Isolation of a D-genome specific repeated DNA sequence fromAegilops squarrosa |
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Authors: | A Lane Rayburn Bikram S Gill |
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Institution: | (1) Department of Plant Pathology, Throckmorton Hall, Kansas State University, 66506 Manhattan, Kansas, (USA);(2) Present address: Department of Agronomy, Oklahoma State University, 74078 Stillwater, Oklahoma |
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Abstract: | Three repeated sequence clones, pAS1(1.0 Kb), pAS2(1.8 Kb) and pAS12(2.5 Kb), were isolated fromAegilops squarrosa (Triticum tauschii). The inserts of the three clones did not hybridize to each other. Two of the clones, pAS2 and pAS12, contain repeated sequences
which were distributed throughout the genome. The clone pAS1 sequence was more restricted and was located in specific areas
on telomeres and certain interstitial sites along the chromosome length. This cloned sequence was also found to be restricted
to the D genome at the level ofin situ hybridization. The pAS1 sequence will be useful in chromosomal identification and phylogenetic analysis. All three clones
will allow assessment of genome plasticity inAegilops squarrosa.
Nuclear DNA content varies over a range of 10,000 fold among all organisms (Nagl et al., 1983). Among angiosperms, at least
a 65-fold range in genome size occurs in diploid species (Sparrow, Price and Underbrink, 1972; Bennett, Smith and Heslop-Harrison,
1982). This DNA variation has been reported within families, genera, and species (Rothfels et al., 1966; Rees and Jones, 1967;
Miksche, 1968; Price, Chambers and Bachmann, 1981). Much of the interspecific variation in genome size among angiosperms appears
to be due to amplification and/or deletion of DNA within chromosomes.
The variation in genome size does not appear to result in changes in the number of coding genes (Nagl et al., 1983). While
the number of coding genes, with the exception of rDNA in specific examples, appears to remain constant, the remaining non-coding
regions are quite flexible. This non-coding DNA encompasses over 99% of the plant genome and consists of sequences that exist
as multiple copies throughout the genome and are identified as repeated DNA sequences (Flavell et al., 1974). Flavell et al.
(1974) have reported that increasing genome size in higher plants is associated with increasing repetitive DNA amounts. Subsequent
reports have substantiated this correlation (Bachmann and Price, 1977; Narayan, 1982). In various cereals, heterochromatin,
which has been demonstrated to be correlated with the location of specific repeated DNA sequences, has been positively correlated
with genome size (Bennett, Gustafson and Smith, 1977; Rayburn et al., 1985).
Furuta, Nishikawa and Makino (1975) found significant DNA content variation among different accessions ofAegilops squarrosa L. This species contains the D genome, a pivotal genome in several polyploid species and also found in hexaploid wheat (AABBDD).
The importance of this genome to the study of bread wheat genomes makes the mechanism(s) of this genomic plasticity of particular
interest. In order to determine which sequences are varying, one must first have a way to identify specific types of chromatin
and/or DNA.
Specific types of chromosome banding such as C- and N-banding have been used to identity types of chromatin in previous studies.
C-banding of the D genome results in very lightly staining bands whose pattern is somewhat indistinct. N-banding alternatively
has been shown to be useful in identifying certain chromosomes of hexaploid wheat but is limited by the lack of major bands
in the D genome (Endo and Gill, 1984). Specific DNA sequences have been isolated fromTriticum aestivum cultivar “Chinese Spring” (hexaploid wheat). However, these sequences are representatives of the A and/or B genomes of hexaploid
wheat and are not found in significant quantities in the D genome (Hutchinson and Lonsdale, 1982). Various other repeated
DNA sequences have been successfully isolated from rye (Bedbrook et al., 1980) and identified on rye chromosomes (Appels et
al., 1981; Jones and Flavell, 1982). Certain of these sequences are found in wheat genomes, but the sequences are representative
of only a minor fraction of the D genome (Bedbrook et al., 1980; Rayburn and Gill, 1985).
The purpose of this report is to describe three distinct repeated DNA sequences isolated fromA. squarrosa (D genome). Two clones appear to be distributed throughout the total genome, and the third clone is restricted to specific
sites along the chromosomes. This latter clone will prove useful in cytologically defining the D genome chromosomes. These
sequences appear representative of two types of repeated DNA genome organization: 1) sequences distributed throughout the
genome and 2) specific arrays of repeated sequences. The availability of such repeated DNA sequence clones along with the
known intraspecific DNA content variation inA. squarrosa will allow the study of genomic plasticity of this species. |
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