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

The first developmental studies of Ascophyllum nodosum are presented i n which zygote development is compared in thalli with and without endo phytic Mycosphaerella ascophylli. In cultures where 24 h A. nodosum zy gotes 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. Morpholog ical differences were apparent within one month when infected thalli w ere ovoid and non-infected thalli were clavate. In addition, infected thalli were longer, had greater apical diameters, more apical hairs, a nd fewer and shorter rhizoids than non-infected thalli, the developmen t of which more closely resembled other fucoid species. Differences be tween infected and non-infected individuals became more pronounced wit h 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 fo r an obligate, mutualistic symbiosis between A. nodosum and M. ascophy lli.