria saccharina, L. hyperborea and L. digitata at Helgoland, North Sea. Sampling was designed to enable analysis of the influence of 4 fixed factors: species, date, wave exposure, and depth. Microscopic examination of, in total. 1224 thalli showed that the prevalence of infection by endophytic algae was 85%, much higher than was inferred by gross lesions alone. One tenth of the hosts, mostly L. saccharina, showed severe morphological changes, such as distorted stipes or a crippled lamina. One third showed weaker symptoms of endophyte disease, such as dark spots on the lamina or warts on the stipe. In most infected thalli, the infection was not evident macroscopically. Prevalence was high throughout the year with a minimum in spring. At a more exposed site, prevalence was higher and disease symptoms stronger than at a sheltered locality Disease symptoms were more severe in shallower than in deeper water. Endophytes, mostly brown algae, were repeatedly isolated and identified in laboratory cultures. Endophytes were not strictly host-specific, but L. saccharina was predominantly infected by Laminarionema elsbetiae, recently detected at Helgoland. This is the first epidemiological study comparing the prevalence and effects of kelp endophytes in different hosts at the same locality.

Three amino acid loop extension homeodomain transcription factors (TALE HD TFs) act as life cycle regulators in green algae and land plants. In mosses these regulators are required for the deployment of the sporophyte developmental program. We demonstrate that mutations in either of two TALE HD TF genes, OUROBOROS or SAMSARA, in the brown alga Ectocarpus result in conversion of the sporophyte generation into a gametophyte. The OUROBOROS and SAMSARA proteins heterodimerise in a similar manner to TALE HD TF life cycle regulators in the green lineage. These observations demonstrate that TALE-HD-TFbased life cycle regulation systems have an extremely ancient origin, and that these systems have been independently recruited to regulate sporophyte developmental programs in at least two different complex multicellular eukaryotic supergroups, Archaeplastida and Chromalveolata.