Abstract
ABSTRACTAhnfeltia plicata (Hudson) Fries, the type species of Ahnfeltia Fries, is currently assigned to the Phyllophoraceae (Gigartinales). Several morphological and biochemical characters distance A. plicata from the Phyllophoraceae but, because sexual reproduction has never been demonstrated, an alternative placement has not been possible. A. plicata now is shown to have a heteromorphic sexual life history. Erect branched gametophytes are dioecious. In male sori, spermatangia are cut off transversely from spermatangial mother cells. Female sori form numerous terminal sessile carpogonia. Following fertilization, several zygotes in each sorus fuse facultatively with undifferentiated intercalary cells of the female sorus and cut off gonimoblast initials obliquely outwards. These initials give rise to branching gonimoblast filaments that fuse with apical and intercalary female sorus cells and with each other, then grow radially outward in the compound external carposporophyte and terminate in carposporangia. Carpospores develop in culture into crustose tetrasporophytes identical to Porphyrodiscus simulans Batters. Field‐collected P. simulans tetraspores grew into erect A. plicata axes. Tetrasporangia are formed by division and enlargement of crust apical cells followed by sequential enlargement and maturation of tetrasporocytes in an erosive process. Monosporangia are formed in sori on male gametophytes. Pit plugs of both gametophyte and tetrasporophyte phases consist of naked plug cores without cap layers of membranes. Gametophytes exhibit both cell fusions and secondary pit connections whereas tetrasporophytes form cell fusions but lack secondary pit connections. On the basis of the unique female and postfertilization reproductive development and in conjunction with the pit plug structure which is unique among florideophytes, the order Ahnfeltiales, containing the family Ahnfeltiaceae, is proposed.
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