Genetic evidence suggests that homosporous ferns with high chromosome numbers are diploid.
Homosporous ferns have usually been considered highly polyploid because they have high chromosome numbers (average n = 57.05). In angiosperms, species with chromosome numbers higher than n = 14 generally have more isozymes than those with lower numbers, consistent with their polyploidy. By extrapolation, homosporous ferns would be expected to have many isozymes. However, ongoing surveys indicate that within fern genera, species having the lowest chromosome numbers have the number of isozymes considered typical of diploid seed plants. Only species above these lowest numbers have additional isozymes. Therefore, homosporous ferns either have gone through repeated cycles of polyploidy and gene silencing or were initiated with relatively high chromosome numbers. The latter possibility represents a radical departure from currently advocated hypotheses of fern evolution and suggests that there may be fundamental differences between the genomes of homosporous ferns and those of higher plants. These hypotheses can be tested by genetic, karyological, and molecular techniques.
Department of Botany, University of Kansas, Lawrence, KS 66045.
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