Ribosomal RNA molecular weights and the phylogeny of Labyrinthula

Abstract

Ribosomal RNA molecular weights were determined for several species of Labyrinthula, members of the Thraustochytriaceae, and other lower fungi in an attempt to clarify phylogenetic relations among these groups of organisms. The molecular weights of both 18 S and 25 S ribosomal RNA components were calculated from their relative electrophoretic mobility in polyacrylamide gels. The ribosomal RNA molecular weights of the Labyrinthula spp. were lighter than any other fungi thus far examined, averaging 1.22 × 106 and 0.69 × 106 for the heavy and light components, respectively. The ribosomal RNA molecular weights of the thraustochytrids, including Labyrinthuloides spp., are distinctly heavier than those of Labyrinthula spp., averaging 1.32 × 106 and 0.71 × 106. Those of Lagenidium sp. and Didymium iridis are heavier still. The results suggest that Labyrinthula is not closely related to the thraustochytrids, the Oomycetes, or the Myxomycetes, and that the thraustochytrids have appropriately been removed from the Oomycetes.