DNA replication dynamics of vole genome and its epigenetic regulation

Background

The genome of some vole rodents exhibit large blocks of heterochromatin coupled to their sex chromosomes. The DNA composition and transcriptional activity of these heterochromatin blocks have been studied, but little is known about their DNA replication dynamics and epigenetic composition.

Results

Here, we show prominent epigenetic marks of the heterochromatic blocks in the giant sex chromosomes of female Microtus cabrerae cells. While the X chromosomes are hypoacetylated and cytosine hypomethylated, they are either enriched for macroH2A and H3K27me3 typical for facultative heterochromatin or for H3K9me3 and HP1 beta typical for constitutive heterochromatin. Using pulse-chase replication labeling and time-lapse microscopy, we found that the heterochromatic block enriched for macroH2A/H3K27me3 of the X chromosome is replicated during mid-S-phase, prior to the heterochromatic block enriched for H3K9me3/HP1 beta, which is replicated during late S-phase. To test whether histone acetylation level regulates its replication dynamics, we induced either global hyperacetylation by pharmacological inhibition or by targeting a histone acetyltransferase to the heterochromatic region of the X chromosomes. Our data reveal that histone acetylation level affects DNA replication dynamics of the sex chromosomes’ heterochromatin and leads to a global reduction in replication fork rate genome wide.

Conclusions

In conclusion, we mapped major epigenetic modifications controlling the structure of the sex chromosome-associated heterochromatin and demonstrated the occurrence of differences in the molecular mechanisms controlling the replication timing of the heterochromatic blocks at the sex chromosomes in female Microtus cabrerae cells. Furthermore, we highlighted a conserved role of histone acetylation level on replication dynamics across mammalian species.

Electronic supplementary material

The online version of this article (10.1186/s13072-019-0262-0) contains supplementary material, which is available to authorized users.

Extensive Sex Chromosome Polymorphism of Microtus thomasi/Microtus atticus Species Complex Associated with Cryptic Chromosomal Rearrangements and Independent Accumulation of Heterochromatin

The sibling species Microtus thomasi and M. atticus represent probably the highest karyotypic diversity within the genus Microtus and are an interesting model for chromosomal evolution studies. In addition to variation in autosomes, they show a high intraspecific variation in the size and morphology of both sex chromosomes. We analyzed individuals with different sex chromosome constitutions using 3 painting probes, 2 from Y chromosome variants and 1 from the small arm of the submetacentric X chromosome. Our comparative painting approach uncovered 12 variants of Y and 14 variants of X chromosomes, which demonstrates that the polymorphism of sex chromosomes is substantially larger than previously reported. We suggest that 2 main processes are responsible for this sex chromosome polymorphism: change of morphology from acrocentric to submetacentric or metacentric chromosomes and increase in size due to accumulation of repetitive DNA sequences, generating heterochromatic blocks. Strong genetic drift in small and fragmented populations of these 2 species could be related to the origin and maintenance of the large polymorphism of sex chromosomes. We proposed that a similar polymorphism variation combined with random drift fixing the biggest sex chromosomes could have occurred in the origin of some of the actual Microtus species with giant sex chromosomes.

Molecular characterization of bot flies, Oestrus spp., (Diptera, Oestridae), from domestic and wild Bovidae hosts

The identification of Oestrus spp. larvae from Bovidae hosts is a difficult task due to the great morphological similarity between species. The lack of unambiguous identification criteria could have also serious epidemiological implications since domestic and wild hosts are sympatric in many natural areas. In order to accurately identify the Oestrus parasitizing hosts, we characterized two different genetic markers, 28S (rRNA) and COI, in larvae collected from domestic sheep and goats, European mouflon and Iberian ibex. Our sequence analyses demonstrate that all samples, except those from Iberian ibex, greatly resembles O. ovis and so we conclude that the species parasitizing this ibex is not O. ovis. Further studies will be needed to confirm whether it is in fact O. caucasicus, as previously suggested, or even a new species.

Epigenetic modifications in sex heterochromatin of vole rodents

The genome of some vole rodents contains large blocks of heterochromatin coupled to the sex chromosomes. While the DNA content of these heterochromatic blocks has been extensively analyzed, little is known about the epigenetic modifications controlling their structure and dynamics. To better understand its organization and functions within the nucleus, we have compared the distribution pattern of several epigenetic marks in cells from two species, Microtus agrestis and Microtus cabrerae. We first could show that the heterochromatic blocks are identifiable within the nuclei due to their AT enrichment detectable by DAPI staining. By immunostaining analyses, we demonstrated that enrichment in H3K9me3 and HP1, depletion of DNA methylation as well as H4K8ac and H3K4me2, are major conserved epigenetic features of this heterochromatin in both sex chromosomes. Furthermore, we provide evidence of transcriptional activity for some repeated DNAs in cultivated cells. These transcripts are partially polyadenylated and their levels are not altered during mitotic arrest. In summary, we show here that enrichment in H3K9me3 and HP1, DNA demethylation, and transcriptional activity are major epigenetic features of sex heterochromatin in vole rodents.

Molecular and physical characterization of the complex pericentromeric heterochromatin of the vole species Microtus thomasi

A new repeated DNA from Microtus thomasi, Mth-Alu2.2, was cloned and characterized and is presented here for the first time. Digestion of genomic DNA from M. thomasi with AluI restriction enzyme revealed a 2.2-kb repetitive DNA sequence with a high AT content (69%). This sequence consists of a tandemly repeated nonanucleotide of the consensus sequence CACAATGTA, which constitutes approximately 93-95% of the total unit length. The location of the Mth-Alu2.2 sequence in the karyotype was determined by FISH, demonstrating strong hybridization signals in the pericentromeric regions of all chromosomes and in the heterochromatin blocks of several X chromosome variants. In addition, the distribution of the 4 pericentromeric repeat sequences Msat-160, Mth-Alu900, Mth-Alu2.2, and interstitial telomeric repeats was analyzed by in situ hybridization in M. thomasi, in order to shed light on the complex composition of the chromosomal pericentromeric regions in this species. The order and organization of these sequences in the pericentromeric regions are conserved, with slight variations in both the degree of overlapping and the amount of each repeated DNA in the chromosomes. Specifically, Mth-Alu2.2 is localized in the terminal regions of the chromosomes, with Msat-160 occupying the immediately inner region, partially intermixed with Mth-Alu2.2. The sequence Mth-Alu900 is found in internal positions below Msat-160, and the interstitial telomeric repeats are located close to the long-arm euchromatin of the chromosomes.

Rapid, independent, and extensive amplification of telomeric repeats in pericentromeric regions in karyotypes of arvicoline rodents

The distribution of telomeric repeats was analyzed by fluorescence in situ hybridization in 15 species of arvicoline rodents, included in three different genera: Chionomys, Arvicola, and Microtus. The results demonstrated that in most or the analyzed species, telomeric sequences are present, in addition to normal telomeres localization, as large blocks in pericentromeric regions. The number, localization, and degree of amplification of telomeric sequences blocks varied with the karyotype and the morphology of the chromosomes. Also, in some cases telomeric amplification at non-pericentromeric regions is described. The interstitial telomeric sequences are evolutionary modern and have rapidly colonized and spread in pericentromeric regions of chromosomes by different mechanisms and probably independently in each species. Additionally, we colocalized telomeric repeats and the satellite DNA Msat-160 (also located in pericentromeric regions) in three species and cloned telomeric repeats in one of them. Finally, we discuss about the possible origin and implication of telomeric repeats in the high rate of karyotypic evolution reported for this rodent group.

Comparative analysis by chromosome painting of the sex chromosomes in arvicolid rodents

Sex chromosome evolution in mammals has been extensively investigated through chromosome-painting analyses. In some rodent species from the subfamily Arvicolinae the sex chromosomes contain remarkable features such as giant size, a consequence of heterochromatic enlargement, or asynaptic behaviour during male meiosis. Here, we have made a comparative study of the sex chromosomes in 6 arvicolid species using different probes from the X and Y chromosomes of 3 species, in order to gain knowledge about intra- or interspecific preservation of euchromatic regions. Our results clearly reveal poor conservation of the euchromatic region of the Y chromosome within these species, while the euchromatin on the X chromosome is extremely well preserved. Furthermore, we detected no clear correlation between the synaptic/asynaptic behaviour of the sex chromosomes, and the presence or absence of sequence homology within their euchromatic regions. Notably, our study has shown a new relationship between the giant sex chromosomes of 2 species, Microtus agrestis and Microtus cabrerae, that is, both X and Y share a novel region of common sequences in the euchromatin that is not present in the other species analysed. This interspecific euchromatic conservation, limited to the giant sex chromosomes, could point towards a common evolutionary origin for the heterochromatic enlargement process that has characterized the evolution of the sex chromosomes in some arvicolid species.

No differences in the Sry gene between males and XY females in Akodon (Rodentia, Cricetidae)

Several species of the South American genus Akodon present fully fertile XY females besides XX ones. To analyze the possibility of a Sry mutation as the cause of sex reversal in A. azarae and A. boliviensis, we determined the sequence of the Sry gene in 2 males and 3 XY females from each of these species. The Sry gene sequence was also studied in A. dolores, a species that does not have XY females. In inter-specific comparisons, the percentage identities with respect to the region analyzed varied between 96.8% and 97.9%. An ORF of 543 nucleotides was identified, and the predicted Sry proteins comprised 180 amino acids, with an HMG domain of 83 amino acids. Our results indicate that female sex reversal in A. azarae and A. boliviensis cannot be explained by sequence differences in the Sry region analyzed here, which includes the complete ORF and, together with previous results concerning the inheritance of the XY condition, show that Sry mutation is not the basis of sex reversal.