Cytological and functional aspects of telomere maintenance

Chromosomal distribution of interstitial telomeric sequences as signs of evolution through chromosome fusion in six species of the giant water bugs (Hemiptera, Belostoma)

Tandem arrays of TTAGG repeats show a highly conserved location at the telomeres across the phylogenetic tree of arthropods. In giant water bugs Belostoma, the chromosome number changed during speciation by fragmentation of the single ancestral X chromosome, resulting in a multiple sex chromosome system. Several autosome–autosome fusions and a fusion between the sex chromosome pair and an autosome pair resulted in the reduced number in several species. We mapped the distribution of telomeric sequences and interstitial telomeric sequences (ITSs) in Belostoma candidulum (2n = 12 + XY/XX; male/female), B. dentatum (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. elegans (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. elongatum (2n = 26 + X₁X₂Y/X₁X₁X₂X₂), B. micantulum (2n = 14 + XY/XX), and B. oxyurum (2n = 6 + XY/XX) by FISH with the (TTAGG)_n probes. Hybridization signals confirmed the presence of TTAGG repeats in the telomeres of all species examined. The three species with reduced chromosome numbers showed additional hybridization signals in interstitial positions, indicating the occurrence of ITS. From the comparison of all species here analyzed, we observed inverse relationships between chromosome number and chromosome size, and between presence/absence of ITS and chromosome number. The ITS distribution between these closely related species supports the hypothesis that several telomere–telomere fusions of the chromosomes from an ancestral diploid chromosome number 2n = 26 + XY/XX played a major role in the karyotype evolution of Belostoma. Consequently, our study provide valuable features that can be used to understand the karyotype evolution, may contribute to a better understanding of taxonomic relationships, and also elucidate the high plasticity of nuclear genomes at the chromosomal level during the speciation processes.

Association of Two Polymorphisms in H2B.W Gene with Azoospermia and Severe Oligozoospermia in An Iranian Population

BACKGROUND

During spermatogenesis, the H2B family, member W (H2B.W) gene, en- codes a testis specific histone that is co-localized with telomeric sequences and has the potential role to mediate the sperm-specific chromatin remodeling. Previously H2B.W genetic variants were reported to be involved in susceptibility to spermatogenesis im- pairment. In the present study, two single nucleotide polymorphisms (SNPs) in 5΄UTR and exon 1 of H2B.W gene were examined to investigate possible association of these polymorphisms with male infertility in Iranian population.

MATERIALS AND METHODS

This case control study was conducted in Royan institute during four-year period (2010-2013). Genetic alteration of two SNPs loci, -9C>T and 368A>G, in H2B.W gene were indicated in 92 infertile men who were divided into two main groups includ- ing azoospermia (n=46) and sever oligozoospermia (n=46), while there was 60 fertile men as control group. Azoosperima was also divided into three sub-groups including sertoli cell only syndrome (SCOS, n=21), complete maturation arrest (CMA, n=17) and hypo spermatogenesis (n=8) according to testicular biopsy. For analysis, polymerase chain reaction-restriction frag- ment length polymorphism (PCR-RFLP) technique was applied.

RESULTS

The frequency of allele -9T was significantly higher in CMA group than in patients with SCOS (P<0.05). The haplotype TA (corresponding to simultaneous occur- rence of -9T and 368A) compared with haplotype CA (corresponding to simultaneous occurrence of -9C and 368A) in patients suffering from CMA significantly increased, compared with patients had SCOS (P<0.05). However, statistical studies indicated that in general, the distribution frequencies of -9C>T and 368A>G had no significant difference between the infertile groups and control (P=0.859 and P=0.812, respectively).

CONCLUSION

This investigation showed that SNP -9C>T might be contribute to CMA in azoo- spermic patients and SNP 368A>G had no correlation with male infertility in Iranian population.

Telomeric and interstitial telomeric sequences in holokinetic chromosomes of Lepidoptera: telomeric DNA mediates association between postpachytene bivalents in achiasmatic meiosis of females

Telomeres, besides their main role in the protection and maintenance of chromosome ends, have several other vital functions in the cell cycle. We studied their role in the achiasmatic meiosis of female Lepidoptera, insects with holokinetic chromosomes. By fluorescence in-situ hybridization (FISH) with the insect telomeric probe, (TTAGG)n, we mapped the distribution of telomeric and interstitial telomeric sequences (ITS) in female meiotic chromosomes of two species, Orgyia antiqua with a reduced chromosome number (2n = 28) and Ephestia kuehniella mutants, possessing a radiation-induced chromosome fusion in the genome (2n = 59). In addition to the strong typical telomeric signals, O. antiqua displayed weaker hybridization signals in interstitial sites of pachytene bivalents. The observed ITS most probably reflect remnants of chromosomal rearrangements and support the hypothesis that the Orgyia karyotype had arisen by multiple fusions of ancestral chromosomes. On the other hand, the absence of ITS in the chromosome fusion of Ephestia indicated the loss of telomeres before the two original chromosomes fused. When the telomeric probe was amplified by enzymatic reaction with tyramid, the number of ITS observed increased in Orgyia, and a few ITS were also observed in several chromosomes of Ephestia but not in the fused chromosome. This suggests that the genomes of both species also contain ITS other than those originating from chromosome fusions. The analysis of female meiotic prophase I revealed non-homologous associations of postpachytene bivalents mediated by telomeric DNA, which were not observed in the pachytene stage. Surprisingly, in early postpachytene nuclei the telomeric associations also involved ITS, whereas later postpachytene nuclei displayed chains of bivalents interconnected only by true telomeres. This finding favours a hypothesis that telomeric associations between bivalents play a role in chromosome segregation in the achiasmatic meiosis of female Lepidoptera.

Aloe spp.--plants with vertebrate-like telomeric sequences

Chromosome termini of most eukaryotes end in tracks of short tandemly repeated GC-rich sequences, the composition of which varies among different groups of organisms. Plant species predominantly contain (TTTAGGG)n repeats at their telomeres. However, a few plant species, including members of Alliaceae and Aloe spp. (Asphodelaceae) were found to lack such Arabidopsis-type (T3AG3)n telomeric repeats. Recently, it has been proposed that the lack of T3AG3 telomeric repeat sequences extends to all species forming the Asparagales clade. Here, we analysed the composition of Aloe telomeres by single-primer PCR and fluorescence in-situ hybridization (FISH) with directly labelled Arabidopsis-type (TTTAGGG)28-43 DNA probe, and with vertebrate-type (TTAGGG)33-50 DNA and a (C3TA2)3 peptide nucleic acid (PNA) probe. It was found that Nicotiana tabacum contained Arabidopsis-type telomeric repeats, while Aloe telomeres lacked the corresponding FISH signals. Surprisingly, FISH with the highly specific vertebrate-type (C3TA2)3 PNA probe resulted in strong T2AG3-specific FISH signals at the ends of chromosomes of both Aloe and Nicotiana tabacum, suggesting the presence of T2AG3 telomeric repeats in these species. FISH with a long (TTAGGG)33-50 DNA probe also highlighted Aloe chromosome ends, while this probe failed to reveal FISH signals on tobacco chromosomes. These results indicate the presence of vertebrate-like telomeric sequences at the telomeres of Aloe spp. chromosomes. However, single-primer PCR with (TAG3)5 primers failed to amplify such sequences in Aloe, which could indicate a low copy number of T2AG3 repeats at the chromosome ends and/or their co-orientation and interspersion with other repeat types. Our results suggest that telomeres of plant species, which were thought to lack GC-rich repeats, may in fact contain variant repeat types.

Human sperm telomere-binding complex involves histone H2B and secures telomere membrane attachment

Telomeres are unique chromatin domains located at the ends of eukaryotic chromosomes. Telomere functions in somatic cells involve complexes between telomere proteins and TTAGGG DNA repeats. During the differentiation of germ-line cells, telomeres undergo significant reorganization most likely required for additional specific functions in meiosis and fertilization. A telomere-binding protein complex from human sperm (hSTBP) has been isolated by detergent treatment and was partially purified. hSTBP specifically binds double-stranded telomeric DNA and does not contain known somatic telomere proteins TRF1, TRF2, and Ku. Surprisingly, the essential component of this complex has been identified as a specific variant of histone H2B. Indirect immunofluorescence shows punctate localization of H2B in sperm nuclei, which in part coincides with telomeric DNA localization established by fluorescent in situ hybridization. Anti-H2B antibodies block interactions of hSTBP with telomere DNA, and spH2B forms specific complex with this DNA in vitro, indicating that this protein plays a role in telomere DNA recognition. We propose that hSTBP participates in the membrane attachment of telomeres that may be important for ordered chromosome withdrawal after fertilization.

Mammalian meiotic telomeres: protein composition and redistribution in relation to nuclear pores

Mammalian telomeres consist of TTAGGG repeats, telomeric repeat binding factor (TRF), and other proteins, resulting in a protective structure at chromosome ends. Although structure and function of the somatic telomeric complex has been elucidated in some detail, the protein composition of mammalian meiotic telomeres is undetermined. Here we show, by indirect immunofluorescence (IF), that the meiotic telomere complex is similar to its somatic counterpart and contains significant amounts of TRF1, TRF2, and hRap1, while tankyrase, a poly-(ADP-ribose)polymerase at somatic telomeres and nuclear pores, forms small signals at ends of human meiotic chromosome cores. Analysis of rodent spermatocytes reveals Trf1 at mouse, TRF2 at rat, and mammalian Rap1 at meiotic telomeres of both rodents. Moreover, we demonstrate that telomere repositioning during meiotic prophase occurs in sectors of the nuclear envelope that are distinct from nuclear pore-dense areas. The latter form during preleptotene/leptotene and are present during entire prophase I.