Genetic evidence suggests thathomosporous ferns withhigh chromosome numbersarediploid (Pteridophyta/diploidy/polyploidy/electrophoresis/isozymes)

Homosporous ferns haveusually beencon- sidered highly polyploid because theyhavehighchromosome numbers (average n = 57.05). Inangiosperms, species with chromosome numbers higher thann= 14generally havemore isozymes thanthose withlower numbers, consistent withtheir polyploidy. Byextrapolation, homosporous ferns wouldbe expected tohavemanyisozymes. However, ongoing surveys indicate thatwithin ferngenera, species having thelowest chromosome numbers havethenumberofisozymes considered typical ofdiploid seedplants. Onlyspecies above these lowest numbers haveadditional isozymes. Therefore, homosporous ferns either havegonethrough repeated cycles ofpolyploidy andgenesilencing orwereinitiated withrelatively high chromosome numbers. Thelatter possibility represents a radical departure fromcurrently advocated hypotheses offern evolution andsuggests thatthere maybefundamental differ- ences between thegenomes ofhomosporous ferns andthose of higher plants. Thesehypotheses canbetested bygenetic, karyological, andmolecular techniques.