Origin and taxonomic affinities of Thelypteris (subgen. Stegnogramma) burksiorum (Thelypteridaceae)

This paper examines the ecology, frond and spore morphology, gametophytes, and molecular biology ofThelypteris burksiorum to reveal its taxonomic affinities and phytogeographic origin. The species is endemic to Winston County in northern Alabama (U.S.A.) and only occurs in cave-like rockhouse habitats. A related species,T. pilosa, is widespread in central and southern Mexico, Guatemala, and Honduras.Thelypteris pilosa is extremely variable and our data suggest that current recognition of the varietiesmajor andpilosa is not warranted.Thelypteris pilosa is clearly closely related and likely sister toT. burksiorum. Thelypteris burksiorum exhibits the unusual production of gametophytic propagules. This, in combination with current levels of genetic diversity and reproductive biology, suggests thatT. burksiorum is an ancient Tertiary relict and not an example of recent long distance dispersal. Selection in rockhouse habitats has driven species away from their normal life cycle to partial or complete reliance on the gametophyte generation. The combination of a reduced yet fertile sporophyte and a gametophyte that may rely on gametophytic proliferation placesT. burksiorum in an intermediate evolutionary position relative to other rockhouse pteridophytes.