Genome size variation in diploid and tetraploid wild wheats

Hakan O zkan 2 Metin Tuna 1 Benjamin Kilian 0 Naoki Mori 4 Shoji Ohta 3 Background aims 0 Leibniz Institute of Plant Genetics and Crop Plant Research (IPK) Genebank/ Genome Diversity , Corrensstrasse 3, 06466 Gatersleben, Germany 1 Department of Field Crops, Faculty of Agriculture, Namk Kemal University , 59030 Tekirdag, Turkey 2 Department of Field Crops, Faculty of Agriculture, University of Cukurova , 01330 Adana, Turkey 3 Department of Bioscience, Fukui Prefectural University , 4-1-1, Kenjojima, Matsuoka, Yoshida, Fukui 910-1185, Japan 4 Graduate School of Agricultural Science, Kobe University , 1 Rokkodai, Nada, Kobe 657-8501, Japan Intra- and interspecific variations of C-values and the relationship between habitat factors and genome size were studied in natural populations of diploid and tetraploid wild wheats. The 1C nuclear DNA content of 376 individual plants representing 41 populations of diploid and tetraploid wild wheats was determined by flow cytometry (FCM) and correlated with geographical and bioclimate variables. Based on analysis of variance, significant differences between diploid and tetraploid Triticum species were found. Differences among populations of T. boeoticum and T. dicoccoides were also statistically significant and argue for isolation between populations, except for T. araraticum. However, the variation among individuals of the same population was not statistically significant. Maximum genome size differences among populations for T. boeoticum (0.143 pg; 2.32 %), T. dicoccoides (0.314 pg; 2.49 %) and T. araraticum (0.116 pg; 0.98 %) argue for genome constancy in these species. There was no significant correlation between intra-population variance and geographical and bioclimate variables for T. boeoticum and T. dicoccoides. In contrast to the limited genome size variation at the intraspecific level, the interspecific variation was large: 0.5 pg/1C (8 %) at the diploid level (T. boeoticum vs. T. urartu) and 1 pg/1C (9.7 %) at the tetraploid level (T. dicoccoides vs. T. araraticum). Low intraspecific genome size variation occurs in diploid and tetraploid wild wheats, and this limited variation is not correlated with geographical and climate variables. However, interspecific variation is significant at the diploid and tetraploid level. It can be concluded that the genome size of wild self-fertilizing Triticum species is generally stable, despite the presence of many potentially active retroelements. In natural habitats, it is very difficult to distinguish wild wheats from each other. However, all four species can be distinguished easily, quickly and unambiguously by using the FCM technique. Methodology Principal results Conclusions Introduction The term genome size refers to the DNA content of the unreplicated reduced nucleus, irrespective of the ploidy level of the taxon, and it is expressed as C value in picograms (Swift, 1950). Since it became possible to measure the DNA content of a single nucleus (1950s), various researchers have reported interspecific variation among different species (Swift, 1950; Price et al., 1981; Bennett and Leitch, 1995, 2005). The 1C nuclear DNA amount of plant species with the same ploidy level differs by several orders of magnitude, from 0.0648 pg/1C for Genlisea margaretae Hutch (Greilhuber et al., 2006) to 132.45 pg/1C for Trillium camschatcense Ker Gawler (Zonneveld, 2010). More recently, it was found that differences in genome size among species are predominantly associated with differences in the amount of repetitive sequences. Particularly, retrotransposons play a dominant role in genome size differences, and most of the variation in genome size in plants can be ascribed to differential accumulation of retrotransposons (Bennetzen, 2000, 2007; Feuillet and Keller, 2002). The occurrence and extent of genome size variation below the species level are still controversial and not satisfactorily analysed yet. Intraspecific variation in plants has been reported for numerous species and was attributed to differences in chromosome number, chromosome size (polyploidy, aneuploidy, B chromosomes, sex chromosomes) and inherent undetected cryptic species (Greilhuber, 1998; Gregory, 2005). However, evidence for intraspecific genome size variation other than chromosome polymorphism and cryptic taxonomic variation is rare and still controversial. A high degree of genome constancy, which is in agreement with the initial notion of constancy in DNA content within individuals and species (Swift, 1950), has been found in many species, including the base calibration standard for estimating C values, Allium cepa L. (Bennett et al., 2000). On the other hand, in several earlier studies, intraspecific variation often based on densitometry or cytofluorometry techniques has been observed, but these results could not be confirmed by subsequent flow cytometry (FCM) analyses that used exactly the same plant material. Therefore, in most cases, intraspecific variation has been explained by taxonomic misclassification or technical artefacts such as suboptimal staining and insufficient standardization (reviewed by Greilhuber, 1998, 2005). However, it seems plausible that genome size may diverge in populations, even in the face of limited gene flow (Kron et al., 2007). Wheat is one of the principal cereal crops in the world. There are two wild diploid Triticum species: T. boeoticum Boiss. (AbAb) and T. urartu Thum. ex Gandil. (AuAu). These species are separated by crossing barriers (Johnson and Dhaliwal, 1976), and differ in their plant morphology (Gandilian, 1972; Dorofeev et al., 1979) and biochemical and molecular marker loci (Johnson, 1975; Kilian et al., 2007a; Konovalov et al., 2010). Triticum boeoticum has been considered to be the progenitor of cultivated diploid einkorn wheat, T. monococcum L. There are also two wild tetraploid Triticum species: Triticum dicoccoides (Korn. ex Aschers. & Graebn.) Schweinf. (BBAuAu) and T. araraticum Jakubz. (GGAuAu). Triticum dicoccoides is the primary wild-type form and has given rise to several tetraploid wheat taxa that are now cultivated. Similarly, T. araraticum is the wild-type progenitor of T. timopheevii Menabde et Ericzjan (GGAuAu). The A genome of T. dicoccoides and T. araraticum was contributed by the diploid wheat T. urartu (Dvorak et al., 1993). The diploid and tetraploid wild wheats are important germplasm sources for cultivated wheat improvement. Interspecific variability in nuclear DNA content has been reported in Triticum L. and the related genus Aegilops L. Furuta et al. (1977) measured the nuclear DNA content in 43 accessions of the genus Aegilops and confirmed that intra- and interspecific variation existed. Recently, Eilam et al. (2007) reported that within the diploid Aegilops species, the 1C DNA amount ranged from 4.84 pg in Ae. markgrafii (Greuter) Hammer ( Ae. caudata L.) to 7.52 pg in Ae. sharonensis Eig. The nuclear DNA content of Ae. geniculata Roth ( Ae. ovata (...truncated)