The joining of different genomes in allotetraploids played a major role in plant evolution, but the molecular implications of this event are poorly understood. In synthetic allotetraploids of Arabidopsis and Cardaminopsis arenosa, we previously demonstrated the occurrence of frequent gene silencing. To explore the involvement of epigenetic phenomena, we investigated the occurrence and effects of DNA methylation changes. Changes in DNA methylation patterns were more frequent in synthetic allotetraploids than in the parents. Treatment with 5-aza-2Ј-deoxycytidine, an inhibitor of DNA methyltransferase, resulted in the development of altered morphologies in the synthetic allotetraploids, but not in the parents. We profiled mRNAs in control and 5-aza-2Ј-deoxycytidine-treated parents and allotetraploids by amplified fragment length polymorphism-cDNA. We show that DNA demethylation induced and repressed two different transcriptomes. Our results are consistent with the hypothesis that synthetic allotetraploids have compromised mechanisms of epigenetic gene regulation.Allotetraploids are formed by hybridization between two species and inherit a complete diploid set of chromosomes from each parental species. Although many established wild and cultivated allopolyploids are fertile, well adapted, and genetically stable, allopolyploids of more recent origin commonly display genomic and phenotypic instability (Soltis and Soltis, 1995;Pikaard, 1999; Comai, 2000).As a consequence of the union of two genomes, abnormal phenotypes have been reported (Comai, 2000;Schranz and Osborn, 2000). The causes of these phenotypes are largely unknown. McClintock (1984) described similar phenomena as "genomic shock," which she defined as a preprogrammed response to an unusual challenge resulting in extensive restructuring of the genome. This "unusual challenge" may involve epigenetic gene silencing, which results from homologous DNA-DNA or DNA-RNA interactions. The hybridization of redundant and diverged homeologous sets of genes in allopolyploids might trigger widespread gene silencing and changes in chromatin structure and DNA methylation patterns.Recent molecular data are consistent with the gene silencing hypothesis. Previously, we have reported about 1% changes in gene expression by comparing synthetic allotetraploids derived by hybridizing 4x Arabidopsis and 4x Cardaminopsis arenosa (also known as Arabidopsis arenosa; Comai et al., 2000). These changes can involve both normal genes and genes related to transposons. The corresponding natural allotetraploid, Arabidopsis suecica, was examined by Lee and Chen (2001), who demonstrated similar silencing levels. Furthermore, they found that silencing was related to methylation and could be reversed by treatment with the DNA demethylating agent 5-aza-2Ј-deoxycytidine (azadC). Instability can also be manifested by genomic rearrangements. Synthetic hybrids of wheat (Triticum aestivum) displayed rapid and widespread loss of DNA sequences and changes in DNA methylation Shaked et al., 2001). These...

Centromeric H3-like histones, which replace histone H3 in the centromeric chromatin of animals and fungi, have not been reported in plants. We identified a histone H3 variant from Arabidopsis thaliana that encodes a centromere-identifying protein designated HTR12. By immunological detection, HTR12 localized at centromeres in both mitotic and meiotic cells. HTR12 signal revealed tissue-and stage-specific differences in centromere morphology, including a distended bead-like structure in interphase root tip cells. The anti-HTR12 antibody also detected spherical organelles in meiotic cells. Although the antibody does not label centromeres in the closely related species Arabidopsis arenosa , HTR12 signal was found on all centromeres in allopolyploids of these two species. Comparison of the HTR12 genes of A. thaliana and A. arenosa revealed striking adaptive evolution in the N-terminal tail of the protein, similar to the pattern seen in its counterpart in Drosophila . This finding suggests that the same evolutionary forces shape centromeric chromatin in both animals and plants. INTRODUCTIONCentromeres are the specialized chromosomal sites necessary for poleward movement during mitosis and meiosis in eukaryotes. Commonly, a centromere is evident as a prominent constriction within the heterochromatin of each metaphase chromosome. The attachment to and movement of chromosomes along the spindle is mediated by the proteinaceous kinetochores, which form at the centromeres during cell division.Despite this highly conserved function, centromeric DNA sequences are not conserved between organisms. For example, human centromeres consist of large blocks (200 kb to several megabases) of tandemly repeated 171-bp ␣ -satellite (Willard, 1998), but the sequences can differ from those of apes on homologous chromosomes (Haaf and Willard, 1997). Similarly, Drosophila melanogaster centromeric regions contain blocks of 5-to 12-bp satellite repeats that do not appear to be shared by homologous centromeres of sibling species (Lohe and Brutlag, 1987).Plant centromeric regions resemble their mammalian counterparts in that both have large arrays of tandem repeats, frequently of approximately nucleosomal size. In centromere regions of maize (Alfenito and Birchler, 1993;Ananiev et al., 1998), pearl millet (Kamm et al., 1994), rice (Dong et al., 1998), sugarcane (Nagaki et al., 1998), sorghum (Zwick et al., 2000), the Australian daisy Brachycome dichromosomatica (Leach et al., 1995), rape (Xia et al., 1993), and the wild beet Beta procumbens (Gindullis et al., 2001), tandem repeats have been found that differ in sequence but that all have lengths in the range of ‫ف‬ 140 to 180 bp. Particular repeat arrays in cereals have been estimated to be hundreds of kilobases long (Dong et al., 1998;Kaszás and Birchler, 1998). Cereal tandem repeat arrays are interrupted by TY3/gypsy -like retrotransposons and other retrotransposons (Ananiev et al., 1998;Kumekawa et al., 2001;Nonomura and Kurata, 2001). The satellite arrays and interspersed retrotransposons o...

We assessed the genetic diversity in Trichloris crinita (Poaceae) varieties from South America, using AFLPs, morphological characters, and quantitative agronomic traits. Owing to the importance of this species for range grazing, we first characterized the varieties based on forage productivity. Biomass production varied 9 fold among the materials evaluated. Analysis of AFLP fingerprints allowed the discrimination of all varieties with a few selected primer combinations. Pair-wise genetic similarities, using marker data, ranged from 0.31 to 0.92 (Jaccard coefficients). Marker-based unweighted pair group method with arithmetic averaging (UPGMA) cluster analysis did not show geographical clustering, but rather grouped the varieties according to their biomass production. We identified 18 markers associated with biomass production, of which 8 showed complete correlation (r = 1.00) with this trait. These DNA markers can be used to assist selection for high forage productivity in T. crinita. Cluster analysis using morphological and quantitative characters revealed 4 distinct groups of varieties, clearly separated according to their biomass yield. The variables foliage height and basal diameter were strongly correlated with biomass production and these phenotypic markers can be used to select productive plants. The relations among the varieties based on AFLP data were significantly correlated with those based on agronomic and morphological characters, suggesting that the 2 systems give similar estimates of genetic relations among the varieties.

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