AN APPRAISAL OF SOME ISSUES IN THE EVOLUTIONARY CYTOGENETICS OF HOMOSPOROUS FERNS

Besides antiquity, the homosporous ferns Comprise the major component of the living pteridophytes, and they are uniquely characterized by two conspicuous features, namely: (1) high basic chromosome numbers of their genera, and (2) their tiny exosporic gametophytes are usually bisexual, or hermaphroditic retaining thereby the possibility of gametophytic Selfing, for establishment in newer habitats through single spores. The latter feature provided the homosporous ferns a distinct edge over heterospory. However, stereotyping of the fern life cycle as based habitually on intragametophytic selfing leading to total homozygosity (a negation of evolutionary requirements), and that the high basic chromosome numbers of the extant genera reflectancient polyploidy (= paleopolyploidy), have generated tremendous inquiry into various facets of the fern genetic systems. Several of the issues, including the hypothesis of homoeologous chromosome pairing, genetic diploidy of the paleopolyploids, the evolutionary potential of the present-day polyploids (= neopolyploids) and their multiple origins are discussed.