STATUS OF POLYPLOIDY IN THE EVOLUTIONARY BIOLOGY OF PTERIDOPHYTES : AN OVERVIEW

In striking contrast to the heterosporous pteridophytes, the homosporous pteridophytes not only far exceed in their numbers and immense variety, but they are characterized also by unusually high basic chromosome numbers, which are thought to have arisen through repeated cycles of polyploidy during their long evolutionary history. This assumed 'palaeopolyploid' status of the present-day diploids coupled with the feature of their potentially monoecious gametophytes gave birth to an intellectually stimulating hypothesis on the evolution of their genetic system as based on polyploidy (duplicated sets of chromosomes) to offset the homozygotizing consequence of presumed habitual gametophytic selfing, by storing heterozygosity at homoeologous loci and its release via occasional to frequent meiotic-pairing between homoeologous chromosomes together with a genetic restriction on chromosome pairing to form only bivalents. The tenets of this all-inclusive hypothesis have not been supported. And instead, evidence derived from electrophoretic approaches revealed, Surprisingly, genetic diploidy of the present-day diploids. Even if these originated through ancient polyploid events, the present-day high basic chromosome numbers appear to be completely genetically and chromosomally diploidized. This revelation has raised the as yet unsolved issue of the origin of high numbers and, if of polyploid origin, the mechanisms involved in the wholesale diploidization of genomes or silencing of duplicated genes. The available data, howsoever meager, on karyotype analyses, multiple origins of polyploids, reproductive biology of the gametophyte generation, and the nuclear DNA amounts hold promise and impress upon further inquiries toward a better understanding of the evolutionary biology of pteridophytes.