Nuclear DNA amount and genome downsizing in natural and synthetic allopolyploids of the generaAegilopsandTriticum

Abstract

Recent molecular studies in the genera Aegilops and Triticum showed that allopolyploidization (interspecific or intergeneric hybridization followed by chromosome doubling) generated rapid elimination of low-copy or high-copy, non-coding and coding DNA sequences. The aims of this work were to determine the amount of nuclear DNA in allopolyploid species of the group and to see to what extent elimination of DNA sequences affected genome size. Nuclear DNA amount was determined by the flow cytometry method in 27 natural allopolyploid species (most of which were represented by several lines and each line by several plants) as well as 14 newly synthesized allopolyploids (each represented by several plants) and their parental plants. Very small intraspecific variation in DNA amount was found between lines of allopolyploid species collected from different habitats or between wild and domesticated forms of allopolyploid wheat. In contrast to the constancy in nuclear DNA amount at the intraspecific level, there are significant differences in genome size between the various allopolyploid species, at both the tetraploid and hexaploid levels. In most allopolyploids nuclear DNA amount was significantly less than the sum of DNA amounts of the parental species. Newly synthesized allopolyploids exhibited a similar decrease in nuclear DNA amount in the first generation, indicating that genome downsizing occurs during and (or) immediately after the formation of the allopolyploids and that there are no further changes in genome size during the life of the allopolyploids. Phylogenetic considerations of the origin of the B genome of allopolyploid wheat, based on nuclear DNA amount, are discussed.

Genome Evolution Due to Allopolyploidization in Wheat

“…exhibit a similar loss, indicating that DNA elimination occurs soon after allopolyploidization (Nishikawa and Furuta 1969;Furuta et al 1974;Eilam et al 2008Eilam et al , 2010. Also, the narrow intraspecific variation in DNA content of the allopolyploids supports that the loss of DNA occurred immediately after the allopolyploid formation and that there was almost no subsequent change in DNA content during the allopolyploid species evolution (Eilam et al 2008).…”

“…Also, the narrow intraspecific variation in DNA content of the allopolyploids supports that the loss of DNA occurred immediately after the allopolyploid formation and that there was almost no subsequent change in DNA content during the allopolyploid species evolution (Eilam et al 2008). In triticale (a synthetic allopolyploid between wheat and rye, Secale cereale), Boyko et al (1984Boyko et al ( , 1988 and Ma and Gustafson (2005) found that there was a major reduction in DNA content in the course of triticale formation, amounting to 9% for the octoploid and 28-30% for the hexaploid triticale.…”

“…Most of the CSSs are noncoding sequences and are present in all the diploid species of Aegilops and Triticum, but occur in only one pair of chromosomes of allopolyploid wheat, suggesting that they were lost during or after allopolyploidization A most surprising discovery is that allopolyploidization in the wheat group causes immediate nonrandom elimination of specific noncoding, low-copy, and high-copy DNA sequences Liu et al 1998a,b;Ozkan et al 2001;Shaked et al 2001;Han et al 2003Han et al , 2005Salina et al 2004). The extent of DNA elimination was estimated by determining the amounts of nuclear DNA in natural allopolyploids and in their diploid progenitors, as well as in newly synthesized allopolyploids and their parental plants (Ozkan et al 2003;Eilam et al 2008Eilam et al , 2010. Natural wheat and related allopolyploids contain 2-10% less DNA than the sum of their diploid parents, and synthetic allopolyploids Figure 1 Schematic of the wheat karyotype.…”

Nuclear DNA Amounts of Mulberries (<i>Morus </i>spp.) and Related Species

“…All diploids mulberry species studied in this report are potential parents for crossbreeding. allopolyploids of several Triticeae species, nuclear DNA amounts are significantly smaller than the sum of their parental species (Eilam et al 2008(Eilam et al , 2009). Leitch & Bennett (2004) identified a tendency for 1Cx values of higher ploidy species to be smaller; however, in mulberry, the 1Cx values of three of four hexaploids were larger than those of diploids, although the 1Cx values of the docosaploid and a hexaploid were smaller and slightly smaller, respectively.…”

“…Thus, an increase in repetitive sequences may lead to an increase in genome size, whereas after polyploidization genome size can be rapidly reduced within one generation (Eilam et al 2008(Eilam et al , 2009). Both events may lead to changes in the 2C DNA value (the unreplicated genome DNA content).…”