Chromosomal localization of 18S rDNA and telomere sequence in the aye-aye, Daubentonia madagascariensis

Telomeres, species differences, and unusual telomeres in vertebrates: presenting challenges and opportunities to understanding telomere dynamics

There has been increasing interest in the use of telomeres as biomarkers of stress, cellular ageing and life-histories. However, the telomere landscape is a diverse feature, with noticeable differences between species, a fact which is highlighted by the unusual telomeres of various vertebrate organisms. We broadly review differences in telomere dynamics among vertebrates, and emphasize the need to understand more about telomere processes and trends across species. As part of these species differences, we review unusual telomeres in vertebrates. This includes mega-telomeres, which are present across a diverse set of organisms, but also focusing on the unusual telomeres traits of marsupials and monotremes, which have seen little to no prior discussion, yet uniquely stand out from other unusual telomere features discovered thus far. Due to the presence of at least two unique telomere features in the marsupial family Dasyuridae, as well as to the presence of physiological strategies semelparity and torpor, which have implications for telomere life-histories in these species, we suggest that this family has a very large potential to uncover novel information on telomere evolution and dynamics.

Genomic analyses of the Formosan harvest mouse (Micromys minutus) and comparisons to the brown Norway rat (Rattus norvegicus) and the house mouse (Mus musculus)

The harvest mouse, Micromys minutus (MMIN), has a very wide range of distribution (from the British Isles across the Euroasian continent to Japan and Taiwan). We studied an isolated population of MMIN in Taiwan, which is at the southeastern margin of the species' geographic distribution, and compared its genetic complement with those of the same species previously reported from other geographic locations and with two model rodent species, the house mouse (Mus musculus) and the brown Norway rat (Rattus norvegicus). The diploid number (2N) of MMIN was 68, consistent with that reported for other populations. However, variations were noted in the fundamental number (FN) and the shape and banding patterns of the individual chromosomes among populations. The FN of MMIN was estimated to be 72, including 2 bi-armed autosomes, 31 one-armed autosomes, and one pair of one-armed sex chromosomes. Here, we propose the first ideogram for MMIN. C-banding, Ag-NOR, and the locations of 18S rRNA gene sequences (MMIN chromosomes no. 10, 14, 19, 29, 31, 33, and X) mapped by fluorescence in situ hybridization (FISH) are also reported. Additionally, we compared the 18S rDNA sequences and performed cross-species X chromosome painting (FISH) for M. minutus, M. musculus, and R. norvegicus. The results indicate that both genetic elements are rather conserved across species. Thus, implications for the phylogenetic position of Micromys were limited.

Telomere and 45S rDNA sequences are structurally linked on the chromosomes in Chrysanthemum segetum L

Some reports have shown that nucleolar organizer regions are located at the telomeric region and have a structural connection with telomeres at the cellular level in many organisms. In this study, we found that all 45S ribosomal DNA (rDNA) signals were located at telomeric regions on the chromosomes in Chrysanthemum segetum L., and the 45S rDNA showed distinct signal patterns on different metaphase chromosome spreads. The bicolor fluorescence in situ hybridization experiment on the extended fibers revealed that telomere repeats were structurally connected with or interspersed into rDNA sequences. The close cytological structure relation between rDNA and telomere sequences led us to use PCR with combinations of the telomere primer and the rDNA primer to obtain some fragments, which were flanked by different rDNA and telomere primer sequences. One representative clone CHS2 contains closely connected rDNA and telomere sequences, suggesting that the telomere sequence invaded into the conserved rDNA sequence. In addition, the sequences of some PCR clones were flanked by the single telomeric primer sequence or the rDNA primer sequence. These results suggested that homologous recombination occurred between tandem repeat units of rDNA sequences or telomere repeats at the chromosome terminus.

The very long telomeres in Sorex granarius (Soricidae, Eulipothyphla) contain ribosomal DNA

Two closely related shrew species, Sorex granarius and Sorex araneus, in which Robertsonian rearrangements have played a primary role in karyotype evolution, present very distinct telomere length patterns. S. granarius displays hyperlong telomeres specifically associated with the short arms of acrocentrics, whereas telomere lengths in S. araneus are rather short and homogenous. Using a combined approach of chromosome and fibre FISH, modified Q-FISH, 3D-FISH, Ag-NOR staining and TRF analysis, we carried out a comparative analysis of telomeric repeats and rDNA distribution on chromosome ends of Sorex granarius. Our results show that rDNA sequences forming active nuclear organizing regions are interspersed with the long telomere tracts of all short arms of acrocentrics. These observations suggest that the major rearrangements that gave rise to today's karyotype in S. granarius were accompanied by a profound reorganization of chromosome ends, which comprised extensive amplification of telomeric and rDNA repeats on the short arms of acrocentrics and finally contributed to the stabilization of telomeres. This is the first time that such telomeric structures have been observed in any mammalian species.

Characterization and evolution of major histocompatibility complex class II genes in the aye-aye, Daubentonia madagascariensis

Major histocompatibility complex genes (Mhc-DQB and Mhc-DRB) were sequenced in seven aye-ayes (Daubentonia madagascariecsis), which is an endemic and endangered species in Madagascar. An aye-aye from a north-eastern population showed genetic relatedness to individuals of a north-western population and had a somewhat different repertoire from another north-eastern individual. These observations suggest that the extent of genetic variation in Mhc genes is not excessively small in the aye-aye in spite of recent rapid destruction of their habitat by human activities. In light of Mhc gene evolution, trans-species and allelic polymorphisms can be estimated to have been retained for more than 50 Ma (million years) based on the time scale of lemur evolution.