We argue that there is a need to maintain the term genome size in a broad sense as a covering term, because it is widely understood, short and phonetically pleasing. Proposals are made for a unified and consensual terminology. In this, 'genome size' should mean the DNA content based on chromosome number x and n, and should be used mainly in a general sense. The necessary distinction of the kinds of genome sizes is made by the adjectives 'monoploid' and the neology 'holoploid'. 'Holoploid genome size' is a shortcut for the DNA content of the whole chromosome complement characteristic for the individual (and by generalization for the population, species, etc.) irrespective of the degree of generative polyploidy, aneuploidies, etc. This term was lacking in the terminology and is for reasons of linguistic consistency indispensable. The abbreviated terms for monoploid and holoploid genome size are, respectively, Cx-value and C-value. Quantitative data on genome size should always indicate the C-level by a numerical prefix, such as 1C, 1Cx, 2C, etc. The proposed conventions cover general fundamental aspects relating to genome size in plants and animals, but do not treat in detail cytogenetic particularities (e.g. haploids, hybrids, etc.) which will need minor extensions of the present scheme in a future paper.

3 1 l e t t e r sThe shift from outcrossing to selfing is common in flowering plants 1,2 , but the genomic consequences and the speed at which they emerge remain poorly understood. An excellent model for understanding the evolution of self fertilization is provided by Capsella rubella, which became self compatible <200,000 years ago. We report a C. rubella reference genome sequence and compare RNA expression and polymorphism patterns between C. rubella and its outcrossing progenitor Capsella grandiflora. We found a clear shift in the expression of genes associated with flowering phenotypes, similar to that seen in Arabidopsis, in which self fertilization evolved about 1 million years ago. Comparisons of the two Capsella species showed evidence of rapid genome-wide relaxation of purifying selection in C. rubella without a concomitant change in transposable element abundance. Overall we document that the transition to selfing may be typified by parallel shifts in gene expression, along with a measurable reduction of purifying selection.

No abstract

Most angiosperms possess small genomes (mode 1C = 0.6 pg, median 1C = 2.9 pg). Those with truly enormous genomes (i.e. ≥ 35 pg) are phylogenetically restricted to a few families and include Liliaceae – with species possessing some of the largest genomes so far reported for any plant as well as including species with much smaller genomes. To gain insights into when and where genome size expansion took place during the evolution of Liliaceae and the mode and tempo of this change, data for 78 species were superimposed onto a phylogenetic tree and analysed. Results suggest that genome size in Liliaceae followed a punctuated rather than gradual mode of evolution and that most of the diversification evolved recently rather than early in the evolution of the family. We consider that the large genome sizes of Liliaceae may have emerged passively rather than being driven primarily by selection.