Complete uniparental chromosome elimination occurs in several interspecific hybrids of plants. We studied the mechanisms underlying selective elimination of the paternal chromosomes during the development of wheat (Triticum aestivum) 3 pearl millet (Pennisetum glaucum) hybrid embryos. All pearl millet chromosomes were eliminated in a random sequence between 6 and 23 d after pollination. Parental genomes were spatially separated within the hybrid nucleus, and pearl millet chromatin destined for elimination occupied peripheral interphase positions. Structural reorganization of the paternal chromosomes occurred, and mitotic behavior differed between the parental chromosomes. We provide evidence for a novel chromosome elimination pathway that involves the formation of nuclear extrusions during interphase in addition to postmitotically formed micronuclei. The chromatin structure of nuclei and micronuclei is different, and heterochromatinization and DNA fragmentation of micronucleated pearl millet chromatin is the final step during haploidization.

SummaryStrong methylation of lysine 4 (K4) and low methylation of lysine 9 (K9) have been proposed as modi®ca-tions of histone H3, typical for transcriptionally active euchromatin and the opposite for inactive heterochromatin. We have analysed the correlation between the global distribution of histone H3, methylated at either lysine 4 or lysine 9, and of microscopically detectable euchromatic or heterochromatic regions in relation to genome size for 24 plant species. Two different distribution patterns of methylated (K9)H3 (Met(K9)H3) were found that depend on genome size. For most species with small genomes (1C <500 Mbp), including Arabidopsis thaliana, strong methylation of (K9)H3 was restricted to constitutive heterochromatin. Species with larger genomes showed a uniform distribution of Met(K9)H3. Contrary to this and regardless of the genome size, methylated (K4)H3 (Met(K4)H3) was found to be enriched within the euchromatin of all species. Transcriptionally less active B chromosomes showed the same patterns as basic A chromosomes. We thus propose that large genomes with high amounts of dispersed repetitive sequences (mainly retroelements) have to silence these sequences and therefore display epigenetic modi®cations such as methylation of DNA and (K9)H3 also within euchromatic regions.

Summary N-terminal modifications of nucleosomal core histones are involved in gene regulation, DNA repair and recombination as well as in chromatin modeling. The degree of individual histone modifications may vary between specific chromatin domains and throughout the cell cycle. We have studied the nuclear patterns of histone H3 and H4 acetylation and of H3 methylation in Arabidopsis. A replication-linked increase of acetylation only occurred at H4 lysine 16 (not for lysines 5 and 12) and at H3 lysine 18. The last was not observed in other plants. Strong methylation at H3 lysine 4 was restricted to euchromatin, while strong methylation at H3 lysine 9 occurred preferentially in heterochromatic chromocenters of Arabidopsis nuclei. Chromocenter appearance, DNA methylation and histone modification patterns were similar in nuclei of wild-type and kryptonite mutant (which lacks H3 lysine 9-specific histone methyltransferase), except that methylation at H3 lysine 9 in heterochromatic chromocenters was reduced to the same low level as in euchromatin. Thus, a high level of H3methylK9 is apparently not necessary to maintain chromocenter structure and does not prevent methylation of H3 lysine 4 within Arabidopsis chromocenters.

Immunolabeling using site-specific antibodies against phosphorylated histone H3 at serine 10 or serine 28 revealed in plants an almost similar temporal and spatial pattern of both post-translational modification sites at mitosis and meiosis. During the first meiotic division the entire chromosomes are highly H3 phosphorylated. In the second meiotic division, like in mitosis, the chromosomes contain high phosphorylation levels in the pericentromeric region and very little H3 phosphorylation along the arms of monocentric species. In the polycentric plant Luzula luzuloides phosphorylation at both serine positions occurs along the whole chromosomes, whereas in monocentric species, only the pericentromeric regions showed strong signals from mitotic prophase to telophase. No phosphorylated serine 10 or serine 28 was detectable on single chromatids at anaphase II resulting from equational segregation of rye B chromosome univalents during the preceding anaphase I. In addition, we found a high level of serine 28 as well as of serine 10 phosphorylation along the entire mitotic monocentric chromosomes after treatment of mitotic cells using the phosphatase inhibitor cantharidin. These observations suggest that histone H3 phosphorylation at serine 10 and 28 is an evolutionarily conserved event and both sites are likely to be involved in the same process, such as sister chromatid cohesion.

Analysis of the grass Zingeria trichopoda (2n = 8, 2C = 5.3 pg) revealed a dynamic evolution with the following characteristics. (i) Genomic in situ hybridization (GISH) demonstrates that Z. trichopoda evolved from an interspecific hybrid involving a species like contemporary Zingeria biebersteiniana (2n = 4) and a second species with a similar low number of chromosomes. The nucleus of Z. trichopoda is spatially organized at the genome level and the two parental genomes occupy distinct and separate domains of lateral arrangements. (ii) The copy number of the Z. biebersteiniana specific pericentromeric tandem repeat family Zbcen1 is drastically reduced in Z. trichopoda. (iii) GISH in combination with labeled rDNA sequences simultaneously discriminated the two parental genomes and the corresponding 5S and 45S rDNA sites. Hence, following allopolyploidization of Z. trichopoda the Z. biebersteiniana like parental chromosomes probably underwent drastic loss of 45S rDNA. This could have arisen either through the loss of Z. biebersteiniana derived 45S rDNA or through Z. trichopoda genome-wide homogenization of Z. biebersteiniana type 45S rDNA and subsequent elimination of 45S rDNA loci from Z. biebersteiniana derived chromosomes. Finally, 5S rDNA loci are present in both subgenomes of Z. trichopoda and the chromosomal position of these loci is similar for both Z. biebersteiniana and the Z. biebersteiniana like parental genome of Z. trichopoda.