ENDOPHYTIC ALGAE OFCHONDRUS CRISPUS(RHODOPHYTA). III. HOST SPECIFICITY1

ABSTRACT Susceptibility of rhodophycean macroalgae to infection by the green endophytes,Acrochaete operculataCorrea & Nielsen andA. heterocladaCorrect & Nielsen was studied. Cross‐infection experiments showed thatA. operculatais host specific and developed only in sporophytic fronds ofChondrus crispusStackh. andIridaea cordata(Turn.) Bory. AlthoughA. operculatapenetrated equally the multilamellar outer cell wall of sporophytic and gametophytic fronds ofC. crispus,subsequent development was arrested in the gametophytic fronds. Susceptibility of the sporophytic phase ofC. crispuswas detected early in the development of the host, at a discoid stage that is structurally distinct from the adult fronds. The evidence strongly suggests that host specificity inA. operculatais determined by cell‐wall composition of the hosts, likely the carrageenan fraction. In contrast,A. heterocladawas not host specific, infecting all offered hosts, including carrageenophytes and agarophytes. Germination occurred on the surface of the hosts and led to the development of an epiphytic stage. Subsequent penetration in many cases involved total displacement of cortical tissue in the infected frond.