A single gene with pleiotropic effects accounts for the Scottish endemic taxon Athyrium distentifolium var. flexile.

• Experiments were conducted to determine the genetic relationship between the alpine fern Athyrium distentifolium and the Scottish endemic taxon A. distentifolium var. flexile. • Segregation of the taxa was studied after growing sporophytes from spore families, and after selfing and crossing gametophytes. The growth of both gametophytes and sporophytes was measured under a range of nutrient regimes. • The distinction between the two taxa is due to variation at a single gene with pleiotropic effects expressed at both the sporophyte and gametophyte stages. A. distentifolium sporophytes (genotypes A&supD;/A&supD; or A&supD;/A&supF; ) are taller, have fewer fronds, and produce fewer spores than A. distentifolium var. flexile sporophytes (genotype A&supF;/A&supF; ). Gametophytes containing the A F allele have a 50% higher growth rate than those with the A&supD; allele on a range of media. • The polymorphism may be maintained by a balance between selection for and against the A&supF; allele at the gametophyte and sporophyte stages of the life cycle, respectively. Implications for the conservation of the A. distentifolium var. flexile taxon are discussed. [ABSTRACT FROM AUTHOR]