Ultrastructural observations of tetrasporangia and conceptacles in Hildenbrandia (rhodophyta: Hildenbrandiales)

Abstract

Ultrastructural techniques were used to investigate tetrasporangial development and conceptacle contents of Hildenbrandia occidentalis Setch. and H. rubra (Sommerf.) Menegh. = H. prototypus Nardo. Tetrasporocytes are formed by transformation of vegetative cells. A distinctive tetrasporangial wall is deposited around the tetrasporocyte. Cleavage of the tetrasporocyte is effected by mucilage-filled furrows initiated successively, but completed simultaneously. Following discharge of the tetraspores, the cell subtending the sporangium becomes a tetrasporocyte. Repetition of this process enlarges the conceptacle. True paraphyses and regenerative stalk cells were not observed in either species. Tetrasporangial walls persist in the conceptacle after spore release and, presumably, these are responsible for some of the reports of paraphyses. The implications of these findings for the intrageneric taxonomy of Hildenbrandia are considered. The unique mode of tetrasporogenesis found in the HIldenbrandiaceae supports segregation of this family into a separate order.