The marine ascomycete genus Haloguignardia occurs endophytically in members of the marine brown algal family Sargassaceae globally. This example of endosymbiosis has been morphologically described in detail: the fungal component internally infects the algal host resulting in prolific host cell tissue growth. The resulting galls, borne on host blades or stipes, house fungal perithecia and filamentous hyphae but are composed chiefly of host algal cells. Using culture-independent molecular techniques, I have sequenced the 18s rDNA gene region for H. irritans and compared these data to published fungal 18s sequences. 18s analysis places the genus Haloguignardia in the class Sordariomycetes and is consistent with the current, morphological taxonomy for this genus. In order to study intraspecific genetic variation in H. irritans I have sequenced the ITS rDNA (ITS 1, 2 and the 5.8s) regions for five separate gall-tissue samples from Santa Rosa Island in southern California and for five samples from Monterey and Carmel in central California. Intraspecific DNA sequence variation in the ITS regions of H. irritans reveals consistent sequence divergence between sites sampled. The fungal ITS regions for H. irritans total 613 bp in length and contain 40 synapomorphic characters for a total of 6.5% variation in informative loci between southern and central Californian sites. This value is similar to those found for the ITS and other gene regions previously used by researchers investigating species boundaries at the intraspecific level in symbiotic, terrestrial fungi. The ITS is one of several gene regions I am currently using to investigate the coevolution of this system via comparison of multiple gene genealogies generated for H. irritans to analogous genealogies constructed for the algal host.

Key words: 18s rDNA, coevolution, ITS, marine fungi, Sargassaceae, symbiosis