Considerations of life history, morphology and taxonomy in Rhodophysema georgii Batters (Rhodophyta, Palmariales)

Abstract

The life history of Rhodophysema georgii from the Atlantic coast of Canada is shown to be exclusively asexual, with direct development of tetrasporophytes from mitotically derived tetraspores. Sexual structures are completely absent and the presence of intercalary sporangial stalk cells provides further morphological evidence of the asexual origin of the tetrasporangia. Chromosome numbers of 17 or 18 were found both in vegetative cells and tetrasporocytes. The occurrence of a large-celled medulla, which characterizes globular thalli of R. georgii, is variable within the species, and the mechanism of its formation, by cellular fusion, is not limited to this species within the genus Rhodophysema. Thus, R. georgii is not fundamentally different in vegetative ontogeny and morphology from other, exclusively crustose, species of Rhodophysema. The morphological features related to life history and medullary cell formation were also observed in lectotype material of R. georgii, thus confirming the identity of the Canadian material.