Heterozygosity for recessive lethals in homosporous fern populations: Thelypteris palustris and Onoclea sensibilis

Sporophytes from natural populations of Thelypteris palustris and Onoclea sensibilis were subjected to breeding tests to reveal the presence of recessive lethal alleles in their genotypes. All sporophytes were found to be heterozygous for recessive sporophytic lethals. Despite the capability for producing completely homozygous genotypes by intragametophytic self-fertilization, ferns in natural populations seem to retain considerable genetic loads of deleterious recessive alleles. It is suggested that natural selection operates to strongly favour heterozygous rather than homozygous sporophytes under most conditions.