Genome size and the phenotype

Abstract

Land plant species (Embryophyta) vary more than 2,300-fold in the size of the holoploid genome (C-value) (see Leitch and Leitch 2012a, this volume) with the extremes at both ends of the scale contributed by angiosperms. At the lower end we find some species of the carnivorous Lentibulariaceae with ultrasmall genomes, e.g. Genlisea aurea with 0.065 pg or 63.5 Mbp (1C) (Greilhuber et al. 2006). This is only about 0.40-fold the size of the genome of Arabidopsis thaliana with 0.16 pg or 156.5 Mbp (1C) (Bennett et al. 2003), a species long considered to be the plant with the smallest reliably determined genome size. At the upper end of the scale stands the monocot octoploid Paris japonica (Melanthiaceae) with 2n = 8x = 40 and 152.23 pg or 148.88 Gbp (1C) (Pellicer et al. 2010). Nevertheless, other monocot species such as Fritillaria davisii (69.45 pg, 1C; 2n = 2x = 24, a possible palaeotetraploid), Trillium apetalon (95.0 pg, 1C; 2n = 4x = 20), and the dicot tree parasite Viscum album (102.9 pg, 1C; 2n = 20, palaeotetraploid?) also rank high on the scale regarding monoploid genome size (i.e. Cx-value = 2C-value divided by ploidy level) (Zonneveld 2010). From these examples it is clear that polyploidy plays only a relatively small role in the origin of the huge genome size differences reported. Instead, it is the accumulation of retroposon-like and other repetitive elements in the genomes which are largely responsible for the huge diversity of genome sizes in plants (Bennetzen et al. 2005; Grover and Wendel 2010; see also Kejnovsky et al. 2012 in Volume 1). (N.B. Recent studies using flow cytometry to estimate genome size in species with enormous genomes have highlighted how the more traditional approach of estimation using Feulgen densitometry may considerably underestimate genome size at this upper end of the scale (Zonneveld 2010).)