Predominant Inbreeding and its Genetic Consequences in a Homosporous Fern Genus, Sceptridium (Ophioglossaceae)

Abstract

Analysis of isozyme variation revealed the common occurrence of inbreeding in the homosporous fern genus Sceptridium. To examine the genetic consequences of frequent intraga- metophytic selfing, the population genetic structure of S. ternatum was analyzed with special ref- erence to multilocus organization within and among populations. The presence of multiple clones in populations was suggested by high levels of gametic phase linkage disequilibrium within pop- ulations. Each population contained 13-14 multilocus gamete genotypes, and 30 genotypes were counted in three populations examined. The universal genotype was rare and 73% of the genotypes were restricted to a single population. This pattern of genotype variation is concordant with the general pattern in agamospermous or vegetatively reproducing plants. This study also indicated that the local endemic S. triangularifolium has no genetic variation and suggested that this species is a monoclonal lineage derived from hybridization between S. nipponicum and S. ternatum. Large-scale surveys of isozyme variation in higher plants have provided evidence of the important role of the mating system in influ- encing the amounts and organization of genetic diversity within and among populations (Ham- rick and Godt 1990; Loveless and Hamrick 1984). Among the great diversity of mating systems in higher plants, the inbreeding of homospo- rous pteridophytes has a unique position, and may give important insights into the genetic consequences of inbreeding. Homosporous pte- ridophytes produce free-living, bisexual ga- metophytes capable of producing completely homozygous sporophytes through intragame-