Applied mutagenesis could improve economically important traits in bitter gourd (Momordica charantia L.).

Mutants with unique characters have played a key role in discovery of gene, mapping, functional genomics and breeding in many vegetable crops, but information on bitter gourd is lacking. Induction of mutation by gamma rays (Co60 source) at five different doses (50 Gy, 100 Gy, 150 Gy, 200 Gy and 250 Gy) was studied in four widely divergent bitter gourd genotypes BG-1346501, Meghna-2, Special Boulder and Selection-1 in M1 generation. Reduction in seed germination percentage, vine length and pollen fertility occurred in M1 generation with the increasing doses of mutagens. LD50 dose for BG-1346501, Meghna-2, Special Boulder and Selection-1 corresponded to 290.76 Gy, 206.12 Gy, 212.81 Gy and 213.49 Gy ᵞ radiation, respectively suggested low to medium doses (200-250 Gy) of gamma rays would be helpful in producing useful and exploitable mutants for further breeding. No remarkable effect of ᵞ radiation on fruit physicochemical characters in M1 generation were observed. M2 generation, raised from two widely divergent genotypes, BG-1346501 and Meghna-2, were screened critically and observed no significant reduction in seed germination and pollen viability, however little damage occurred particularly in vine length. There is possibility of isolating segregates in M2 generation with enhanced nutrient contents at low radiation dose. Highest mutation frequency resulted by treating Meghna-2 at 200 Gy and BG-1346501 at 100 Gy. Both genotype and mutagenic doses influenced mutagenic effectiveness. Spectrum of mutation was very low; number of putative mutants isolated from M2 generation was five in Meghna-2 and three in BG-1346501. Among six putative macro mutants isolated from M3 generation, we could identify two putative mutants, namely Meghna-2 with gynoecious sex form and BG-1346501 with high charantin, appreciable ß-carotene and high ascorbic acid contents having ample promise for further utilization in bitter gourd breeding after critical testing in subsequent generations for estimation of genetic gain and trait heritability to confirm the mutant stability.

Flaws in the LNT single-hit model for cancer risk: An historical assessment.

The LNT single-hit model was derived from the Nobel Prize-winning research of Herman J. Muller who showed that x-rays could induce gene mutations in Drosophila and that the dose response for these so-called mutational events was linear. Lewis J. Stadler, another well-known and respected geneticist at the time, strongly disagreed with and challenged Muller's claims. Detailed evaluations by Stadler over a prolonged series of investigations revealed that Muller's experiments had induced gross heritable chromosomal damage instead of specific gene mutations as had been claimed by Muller at his Nobel Lecture. These X-ray-induced alterations became progressively more frequent and were of larger magnitude (more destructive) with increasing doses. Thus, Muller's claim of having induced discrete gene mutations represented a substantial speculative overreach and was, in fact, without proof. The post hoc arguments of Muller to support his gene mutation hypothesis were significantly challenged and weakened by a series of new findings in the areas of cytogenetics, reverse mutation, adaptive and repair processes, and modern molecular methods for estimating induced genetic damage. These findings represented critical and substantial limitations to Muller's hypothesis of X-ray-induced gene mutations. Furthermore, they challenged the scientific foundations used in support of the LNT single-hit model by severing the logical nexus between Muller's data on radiation-induced inheritable alterations and the LNT single-hit model. These findings exposed fundamental scientific flaws that undermined not only the seminal recommendation of the 1956 BEAR I Genetics Panel to adopt the LNT single-hit Model for risk assessment but also any rationale for its continued use in the present day.

Further assessment of exome-wide UVR footprints in melanoma and their possible relevance.

C>T substitutions at dipyrimidine sites dominate the melanoma genome. We recently analyzed the exomes of spontaneous and neonatal UVR-induced murine melanomas, noting a dramatic change in the genomic footprint at C>T substitutions in the latter. Here we re-analyzed published exome-wide footprints in human melanomas stratified in terms of likely previous sun exposure. Acral and mucosal melanomas were heterogeneous in terms of base substitution types, but most C>Ts occurred in the context of 3'G, probably resulting from spontaneous deamination of the cytosine. C>Ts in sun-exposed melanomas were statistically different from acral/mucosal lesions only in preferring an adjacent 5'T and 3'C. Pyrimidine dimer adducts can form between any pyrimidine (TT, TC, CT, CC). Hence in melanoma C>Ts are overwhelmingly induced at TC or CC photoproducts, or, there are peculiarities in DNA repair that favor the mutation of cytosines with these two pyrimidines adjacent. If melanoma UVR footprints at C>Ts reflect a specific dimer type (eg, 6-4 photoproduct or cyclobutane pyrimidine dimer), these could be removed post UVR, for instance using photolyases, to potentially reduce melanoma risk. If specific modes of DNA repair and/or replication cause these footprints, methodically downregulating selected DNA polymerases in UVR-induced animal models of melanoma, combined with exome sequencing, could begin to assess this. Finally, a preponderance of TpCpC as opposed to NpCpG at C>Ts exome-wide is likely to be a good indicator of whether a melanoma has incurred even a small amount of sun damage. This information will assist epidemiological studies in predicting individual levels of sun exposure.

Costs and benefits of natural transformation in Acinetobacter baylyi.

BACKGROUND: Natural transformation enables acquisition of adaptive traits and drives genome evolution in prokaryotes. Yet, the selective forces responsible for the evolution and maintenance of natural transformation remain elusive since taken-up DNA has also been hypothesized to provide benefits such as nutrients or templates for DNA repair to individual cells. RESULTS: We investigated the immediate effects of DNA uptake and recombination on the naturally competent bacterium Acinetobacter baylyi in both benign and genotoxic conditions. In head-to-head competition experiments between DNA uptake-proficient and -deficient strains, we observed a fitness benefit of DNA uptake independent of UV stress. This benefit was found with both homologous and heterologous DNA and was independent of recombination. Recombination with taken-up DNA reduced survival of transformed cells with increasing levels of UV-stress through interference with nucleotide excision repair, suggesting that DNA strand breaks occur during recombination attempts with taken-up DNA. Consistent with this, we show that absence of RecBCD and RecFOR recombinational DNA repair pathways strongly decrease natural transformation. CONCLUSIONS: Our data show a physiological benefit of DNA uptake unrelated to recombination. In contrast, recombination during transformation is a strand break inducing process that represents a previously unrecognized cost of natural transformation.

Reflections on the impact of advances in the assessment of genetic risks of exposure to ionizing radiation on international radiation protection recommendations between the mid-1950s and the present.

Efforts at protecting people against the harmful effects of radiation had their beginnings in the early 1900s with the intent of protecting individuals in medicine and associated professions. Such efforts remain vital for all of us more than 100 years later as part of our 'learning to live with ionizing radiation.' The field of radiation protection has evolved slowly over time with advances in knowledge on hereditary (i.e., genetic) and carcinogenic effects of radiation continually improving our ability to make informed judgments about how best to balance risks against benefits of radiation exposure. This paper examines just one aspect of these efforts, namely, how advances in knowledge of genetic effects of radiation have impacted on the recommendations of the International Commission on Radiological Protection (ICRP). The focus is on the period from the mid-1950s (when genetic risk estimates were first made) to 2007. This article offers a detailed historical analysis and personal perspective, and concludes with a synopsis of key developments in radiation protection.

Mutations in mammalian cells induced by heavy charged particles: an indicator for risk assessment in space.

Induction of mutations at the HPRT locus in V79 Chinese hamster cells by heavy charged particles is reviewed. A unique dependence of mutation induction cross sections on LET or any other physical parameter of the particles cannot be found. There is, however, a general trend showing an increase up to about 100 keV/microm and an indication of a saturation with higher LET. An empirical expression is given approximating the data which may be useful for practical purposes. On the basis of this expression, it is suggested that the formula used by the ICRP for the quality factor overestimates the risk for very heavy particles. This may have implications for the assessment of the risk of exposure to iron ions in space.

A comparison of the 8-hydroxydeoxyguanosine and micronucleus techniques for the assessment of X-ray and UV induced genotoxicity.

A comparison was made of the relative sensitivities of the cytokinesis-block micronucleus and the 8-hydroxydeoxyguanosine assays for the assessment of X-ray and UV-induced genotoxicity in mouse splenocytes in vitro. A detectable (p < 0.05) increase in micronuclei occurred at about 1/5 the X-ray exposure required to induce a significant increase in the level of 8-hydroxydeoxyguanosine. With UV radiation a significant (p < 0.05) rise in micronuclei was achieved at about 1/10th the dose needed to produce a detectable increase in 8-hydroxydeoxyguanosine levels. The data confirm the value of the cytokinesis-block micronucleus technique for the detection of genotoxicity at low-level X-irradiation, and indicate that it is more sensitive for that purpose than the 8-hydroxydeoxyguanosine assay. The data also demonstrate, for the first time, the production of 8-hydroxydeoxyguanosine by UVA/B-irradiation of intact cells, but point again to a more sensitive assessment of UV-related genotoxicity by the cytokinesis-block micronucleus method.