The Mre11/Rad50/Nbs1 complex is involved in many aspects of chromosome metabolism. Aberrant function of the complex is associated with defects in the DNA checkpoint, double-strand break repair, meiosis, and telomere maintenance. In this article, we report the consequences of Mre11 dysfunction for the stability of mitotic and meiotic chromosomes in Arabidopsis thaliana. Although plants homozygous for a T-DNA insertion in a conserved region of the MRE11 gene are viable, they exhibit growth defects and are infertile. Analysis of mitotic chromosomes prepared from the mutant plants revealed abundant dicentric chromosomes and chromosomal fragments. Fluorescence in situ hybridization showed that anaphase bridges are often formed by homologous chromosome arms. The frequency of chromosome fusions was not reduced in mre11 ku70 double mutants, suggesting that plants possess DNA end-joining activities independent of the Ku70/80 and Mre11 complexes. Cytogenetic examination of pollen mother cells revealed massive chromosome fragmentation and the absence of synapsis in the initial stages of meiosis. The fragmentation was substantially suppressed in mre11 spo11-1 double mutants, indicating that Mre11 is required for repair but not for the induction of Spo11-dependent meiotic DNA breaks in Arabidopsis

that is characterized by delayed chromosome decondensation and aberrant rearrangement of the meiotic spindle. The smg7 phenotype was mimicked by exposing meiocytes to the proteasome inhibitor MG115. Together, these data indicate that SMG7 counteracts cyclin-dependent kinase (CDK) activity at the end of meiosis, and reveal a novel link between SMG7 and regulation of the meiotic cell cycle. Supplementary material available online at

Most flowering plant species are hermaphroditic, but a small number of species in most plant families are unisexual (i.e., an individual will produce only male or female gametes). Because species with unisexual flowers have evolved repeatedly from hermaphroditic progenitors, the mechanisms controlling sex determination in flowering plants are extremely diverse. Sex is most strongly determined by genotype in all species but the mechanisms range from a single controlling lacus to sex chromosomes bearing several linked loci required for sex determination. Plant hormones also influence sex expression with variable effects from species to species. Here, we review the genetic control of sex determination from a number of plant species to illustrate the variety of extant mechanisms. We emphasize species that are now used as models to investigate the molecular biology of sex determination. We also present our own investigations of the structure of plant sex chromosomes of white campion (Silene latifolia = Melandrjum album). The cytogenetic basis of sex determination in white campion is similar to mammals in that it has a male-specific Y-chromosome that carries dominant male determining genes. If one copy of this chromosome is in the genome, the plant is male. Otherwise it is female. Like mammalian Y-chromosomes, the white campion Y-chromosome is rich in repetitive DNA. We isolated repetitive sequences from microdissected Y-chromosomes of white campion to study the distribution of homologous repeated sequences on the Y-chromosome and the other chromosomes. We found the Y to be especially rich in repetitive sequences that were generally dispersed over all the white campion chromosomes. Despite its repetitive character, the Y-chromosome is mainly euchromatic. This may be due to the relatively recent evolution of the white campion sex chromosomes compared to the sex chromosomes of animals.

Telomere lengths and telomerase activity were studied during the development of a model dioecious plant, Melandrium album (syn Silene latifolia ). Telomeric DNA consisted of Arabidopsis-type TTTAGGG tandem repeats. The terminal positions of these repeats were confirmed by both Bal31 exonuclease degradation and in situ hybridization. Analysis of terminal restriction fragments in different tissues and ontogenetic stages showed that telomere lengths are stabilized precisely and do not change during plant growth and development. Telomerase activity tested by using a semiquantitative telomerase repeat amplification protocol correlated with cell proliferation in the tissues analyzed. Highest activity was found in germinating seedlings and root tips, whereas we observed a 100-fold decrease in telomerase activity in leaves and no activity in quiescent seeds. Telomerase also was found in mature pollen grains. Telomerase activity in tissues containing dividing cells and telomere length stability during development suggest their precise control during plant ontogenesis; however, the telomere length regulation mechanism could be unbalanced during in vitro dedifferentiation.

BackgroundIn flowering plants and animals the most common ribosomal RNA genes (rDNA) organisation is that in which 35S (encoding 18S-5.8S-26S rRNA) and 5S genes are physically separated occupying different chromosomal loci. However, recent observations established that both genes have been unified to a single 35S-5S unit in the genus Artemisia (Asteraceae), a genomic arrangement typical of primitive eukaryotes such as yeast, among others. Here we aim to reveal the origin, distribution and mechanisms leading to the linked organisation of rDNA in the Asteraceae by analysing unit structure (PCR, Southern blot, sequencing), gene copy number (quantitative PCR) and chromosomal position (FISH) of 5S and 35S rRNA genes in ~200 species representing the family diversity and other closely related groups.ResultsDominant linked rDNA genotype was found within three large groups in subfamily Asteroideae: tribe Anthemideae (93% of the studied cases), tribe Gnaphalieae (100%) and in the "Heliantheae alliance" (23%). The remaining five tribes of the Asteroideae displayed canonical non linked arrangement of rDNA, as did the other groups in the Asteraceae. Nevertheless, low copy linked genes were identified among several species that amplified unlinked units. The conserved position of functional 5S insertions downstream from the 26S gene suggests a unique, perhaps retrotransposon-mediated integration event at the base of subfamily Asteroideae. Further evolution likely involved divergence of 26S-5S intergenic spacers, amplification and homogenisation of units across the chromosomes and concomitant elimination of unlinked arrays. However, the opposite trend, from linked towards unlinked arrangement was also surmised in few species indicating possible reversibility of these processes.ConclusionsOur results indicate that nearly 25% of Asteraceae species may have evolved unusual linked arrangement of rRNA genes. Thus, in plants, fundamental changes in intrinsic structure of rDNA units, their copy number and chromosomal organisation may occur within relatively short evolutionary time. We hypothesize that the 5S gene integration within the 35S unit might have repeatedly occurred during plant evolution, and probably once in Asteraceae.

The plant genus Silene has become a model for evolutionary studies of sex chromosomes and sex-determining mechanisms. A recent study performed in Silene colpophylla showed that dioecy and the sex chromosomes in this species evolved independently from those in Silene latifolia, the most widely studied dioecious Silene species. The results of this study show that the sex-determining system in Silene otites, a species related to S. colpophylla, is based on female heterogamety, a sex determination system that is unique among the Silene species studied to date. Our phylogenetic data support the placing of S. otites and S. colpophylla in the subsection Otites and the analysis of ancestral states suggests that the most recent common ancestor of S. otites and S. colpophylla was most probably dioecious. These observations imply that a switch from XX/XY sex determination to a ZZ/ZW system (or vice versa) occurred in the subsection Otites. This is the first report of two different types of heterogamety within one plant genus of this mostly nondioecious plant family. K E Y W O R D S :Evolution, sex chromosomes, sex determination, XY, ZW.

Melandrium album (syn. Silene latifolia) is a model dioecious species in which the Y chromosome, present only in heterogametic males, plays both a male-determining and a strict female-suppressing role. We showed that treatment with 5-azacytidine (5-azaC) induces a sex change to androhermaphroditism (an-dromonoecy) in about 21% of male plants, while no apparent phenotypic effect was observed in females. All of these bisexual androhermaphrodites (with the standard male 24, AA + XY karyotype) were mosaics possessing both male and hermaphrodite flowers and, moreover, the hermaphrodite flowers displayed various degrees of gynoecium development and seed setting. Southern hybridization analysis with a repetitive DNA probe showed that the 5-azacytidine-treated plants were significantly hypomethylated in CG doubles, but only to a minor degree in CNG triplets. The bisexual trait was transmitted to two successive generations, but only when androhermaphrodite plants were used as pollen donors. The sex reversal was inherited with incomplete penetrance and varying expressivity. Based on the uniparental inheritance pattern of androhermaphroditism we conclude that it originated either by 5-azaC induced inhibition of Y-linked female-suppressing genes or by a heritable activation of autosomal female-determining/promoting genes which can be reversed, on passage through female meiosis, by a genomic imprinting mechanism. The data presented indicate that female sex suppression in M. album XY males is dependent on methylation of specific DNA sequences and can be heritably modified by hypomethylating drugs.

The ends of eukaryotic chromosomes are capped with special nucleoprotein structures called telomeres. Telomere shortening due to telomerase inactivation may result in fusion of homologous or heterologous chromosomes, leading to their successive breakage during anaphase movement, followed by fusion of broken ends in the next cell cycle, i.e. the breakage-fusion-bridge (BFB) cycle. Using fluorescence in situ hybridization (FISH) with 25S rDNA and specific bacterial artificial chromosome (BAC) probes we demonstrate participation of chromosomes 2 and 4 of Arabidopsis thaliana AtTERT null plants in the formation of anaphase bridges. Both homologous and non-homologous chromosomes formed transient anaphase bridges whose breakage and unequal separation led to genome rearrangement, including non-reciprocal translocations and aneuploidy. The 45S rDNA regions located at the ends of chromosomes 2 and 4 were observed in chromosome bridges at a frequency approximately ten times higher than expected in the case of random fusion events. This outcome could result from a functional association of rDNA repeats at nucleoli. We also describe increased variation in the number of nucleoli in some interphase cells with supernumerary rDNA FISH signals. These data indicate that dysfunctional telomeres in Arabidopsis lead to massive genome instability, which is induced by multiple rounds of the BFB mechanism.