Homogenisation of telomere length in a transmissible cancer, devil facial tumour 2

Telomeres comprise tandem repeats of non-coding DNA that protect the ends of chromosomes from degradation during cell division. Telomeres have been considered in senescence and mortality because they tend to shorten with growth, sex, and age. Here, age-specific telomere length in female and male Siamese cobra (Naja kaouthia) was investigated for possible relation to morphological condition. Measurements of relative telomere length (RTL) were performed on erythrocytes of 80 cobras (29 females and 51 males, age range 3 weeks – 11 years) using quantitative real-time polymerase chain reaction (qPCR). Total length (TTL) and snout-vent length (SVL) of cobras were also examined. Results showed association between age and SVL (female, R2=0.30 and male, R2=0.31), and between age and TTL (R2= 0.28 for both female and male). Two simple regression models between age and RTL were then constructed in accordance with two age groups (A: ≤ 4 years and B: > 4 years). Interestingly, in Group A, increase of telomere length was observed in females (p .05, R2=0.91) but not in males (p=0.62, R2=0.03). By contrast, in Group B, telomere length decreased with age in both sexes (female, R2=0.38 and male, R2=0.21) suggesting that increasing age above 4 years resulted in cobra telomere shortening. No evidence for quadratic or higher-order equations was found for the relationship between RTL and SVL or TTL. Observation with chromosome mapping of telomeric repeat (TTAGGG)n, indicated that the female W chromosome showed degeneration and remarkable (TTAGGG)n amplification, although interstitial telomeric sites were found on autosomes in either sex. This might relate to the increase of telomere length in females. Telomere dynamics may affect the aging process differently in females and males given their divergent reproductive strategies. Further research is required regarding sex chromosome constitution, in particular, the degenerate W chromosome and links to female fitness in snakes 

Abstract: Sex determination systems are exceptionally diverse and have undergone multiple and independent evolutionary transitions among species, particularly reptiles. However, the mechanisms underlying these transitions have not been established. Here, we tested for differences in sex-linked markers in the only known reptile that is polymorphic for sex determination system, the spotted snow skink, Niveoscincus ocellatus, to quantify the genomic differences that have accompanied this transition. In a highland population, sex is determined genetically, whereas in a lowland population, offspring sex ratio is influenced by temperature. We found a similar number of sex-linked loci in each population, including shared loci, with genotypes consistent with male heterogamety (XY).However, population-specific linkage disequilibrium suggests greater differentiation of sex chromosomes in the highland population. Our results suggest that transitions between sex determination systems can be facilitated by subtle genetic differences.

Numerous chemical and physical agents can affect the structure of DNA and cause double-strand breaks, which may result in chromosome aberrations and the formation of micronuclei, which eventually leads to DNA loss. Nowadays, many studies on their genotoxicity are based on the scoring of micronuclei in plants. The application of a micronucleus test combined with fluorescence in situ hybridisation (FISH) permits the involvement of specific chromosomes or chromosome fragments in micronuclei formation to be determined. However, due to the dearth of FISH probes that can specifically target individual chromosomes in plants, until now the distribution of chromosome aberrations was usually based on using repetitive DNA such as centromeric, telomeric and rDNA sequences. Chromosome painting (CP), which permits the selective visualisation of entire chromosomes or their specific segments during both cell division and interphase, is one of the most advanced FISH-based approaches. In plants, CP is limited to several small-genome species, including the model grass Brachypodium distachyon (Brachypodium).

 

Here, we used multicolour FISH and CP to characterise the composition and to better understand the origin of the micronuclei that are induced in Brachypodium root-tip meristematic cells, which are caused by the application of chemical (maleic hydrazide) and physical (X-ray) mutagens. We show that this approach permits effective qualitative and quantitative analyses of the micronuclei and we attempted to use it to detect possible ‘hot spots’ for DNA breaks. We demonstrate that CP provides a significant improvement in the sensitivity of the ‘standard’ micronucleus test, which is commonly used in testing genotoxicity using plant cells. It appears that the application of mutagenic treatments to Brachypodium combined with its well-developed cytomolecular resources makes it a very promising, if not unique, model system for studying mutagenesis among all monocotyledonous plants.

 

This study was supported by the National Science Centre Poland (grant no. DEC-2012-04-A-NZ3-00572, DEC-2015-18-M-NZ2-00394).

A decade after the appearance of the first devil facial tumour (DFT1), first reported in 1996, a suite of genetics work was done to understand the transmissible cancer. These revealed that though the cancer was slowly evolving and capable of changing in reaction to its ‘environment’, it had reached more a less a point of ‘genomic stability’. In 2014, a new independent transmissible cancer was found in Tasmanian in devils, devil facial tumour 2 (DFT2). This has presented us with a chance to study the cancer in its ‘early’ stages, an opportunity missed with DFT1. We present work done investigating telomeric changes in DFT2. Telomeres, the ends of chromosomes, erode over time, and a lack of telomeric ends may cause massive genomic rearrangements through a breakage-fusion cycle, potentially leading to oncogenesis. There is evidence of this occurring between chromosomes 1 and X in DFT1, and likewise evidence of a chromosome fusion occurring between chromosomes 1 and 6 in DFT2. We know that Tasmanian devils, and dasyurids in general, have a telomeric length dimorphism; for each chromosome pair, the maternal chromosome has short, and the paternal long, telomeres. This dimorphism had disappeared in DFT1 by the time its telomeres were investigated, but is still present in DFT2. By looking at long term cell culture of DFT2 over 200 population doublings, we present evidence that the telomeres of DFT2 are likely to homogenise, as in DFT1. The long telomeres shorten over time, yet the short telomeres are maintained, likely by the telomerase enzyme. This indicates that telomere length is stabilising in DFT2, and is likely to end up homogeneous as in DFT1. Without telomeric erosion and instability, DFT2 is unlikely to undergo new chromosome fusions. These results give a glimpse as to how DFT1 could have lost the telomere length dimorphism.