INTRASPECIFIC VARIATION IN SPORE-SIZE OF HOMOSPOROUS FERNS AND ITS IMPLICATIONS ON FERN MATING SYSTEMS

In the homosporous ferns, sex determination is delayed until spore germination. Published data reveal a wide variety of adaptations that help avoid intragametophytic selfing, despite the potential bisexuality or hermaphroditism of their gametophytes. The major developmental adaptation appears to be asynchronous germination of spores, originating even from a single sporophyte. Several studies have entertained the possibility of differential capacities of fern spores as a causative factor, suspecting some kind of a "genetic" basis for such inequality.

We have examined, therefore, the factor of spore-size variation in six diploid species of homosporous ferns based on statistical analyses of a random sample of 1,000 spores in each of the species. There is observed enormous variation in spore-size within each of the six species. More importantly, the observed variation simulates, more or less anormal distribution. Such a kind of distribution reflects variation due to "uncontrolled factors" during sporogenesis. It is proposed that such between-spores variation in sporesize constitutes the principal basis of asynchrony in spore germination and early gametophyte development, which, in turn, would generate a competitive system of gametophyte populations where intergametophytic interactions coupled with other environmental factors begin to operate, and the gametophyte populations get structured sexually.