GAMETOPHYTE ONTOGENY, SEX EXPRESSION, AND GENETIC LOAD AS MEASURES OF POPULATION DIVERGENCE IN BLECHNUM SPICANT

Early gametophyte ontogeny was quantitatively distinct for Olympic Peninsula, Alaskan, and disjunct Idaho populations of the homosporous fern Blechnum spicant (L.) J. Sm. Although variable, ganmetophyte sex expression was shown to have a genetic component. Statistically different patterns of sex expression characterize each population. The Olympic Peninsula populations were distinct from each other but consistent in having a predominantly unisexual pattern. The disjunct Idaho population was predominantly bisexual at the time when comparable field collected gametophytes bear sporophytes. Preliminary experiments suggest that an antheridogen operates in this species. Increased sowing density favors maleness, and an extract from soil cultures of gametophytes shifts cultures to an exclusively male pattern after a dramatic suppression of growth. Mating experiments revealed that all populations are interfertile, although fertility was highest when the test Idaho population underwent intergametophytic-selfing. The Idaho population evidenced a low level of genetic load consistent with predictions based on its sex expression. Although Olympic Peninsula populations evidenced apparent high genetic load in some experiments, failure to produce abundant sporophytes in other experiments suggested that additional cultural factors operated to reduce sporophyte formation. Moderate density mating experiments produced single sporophytes that were comparable to field collections. Isolated gametophytes underwent polyembryony after a time delay and gametophyte proliferation. Cultural conditions which allow sporophyte formation on isolated gametophytes without this delay or proliferation must be sought before further genetic analysis is undertaken. RECENT WORK has revealed that the gametophyte biology of pteridophytes varies intraspecifically and a new emphasis requiring population samples for gametophyte studies is apparent (Pray, 1968; Klekowski, 1970a, 1972, 1973a; Saus and Lloyd, 1976). Pray (1968) discovered substantial variation in early ontogeny of sexual and apogamous populations of Pellaea andromedaefolia gametophytes. One insular population was especially distinct, and differences among populations were not correlated with differences in the sporophyte generation. This suggests the potential for a degree of independent evolution of the two generations (Cousens, in prep.). When population samples are extensive, substantial variation in the proportions of sex expression types (Lloyd and Gregg, 1975) and genetic load (Saus and Lloyd, 1976) may characterize fern species. Galapagos populations of Pteridium aquilinum (L.) Kuhn ssp. caudatum (L.) Bonap. var. arachnoideum (Kaulf.) Brade 1 Received for publication 8 December 1977; revision accepted 14 August 1978. Support for this work was provided by a N.D. E.A. predoctoral fellowship awarded through the Botany Department, Washington State University. Appreciation is extended to Dr. David Wagner for Alaskan material. were demonstrated to be genetically distinct from South American populations, although they are morphologically similar (Klekowski, 1973b). The hypothesis that the gametophyte generation of the homosporous fern Blechnum spicant (L.) J. Sm. may vary adaptively among stands that represent a range for sporophyte vigor and vitality will be introduced elsewhere (Cousens, in prep.). In that study, vegetation for four Olympic Peninsula, Washington stands, and a single disjunct northern Idaho stand is described, and observations on natural populations of fern gametophytes for the Lake Quinault (LQ) stand, which represented a modal site for vigor and vitality of the species, are presented. In addition to the modal stand (LQ), and the North Fork, Idaho stand (NF), the Twanoh stand (T) which is optimal for the species, the Hoh stand (H), characterized by dwarfed sporophytes, and the Mason Mtn. (MM) stand, which is at the xeric edge of tolerance for the species, are described. Spores from 9-21 sporophytes were collected to represent populations of Blechnum spicant at these five stands, and with the addition of a single Alaskan spore cQllection, results of laboratory comparisons of gametophyte ontogeny, sex expression, and assessment of genetic load are presented here.