On the Phylogenetic Position of Cystodium: It's Not a Tree Fern – It's a Polypod!

Abstract

The phylogenetic position of Cystodium J. Sm. is studied here for the first time using DNA sequence data. Based on a broad sampling of leptosporangiate ferns and two plastid genes (rbcL and atpB), we show that Cystodium does not belong to the tree fern family Dicksoniaceae, as previously thought. Our results strongly support including Cystodium within the large polypod clade, and suggest its close relationship to the species-poor grade taxa at the base of the polypod topology (Sphenomeris and Lonchitis ,o rSaccoloma in this study). Further studies, with an expanded taxon sampling within polypods, are needed to fully understand the more precise phylogenetic relationships of Cystodium. Cystodium J. Sm., with its single species C. sorbifolium (Sm.) J. Sm., has traditionally been included in Dicksoniaceae (Christensen, 1938; Pichi Sermolli, 1977; Tryon and Tryon, 1982; Kramer, 1990; Stevenson and Loconte, 1996) and, more specifically, has been considered to be closely related to Dicksonia L'Her. (Hooker, 1844; Copeland, 1947; Holttum and Sen, 1961). Cystodium grows in lowland rainforests from Borneo to New Guinea and adjacent islands such as the Bismarck and Louisiade Archipelagos, and the Solomon Islands (Holttum, 1963; Croft, 1986; Kramer, 1990). It has a creeping, hairy, dictyostelic rhizome with large, bipinnate leaves that are two meters or more in length (Croft, 1986). The terminal sori are covered by a true indusium