The Expanding Universe Of Genome Size(s) -- New Light and Black Holes

Since the role of DNA was discovered in 1953, interest in the genome has expanded each decade, including a growing focus on the extent, nature and significance of genome size.  The amount of DNA in an unreplicated gametic nucleus (1C-value) is a key biodiversity character of fundamental significance with many practical and predictive uses.  Methods for estimating genome size have evolved from slow chemical extraction, to microdensitometry and flow cytometry.  By 1963 C-values were known for 30 angiosperms, rising to 273 by 1972, and 3,493 taxa by 2003.  The Plant DNA C-value database (release 6.0, December 2012) includes estimates for 7,542 angiosperms (with 1,255 compiled since 2010).  Yet estimates remain poorly representative, with none for 98% of taxa.  Estimates made by non-molecular methods are subject to technical error (typically 5-10%), but complete genome sequencing was hoped to provide exact measures.  However, the first for Arabidopsis thaliana (125Mb) was 20% below estimates made using other methods.  Today results for 20 species show that in reality the completeness of the sequences generated (typically 67-90%) can vary considerably due to unsolved problems with the assembly of repeat regions.  While such gaps remain genome size estimates from ‘complete’ genome sequencing should not be used in preference to prime values obtained by classical methods which serve to check the completeness of sequencing and assembly, rather than the reverse.  One man’s junk DNA contains another man’s regulome, so realistic plans are needed to improve representation and reveal an unabridged book of life.