THE ASCOPHYLLUM POLYSIPHONIA MYCOSPHAERLLA SYMBIOSIS .4. MUTUALISM IN THE ASCOPHYLLUM MYCOSPHAERELLA INTERACTION

The first developmental studies of Ascophyllum nodosum are presented in which zygote development is compared in thalli with and without endophytic Mycosphaerella ascophylli. In cultures where 24 h A. nodosum zygotes were inoculated with ascospores of M. ascophylli, 20-50% of the zygotes developed differently from control cultures without ascospores, with later infections (at 1-3 weeks) having little effect. Morphological differences were apparent within one month when infected thalli were ovoid and non-infected thalli were clavate. In addition, infected thalli were longer, had greater apical diameters, more apical hairs, and fewer and shorter rhizoids than non-infected thalli, the development of which more closely resembled other fucoid species. Differences between infected and non-infected individuals became more pronounced with development. After eight months there was a four-fold difference in thallus length, and morphologically transformed thalli had endophytic fungi. This transformation provides the first experimental evidence for an obligate, mutualistic symbiosis between A. nodosum and M. ascophylli.