Inferring genetic relationship among haplomes in Triticeae: The utility of the 5S DNA units with examples

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Elimination of 5S DNA unit classes in newly formed allopolyploids of the genera Aegilops and Triticum

Two classes of 5S DNA units, namely the short (containing units of 410 bp) and the long (containing units of 500 bp), are recognized in species of the wheat (the genera Aegilops and Triticum ) group. While every diploid species of this group contains 2 unit classes, the short and the long, every allopolyploid species contains a smaller number of unit classes than the sum of the unit classes of its parental species. The aim of this study was to determine whether the reduction in these unit classes is due to the process of allopolyploidization, that is, interspecific or intergeneric hybridization followed by chromosome doubling, and whether it occurs during or soon after the formation of the allopolyploids. To study this, the number and types of unit classes were determined in several newly formed allotetraploids, allohexaploids, and an allooctoploid of Aegilops and Triticum. It was found that elimination of unit classes of 5S DNA occurred soon (in the first 3 generations) after the formation of the allopolyploids. This elimination was reproducible, that is, the same unit classes were eliminated in natural and synthetic allopolyploids having the same genomic combinations. No further elimination occurred in the unit classes of the 5S DNA during the life of the allopolyploid. The genetic and evolutionary significance of this elimination as well as the difference in response to allopolyploidization of 5S DNA and rDNA are discussed.

Xenopus transcription factor IIIA and the 5S nucleosome: development of a useful in vitro system

5S RNA genes in Xenopus are regulated during development via a complex interplay between assembly of repressive chromatin structures and productive transcription complexes. Interestingly, 5S genes have been found to harbor powerful nucleosome positioning elements and therefore have become an important model system for reconstitution of eukaryotic genes into nucleosomes in vitro. Moreover, the structure of the primary factor initiating transcription of 5S DNA, transcription factor IIIA, has been extensively characterized. This has allowed for numerous studies of the effect of nucleosome assembly and histone modifications on the DNA binding activity of a transcription factor in vitro. For example, linker histones bind 5S nucleosomes and repress TFIIIA binding in vitro in a similar manner to that observed in vivo. In addition, TFIIIA binding to nucleosomes assembled with 5S DNA is stimulated by acetylation or removal of the core histone tail domains. Here we review the development of the Xenopus 5S in vitro system and discuss recent results highlighting new aspects of transcription factor – nucleosome interactions.Key words: nucleosomes, 5S genes, transcription factor IIIA, chromatin.

The South African Hordeum capense is more closely related to some American Hordeum species than to the European Hordeum secalinum: a perspective based on the 5S DNA units (Triticeae: Poaceae)

Several authors have proposed that the European Hordeum secalinum and the morphologically similar South African Hordeum capense are conspecific. In this paper we provide evidence that the two species differ in their 5S DNA unit class composition. We also report on the diversity of 5S DNA units in Hordeum muticum, a South American species. When the 5S rDNA unit class composition for these three species is compared with the unit class composition for all Hordeum species thus far investigated, it appears that H. capense is more closely related to the American Hordeum species containing the long Y2 unit class, than to H. secalinum, which lacks the long Y2 unit class but contains the long X2 unit class found in H. marinum. This analysis suggests H. capense may have originated from a stock common to the South American species, such as H. muticum.Key words: 5S DNA unit class, Hordeum capense, Hordeum secalinum, Hordeum muticum, continental drift.

A comparison of the 5S rDNA diversity in the Hordeum brachyantherum–californicum complex with those of the eastern Asiatic Hordeum roshevitzii and the South American Hordeum cordobense (Triticeae: Poaceae)

Amplification of the 5S rDNA gene by the polymerase chain reaction, followed by cloning and sequencing, was used to generate data from 23 seed accessions of Hordeum brachyantherum Nevski, Hordeum californicum Covas et Stebbins, Hordeum cordobense Bothmer, Jacobsen et Nicora, and Hordeum roshevitzii Bowden. One hundred and fourteen clones were analyzed, resulting in the detection of four different 5S DNA unit classes. Three of them, long H1, long H2, and long Y2, had been previously reported. The long H3 class, described for the first time, is present only in H. roshevitzii but can be grouped with previously unassigned units of Hordeum bulbosum L. and Hordeum spontaneum C. Koch. Based upon the analyses of 5S rDNA sequences, we found that (i) the long H2 unit class was not found in the Asiatic H. roshevitzii and therefore may be restricted to the American species, (ii) there is no strong support that H. brachyantherum and H. californicum are worthy of species recognition, and (iii) cladistic analysis of the consensus sequences of the four paralogous unit classes demonstrated that long Y2 is the most distant from the three long H classes.Key words: 5S DNA gene, Hordeum, unit classes.