The number of angiosperm species for which nuclear DNA amount estimates have been made has nearly trebled since the last collected lists of such values were published, and therefore, publication of a more comprehensive list is overdue. This paper lists absolute nuclear DNA amounts for 753 angiosperm species. The data were assembled primarily for reference purposes, and so the species are listed in alphabetical order, as this was felt to be more helpful to cyto- and biochemists whom, it is anticipated, will be among its major users. The paper also reviews aspects of the history, nomenclature, methods, accuracy and problems of nuclear DNA estimation in angiosperms. No attempt is made to reconsider those aspects of nuclear DNA estimation which have been fully revised previously, although the bibliography of such aspects is given. Instead, the paper is intended as a source of basic information regarding the terminology, practice and limitations of nuclear DNA estimation, especially by Feulgen microdensitometry, as currently practiced.

Nuclear DNA Amounts in Angiosperms

Nuclear DNA amounts have been estimated for more than 200 angiosperm species since the last collected list of such values for about 750 species was published by Bennett & Smith in 1976 (Phil. Trans. R. Soc. Lond. B 274, 227-274). These new estimates are either scattered in a wide range of scientific journals or, in many cases, unpublished; so they are not readily accessible. A publication, collecting these data in a single list is required. This paper contains a supplementary list of absolute DNA values, including estimates for 240 angiosperm species not listed by Bennett & Smith in 1976, as well as additional estimates for 41 species already listed by them. These data are assembled primarily for reference purposes. Consequently, the species are listed in alphabetical order, as this was felt to be more helpful to cyto- and biochemists, who it is anticipated will be among the major users.

The 40,000-fold variation in eukaryote haploid DNA content is unrelated to organismic complexity or to the numbers of protein-coding genes. In eukaryote microorganisms, as well as in animals and plants, DNA content is strongly correlated with cell volume and nuclear volume, and with cell cycle length and minimum generation time. These correlations are simply explained by postulating that DNA has 2 major functions unrelated to its protein-coding capacity: (1) the control of cell volume by the number of replicon origins, and (2) the determination of nuclear volume by the overall bulk of the DNA: cell growth rates are determined by the cell volume and by the area of the nuclear envelope available for nucleocytoplasmic transport of RNA, which in turn depends on the nuclear volume and therefore on the DNA content. During evolution nuclear volume, and therefore DNA content, has to be adjusted to the cell volume to allow reasonable growth rates. The great diversity of cell volumes and growth rates, and therefore of DNA contents, among eukaryotes results from a varying balance in different species between r-selection, which favours small cells and rapid growth rates and therefore low DNA C-values, and K-selection which favours large cells and slow growth rates and therefore high DNA C-values. In multicellular organisms cell size needs to vary in different tissues: size differences between somatic cells result from polyteny, endopolyploidy, or the synthesis of nucleoskeletal RNA. Conflict between the need for large ova and small somatic cells explains why lampbrush chromosomes, nurse cells, chromatin diminution and chromosome elimination evolved. Similar evolutionary considerations clarify the nature of polygenes, the significance of the distribution of haploidy, diploidy and dikaryosis in life cycles and of double fertilization in angiosperms, and of heteroploidy despite DNA constancy in cultured cells, and other puzzles in eukaryote chromosome biology. Eukaryote DNA can be divided into genic DNA (G-DNA), which codes for proteins (or serves as recognition sites for proteins involved in transcription, replication and recombination), and nucleoskeletal DNA (S-DNA) which exists only because of its nucleoskeletal role in determining the nuclear volume (which it shares with G-DNA, and performs not only directly, but also indirectly by coding for nucleoskeletal RNA). Mechanistic and evolutionary implications of this are discussed.

Nuclear volume control by nucleoskeletal DNA, selection for cell volume and cell growth rate, and the solution of the DNA C-value paradox.

Variation in DNA content has been largely ignored as a factor in evolution, particularly following the advent of sequence-based approaches to genomic analysis. The significant genome size diversity among organisms (more than 200 000-fold among eukaryotes) bears no relationship to organismal complexity and both the origins and reasons for the clearly non-random distribution of this variation remain unclear. Several theories have been proposed to explain this ‘C-value enigma’ (heretofore known as the ‘C-value paradox’), each of which can be described as either a ‘mutation pressure’ or ‘optimal DNA’ theory. Mutation pressure theories consider the large portion of non-coding DNA in eukaryotic genomes as either ‘junk’ or ‘selfish’ DNA and are important primarily in considerations of the origin of secondary DNA. Optimal DNA theories differ from mutation pressure theories by emphasizing the strong link between DNA content and cell and nuclear volumes. While mutation pressure theories generally explain this association with cell size as coincidental, the nucleoskeletal theory proposes a coevolutionary interaction between nuclear and cell volume, with DNA content adjusted adaptively following shifts in cell size. Each of these approaches to the C-value enigma is problematic for a variety of reasons and the preponderance of the available evidence instead favours the nucleotypic theory which postulates a causal link between bulk DNA amount and cell volume. Under this view, variation in DNA content is under direct selection via its impacts on cellular and organismal parameters. Until now, no satisfactory mechanism has been presented to explain this nucleotypic effect. However, recent advances in the study of cell cycle regulation suggest a possible ‘gene–nucleus interaction model’ which may account for it. The present article provides a detailed review of the debate surrounding the C-value enigma, the various theories proposed to explain it, and the evidence in favour of a causal connection between DNA content and cell size. In addition, a new model of nucleotypic influence is developed, along with suggestions for further empirical investigation. Finally, some evolutionary implications of genome size diversity are considered, and a broadening of the traditional ‘biological hierarchy’ is recommended.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-185X.2000.tb00059.x

Coincidence, coevolution, or causation? DNA content, cell size, and the C-value enigma.

The reannealing kinetics of denatured DNA fragments from 23 species of higher plants have been studied, using hydroxylapatite chromatography to distinguish reannealed from single-stranded DNA. The 2C nuclear DNA contents of the species varied between 1.7 and 98 pg. The proportions of DNA in species with a nuclear DNA mass above 5 pg that reannealed with the kinetics of sequences present in more than 100 copies were high (69–92% with a mean of 80±2.0%). For species with less than 4 pg of DNA, the mean proportion of repeated-sequence DNA was 62±2.9%. It is concluded that most of the variation in nuclear DNA mass in higher plant chromosomes can be accounted for by variation in repeated-sequence DNA. The consequences of altering the adapted DNA content of a species by the addition of families of repeated sequences are discussed in relation to the proportion of repeated-sequence DNA.

Genome size and the proportion of repeated nucleotide sequence DNA in plants.

Publisher Summary This chapter discusses the B-chromosome systems in flowering plants and animal species. The term B chromosome is introduced to describe extra chromosome which have little if any effect on visible characters of the plant, which are not homologous with the A chromosomes of the normal complement, and which are extremely irregular in their meiotic distribution. B Chromosomes are now known under a variety of different names, including supernumerary and accessory, which are the most common alternatives. B chromosomes are dispensable and nonhomologous with A chromosomes. These two characteristics above all others most sharply differentiate them from members of the basic A-chromosome complement. The distribution of B chromosome in plant and animal species is discussed. In many plants and animals the B chromosomes are completely stable during the cell cycle and are inherited in a constant and unchanging form along with the A chromosomes. Some plant species have an apparent order in their instability, which results in exclusion and/or accumulation of B chromosomes in specific tissues and organs. The significance of B-chromosome effects in flowering plants and animals is discussed.

B-Chromosome Systems in Flowering Plants and Animal Species

Publisher Summary This chapter discusses the origin of the wide species variation in nuclear DNA content. Evolution depends upon the selection of phenotypes displaying adaptive changes of a heritable nature. The generation of such phenotypes depends on the alteration of genetic information embodied within the DNA of the chromosomes. As for evolutionary change in DNA amount , there is a progression from low DNA content in primitive phyla, such as bacteria to high DNA content in cells of sophisticated higher plants and animals. The causes of change in DNA amount are illustrated in the chapter. Polyploidy, common in plant groups such as the angiosperms and pteridophytes, is a special case as it involves amplification of all genes and all base sequences of the haploid complement. There is an extensive and widespread variation in DNA amount, which is independent of alteration in chromosome number. This is attributable to the amplification or reduction of DNA segments within chromosomes.

R.N. Jones

Papers

B-Chromosome Systems in Flowering Plants and Animal Species

The origin of the wide species variation in nuclear dna content