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ABSTRACT
Eocronartium muscicola (Fr.) Fitz., a heterobasidiomycetous moss parasite, has been hypothesized to occupy a pivotal taxonomic position between the Uredinales and the simple-septate auricularioid phytoparasites occurring on archaic hosts. Sporulating fruitbodies were fixed in the field for transmission electron microscopy, and epifluorescence microscopy was used to preselect representative nuclei in precise meiotic stages with which secondary characters could be correlated. Ultrastructural features of nuclear division, spindle pole body (SPB), septa and patterns of basidial development were analyzed in E. muscicola and compared to findings in the Uredinales, Auriculariales sensu stricto, and other simple-septate Heterobasidiomycetes. The septal pore apparatus in E. muscicola is similar to that found in the Uredinales, differing significantly from the Auriculariales sensu stricto. However, ultrastructural details of meiosis, such as the SPB and its relation to the nuclear envelope during division, the presence of endoplasmic reticulum caps at the spindle poles, nucleolus behavior and wall breakage phenomena associated with vegetative branch initiation serve to distinguish E. muscicola from the Uredinales. The host-parasite interface and the phylogenetic antiquity of the mosses parasitized are discussed in relation to rust evolution.