Genome Size Evolution In Polyploid Angiosperms
MD Bennett, L Hanson and IJ LeitchJodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK
Polyploidy has played a major role in the evolution of angiosperms. Genome sequencing of Arabidopsis and Oryza suggests that both are polyploids. Indeed, most angiosperms are considered to be polyploid. Levels of polyploidy can be extremely high, with the highest recorded in the eudicotyledon Sedum suaveolens (2n = c. 640) which may be 80-ploid. All else being equal, nuclear DNA amounts for polyploids are expected to increase in direct proportion to ploidy level. This expectation is obeyed in many polyploid series, especially those newly formed, but examples are found where genome size in polyploids is smaller than expected. Kew has long investigated genome evolution in polyploids. In 2000 we reported that mean DNA C-values for diploids and polyploids were much more similar than expected1. Recent comparison of genome sizes in 2,147 diploid and 812 polyploid species from the Angiosperm DNA C-values database (http://www.rbgkew.org.uk/cval/homepage/html) confirms this. Analysis revealed striking results, deviating from expectation, thus: (1) mean DNA amount did not increase in direct proportion with ploidy level and (2) the mean DNA amount per genome (x) tended to decrease with increasing ploidy level in monocots, eudicots and individual families (e.g. Poaceae and Fabaceae). Loss of DNA reported for a few species following polyploid formation2 is probably a very common phenomenon.
1. Bennett MD, Bhandol P, Leitch IJ. (2000). Annals of Botany 86: 859-909.
2. Wendel JF et al. (2002). Genetica 115: 37-47.