GENOMIC OBESITY AND EVOLUTIONARY BIOLOGY IN PTERIDOPHYTES

Homosporous pteridophytes comprise the ferns (including Psilotum and Tmesipteris), Equisetum and homosporous lycophytes, all characterized by a homosporous life cycle. All of these groups are the descendants of ancient plant lineages that extend back to the Devonian Period. Study of chromosomes, hybrids, and breeding systems has revealed much of value in their understanding. Homosporous monilophytes and lycophytes are uniquely endowed with high chromosome numbers; the lowest recorded "haploid' chromosome numbers (= base chromosome numbers!) of most homosporous ferns and fern-allies are substantially high (>27), when compared to the heterosporous pteridophytes and the seed plants. But the question of how such high numbers originated remains unanswered. Solely on the basis of the distribution of chromosome numbers among vascular plants, it is reasonable to speculate that the present-day high basic chromosome numbers in homosporous pteridophytes evolved from cycles of ancient polyploidy (palaeopolyploidy) !

Enzyme electrophoresis has been used to test the hypothesis that the so-called diploid species of extant ferns and fern-allies are ancient polyploids. Homosporous monilophytes and lycophytes have extremely high chromosome numbers, and yet the species with the lowest recorded chromosome numbers show diploid gene expression patterns, suggesting that they may be diploidized ancient polyploids. The paradoxical combination of high chromosome numbers and diploid gene expression has been explained essentially in two ways: (1) homosporous pteridophytes initially had high chromosome numbers, or (2) duplicated genes resulting from ancient polyploidization events have been silenced ! The most widely accepted hypothesis holds that there have been repeated cycles of polyploidization during the evolution of homosporous ferns, followed by extensive diploidization. Gene silencing remains an under-investigated area of polyploid research. There is, relatively speaking, very little information on nuclear DNA amounts and genome size in pteridophytes, but the record of very small 1C DNA value of 0.16 pg, in heterosporous Selaginella kraussiana, does suggest that the ancestors of some homosporous fern-lineages may have possessed very small genomes. We have virtually no knowledge of how fern genomes are organized and evolve. A number of questions relating to genomic obesity still need to be probed, as also the genetic basis of phenotypic differences between populations and Species.