Genome size in liverworts.

Liverworts are poorly represented in the record of DNA C-values. Data for not more than nine species are reported in the literature. Here we present flow cytometric measurements of genome size for 32 foliose and 11 thallose species from 22 out of 83 families. The main method used in this study was flow cytometry using propidium iodide as the DNA stain. Feulgen densitometrywas applied as a supplementary method but it proved less suitable because the rigid cellwalls of liverwort tissue are resistant to maceration and apparently often inhibit the diffusion of reagents, which results in low estimates of DNA content. The precise or approximate number of chromosomes were counted, where possible. Among the thallose liverworts, the lowest 1C-value was recorded for Marchantia polymorpha (0.293 pg) and the highest for diploid Pellia epiphylla (7.401 pg). Haploid P. epiphylla (1C = 3.803 pg) had the largest genome among the haploid thalloid liverworts. Among the foliose liverworts, Lejeunea cavifolia with a value of 0.211 pg (1C) was ranked the lowest and Mylia taylorii, a haploid with 7.966 pg (1C) and a large amount of dense heterochromatin, concentrated in one big spherical chromocentre, the highest. This 38-fold variation covers the extremes of the whole sample and exceeds the ca 12-fold variation recorded in mosses (0.174–2.160 pg, 1C). This variation is nevertheless low compared to the 2000-fold interspecific variation found in angiosperms. Several instances of intraspecific variation in DNA ploidy (x and 2x) were found – in Radula complanata, Pellia epiphylla and Metzgeria furcata. In Lophocolea heterophylla, accessions differed 3.37-fold in C-value at haploid chromosome number. This points to cryptic taxonomic differentiation and warns against premature statements about ploidy levels based only on DNA measurements. Significant intraspecific intraploidal variation in C-value was also observed in certain instances. In Frullania dilatata, female plants with two large heterochromatic sex-chromosomes have a 1.35-fold higher C-value than male plants with only one sex chromosome. In most other cases of intraspecific variation the role of sex differences remains to be clarified.