Genome downsizing in polyploid plants

Abstract

All else being equal, polyploids are expected to have larger C-values (amount of DNA in the unreplicated gametic nucleus) than their diploid progenitors, increasing in direct proportion with ploidy. This expectation is observed in some polyploid series, especially those newly formed, but there are examples suggesting that C-values in particular polyploids are less than expected. The availability of the Angiosperm DNA C-values database (http:// www.rbgkew.org.uk/cval/homepage.html) has allowed this question to be addressed across a broad range of angiosperms and has revealed striking results deviating from expectation: (i) mean 1C DNA amount did not increase in direct proportion with ploidy, and (ii) mean DNA amount per basic genome (calculated by dividing the 2C value by ploidy) tended to decrease with increasing ploidy. These results suggest that loss of DNA following polyploid formation, or genome downsizing, may be a widespread phenomenon of considerable biological significance. Recent advances in our understanding of the molecular events that take place following polyploid formation together with new data on how DNA amounts can both increase and decrease provide some insights into how genome downsizing may take place. The nature of the evolutionary forces that may be driving DNA loss are also discussed.

Geographical parthenogenesis, genome size variation and pollen production in the arctic-alpine species Hieracium alpinum

“…In our study, we confirmed a general trend of genome downsizing in polyploids with respect to their diploid progenitors (Leitch and Bennett 2004). This contrasts with the closely related genus Pilosella, where downsizing was not apparent in triploids, but only at higher ploidy levels (Suda et al 2007).…”

“…This contrasts with the closely related genus Pilosella, where downsizing was not apparent in triploids, but only at higher ploidy levels (Suda et al 2007). Several mechanisms may lead to genome size reduction in polyploids: (1) unequal homologous recombination, (2) elimination of specific DNA sequences (including sequences in low-and high copy number from both coding and non-coding regions), and (3) change in transposons activity (Leitch and Bennett 2004).…”

The shrunken genome of Arabidopsis thaliana

“…One proposed mechanism is the loss of whole chromosomes or genes following polyploidization events (Leitch and Bennett 2004). But our data reveal no correlation between chromosome number and genome size from an evolutionary perspective.…”

Genome size in aquatic and wetland plants: fitting with the large genome constraint hypothesis with a few relevant exceptions

“…Ranunculus lingua, the species with the largest genome size among those studied in this paper, is likely a high polyploid, as its commonest chromosome The two taxa for which two populations were studied also could likely present different ploidy levels. For Caltha palustris, 2C-values (19.54 and 19.62 (Leitch and Bennett 2004). One of the previous records from this taxon (2C = 4.20, 11.05 pg; Table 1) would suggest still one more ploidy level.…”