2C or not 2C: a closer look at cell nuclei and their DNA content

Abstract

The life cycle of animals and plants involves changes in chromosome number (nuclear phase) and sometimes even the karyotype, and consequently the DNA content of a nuclear genome is not static in time. Thus, in order to interpret DNA content data, it is important that the status of the materials from which DNA content is estimated be precisely defined. The previously proposed distinction between "holoploid" (C) and "monoploid" (Cx) genome size covers the most frequent states of plant and animal nuclear genomes. However, restricting nomenclature to just C and Cx still leaves a number of unresolved problems. Here, we propose an extension of the C-value terminology to handle a range of cytogenetic conditions, life cycle segments, and nuclear phases. A set of superscripts and subscripts are used in a formal way to identify life cycle segments and to express the quantitative relationship between these segments. A revision of the current usage of the holoploid chromosome number n was necessary to maintain the intimate link between n and C-value and between the monoploid chromosome number x and Cx-value. In this revision, haplophase individuals (i.e., "haploid" animals and "haploid" spontaneous or experimentally induced land plant sporophytes) have chromosome number n (not 2n, as is the current tradition) and thus nuclear DNA contents based on 1C. However, to avoid an unlimited progression of n levels due to generative polyploidy, zygotic individuals are assigned as 2n starting from the zygote, whatever their ploidy level. Their ploidy is indicated by multiples of the basic chromosome number x. The extended terminology for genome size should eliminate ambiguities in reporting DNA contents in both plants and animals.

Differential effects of polyploidy and diploidy on fitness of apomictic Boechera

“…For ploidies, the points where the angled sides of the box reach the maximum box width are equivalent to 95% confidence intervals around the medians. If the notches of two plots do not overlap, this is strong evidence that the two medians differ (Chambers et al 1983) The fraction of seeds that deviated from the expected apomictic embryo/endosperm ratio were either specifically described under ''Event'', or clustered as ''Unknown'' if ploidy measurement of either embryo or endosperm was questionable, Emb = embryo, E = endosperm after Greilhuber and Doležel (2009) This case could be attributed to three possibilities: (i) fertilization of unreduced egg nucleus by 2 unreduced sperm nuclei, pseudogamous endosperm formation by fusion of two 3Cx polar nuclei with two 3Cx sperm nuclei, 1m:1p contribution to endosperm (Voigt et al 2007, indication of pollen grains with 4 sperm cells and 2 vegetative cells), Emb(2n=9x) 9Cx: E(8n=12x) 12Cx; (ii) fertilization of unreduced egg cell by one 6Cx sperm cell, pseudogamous endosperm formation by fusion of two 3Cx polar nuclei with one 6Cx sperm cell, Emb(2n=9x) 9Cx: E(6n=12x) 12Cx, or (iii) fertilization of 6Cx egg nucleus (somatic doubling) with one unreduced 3Cx sperm nucleus, autonomous endosperm formation by fusion of two 6Cx polar nuclei, lack of paternal contribution to endosperm, Emb(2n=9x) 9Cx: E(4n=12x) 12Cx…”

“…The relative DNA content of embryo and endosperm cells is illustrated following Greilhuber and Doležel (2009), whereby unreduced gametes are always diplophasic (2n, sporophytic life cycle), and their ploidy depends on the number of monoploid chromosome sets (x = 7). A superscript is attached to C and Cx values to specify embryo (Emb) and endosperm (E) chromosome constitution, a subscript is attached to specify maternal( m ) and paternal( p ) contribution to endosperm formation.…”

Ploidy level determination within the context of in vitro breeding

“…Therefore, for a typical DNA histogram of euploid material, two peaks will be produced, one more prominent peak representing the G1 phase (unreplicated, 2C) and the other (which will be formed at twice the channel value) representing the G2/M phase (replicated, 4C) of the cell cycle (Fig. 2;Ochatt 2008;Greilhuber and Doležel 2009). Additionally, endopolyploid/ endoreduplicated peaks with 8C, 16C, etc.…”

Nuclear genome size: Are we getting closer?

“…Thus, the mass of nuclear DNA in the whole chromosome complement (with chromosome number n) was often referred to as genome size irrespective of ploidy and number of basic chromosome sets (x) the chromosome complement comprised. Only recently, Greilhuber et al (24,25) proposed a coherent terminology in which the term genome size is used as a covering term in the wide sense, irrespective of ploidy. The necessary distinction of the kinds of genome sizes is made by the adjectives ''monoploid" (one chromosome set of an organism having the chromosome base number x) and ''holoploid'' (the whole complement of chromosomes with chromosome number n characteristic for the organism).…”