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Abstract

This study employs molecular and field sampling techniques to understand the complex life history of the filamentous green alga, Acrosiphonia, in southern British Columbia, Canada. The DNA sequences of the nuclear ribosomal internal transcribed spacer (ITS) regions conclusively identify the unicellular green algal endophytes, 'Chlorochytrium inclusum' and 'Codiolum petrocelidis', as the alternate life history phases of one or more Acrosiphonia species. 'Chlorochytrium inclusum', a spherical unicell, was found abundantly in the foliose red alga Mazzaella splendens, whereas 'Codiolum petrocelidis', a stalked unicell, densely colonises 'Petrocelis franciscana' (= crustose tetrasporophytic phase of Mastocarpus papillauts). The DNA sequence data supported previous culture studies and Kornmann's hypothesis that the two morphologically different endophytes are alternate phenotypes of the sporophyte of a single Acrosiphonia species. The relationship of Acrosiphonia's endophytic sporophyte and filamentous, free-living gametophyte in nature revealed similar dynamics for three environmentally variable field sites. Filamentous Acrosiphonia plants were seasonally abundant (spring and summer) with fertile cells developing almost immediately after Acrosiphonia's appearance in the rocky intertidal zone. The unicellular sporophytes colonised M. splendens and 'Petrocelis' one to three months later, and showed higher tolerance to abiotic factors than Acrosiphonia's gametophyte: high summer temperatures (which correlated with death of the filamentous free-living plants) were survived and they overwintered in their hosts. Endophytes matured primarily in winter, zoospore release occurred throughout winter and spring and Acrosiphonia's life cycle is completed with subsequent zoospore germination and establishment of filamentous gametophytic plants. The two red algal hosts, M. splendens and 'Petrocelis1, were abundantly available for endophyte colonisation in spring and summer. A number of factors, e.g. herbivory, winter storms and senescence, however, were identified to produce fluctuating seasonal abundance patterns of the hosts, thus potentially affecting endophyte survival. I suggest the endophytes have evolved a strategy whereby duration in the host is synchronised with seasonality of the host. An investigation of possible hosts for Acrosiphonia's sporophyte established a wide range of hosts. However, 'Codiolum' showed a greater affinity for 'Petrocelis' than for other crusts, and 'Chlorochytrium' colonised foliose red algae characterised by loosely compacted cells in the cortex and medulla and carrageenansand carragars as cell wall constituents e.g. M. splendens, M. heterocarpa and Schizymenia pacifica, more readily than others. A bet-hedging strategy is proposed for Acrosiphonia's life history. Not only have two morphologically different phases adapted to a seasonally variable environment, but the sporophytic phase of at least one Acrosiphonia species can colonise two alternate hosts (crustose and foliose red algae), and low host specificity is evident for both endophytes.