COMPARTMENTALIZATIONS OF THE FERN GENOME AND THE GENE BLOCK HYPOTHESIS REVISITED

The regular alternation of morphologically distinct and independent gametophytic ("haploid”) and sporophyte ("diploid”) generations in the life cycle of homosporous ferns offers exceptional opportunities to fathom the causal factors of the phase change and to dissect their genomes into the sporophytic and gametophytic components. The added advantage is provided by the possibility of induced apospory and induced apogamy, which may serve to answer questions relating to the control of the life cycle without the obvious complications of the haploid-diploid chromosome cycle. The genome of ferns is compartmentalized, as evidenced from genetic load in the form of recessive lethal genes operative only in the sporophytic generation. The issues relating to phase change with particular reference to Mehra's Gene-Block hypothesis are reviewed briefly.