Phylogenetic relationships in the Desmarestiales (Phaect phy ceae) were infmed among the monotypic Arthrocladia {Arthrocladiaceae) and 2 7 isolates j-om Desmarestiaceae, representing 17 taxa of Desmarestia and the monotypic Antarctic genera Himantothallus and Phaeurus. Phaeurus and Arthrocladia were used as outgroups. Parsimony analyses of nuclear ribosomal DNA internal transm'bed spacer (ITS1 and ITS2) sequences, in which gaps were both included and excluded, yielded well-resolved trees with a consistent general branching pattern. A parallel analysis of nine morphologzcal and life-histoly characters and three ecolopcal characters yielded a similar tree but provided little resolution in the terminal clades. The position of the monotypic Arthrocladia villosa within the Desmarestiaks is consistent with monophyly for the order, but its position as the most primitive desmarestialean is not resoluable from the molecular data set. The basal position of Phaeurus, the Antarctic Desmarestia species, and Himantothallus is consistent with the hypothesis of a Suuthern Hemisphere origin for the family Desmarestiaceae. The more recent Nmthern H~i s p h e r e "aculeata" clade evoluedj-om an Antarctic ancestor. A "D. aculeatalike" species was ancestral to a lineage characterized by annual sporophytes with high sulfuric acid content, which radiated into many species, widely distributed in both hemispheres. Mapping of morphological and ecological characters onto the molecular tree confirm the informativeness of sulfuric acid-containing vacuoles and unilocular sporangzal types. There is good congruence between phylogenetic tree topology and temperature impints in relation to biogeographic distribution, supporting the the09 that temperature tolerance is a conservative trait.
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