Genome size evolution in holoparasitic Orobanche (Orobanchaceae) and related genera

Genome size was estimated using Feulgen densitometry for 76 accessions of 40 taxa of Orobanche and two taxa each of the related genera Phelypaea and Cistanche, providing the first data set for any group of nonphotosynthetic angiosperms. The 2C-values were 16.8-19.9 pg in Cistanche, 2.9-11.6 pg in Orobanche sect. Orobanche, 6.8-10.8 pg in sect. Trionychon, 4.3-5.1 pg in sect. Myzorrhiza, and 4.9-5.8 pg and 10.5 pg in the two diploid species Phelypaea coccinea and P. tournefortii, respectively. Distribution of genome size is congruent with phylogenetic lineages identified by analyses of nuclear ITS sequence data, in particular regarding the distinctness of O. anatolica from the rest of sect. Orobanche. With the exception of tetraploid O. transcaucasica, polyploid taxa of sect. Orobanche are among those with the smallest C-values, suggesting substantial genome downsizing after polyploidization (as in other angiosperm groups). In O. sect. Orobanche, genome size evolved more rapidly in earlier stages of its evolution than in later stages. This might be indicative of adaptive radiation, but this hypothesis requires corroboration in the form of genome size estimates on more taxa and accessions.