Evolution of genome size by DNA doublings.

Logarithmic distributions of nucleic acid contents per genome of species within major phylogenetic groups of organisms tend to form several peaks. These peaks appear to represent intragroup doublings of DNA or RNA which, in the case of eukaryotes, are independent of polyploidy. This phenomenon has been termed cryptopolyploidy. There are numerical similarities in peak values for different taxonomic groups. A high degree of order is suggested when minimum values for the major phylogenetic groups are plotted against a series of theoretical doublings. These data demonstrate the apparent existence of an exponential periodicity over eight orders of magnitude, leading us to suggest an evolutionary continuity of doublings of a basic ancestral genome (of about 300 nucleotides), these doublings being independent of both chromosome number and ploidy level. This proposed continuity encompasses most major life forms and is generally concomitant with increasing evolutionary complexity, particularly in the prokaryotes and lower eukaryotes. Our interpretation of the data presented here must currently be viewed as speculative, and we do not propose that genome doubling is the only mechanism for genome evolution. However, we feel that the evidence is sufficient to warrant serious scrutiny of our proposals. We hope that this approach to a synthesis of available data will provoke discussion and will stimulate further work toward either supporting, modifying, or disproving our hypothesis.

Mutations induced in Tradescantia by small doses of x-rays and neutrons: analysis of dose-Response curves.

Dose-response curves for pink somatic mutations in Tradescantia stamen hairs were analyzed after neutron and x-ray irradiation with doses ranging from a fraction of a rad to the region of saturation. The dose-effect relation for neutrons indicates a linear dependence from 0.01 to 8 rads; between 0.25 and 5 rads a linear dependence is indicated for x-rays also. As a consequence the relative biological effectiveness reaches a constant value (about 50) at low doses. The observations are in good agreement with the predictions of the theory of dual radiation action and support its interpretation of the effects of radiation on higher organisms. The doubling dose of x-rays was found to be nearly I rad.

Woody plants: changes in survival in response to long-term (8 years) chronic gamma irradiation.

The number of plant deaths which occurred over 8 years of chronic gamma irradiation (20 hours/day) of 11 species of woody plants indicated a decline in the rate of death with increasing exposure time. This suggests that a highly effective repair system may develop, at least in the range of exposure which reduces survival by 50 percent. The inverse relationship previously found between interphase chromosome volume and radiosensitivity for single 16-hour exposures was confirmed for chronic exposures by construction of appropriate regressions. Radiosensitivity of a species can be predicted from these regressions if the interphase chromosome volume is known. The distributions of interphase chromosome volumes and predicted sensitivities are given for 215 species of woody plants.

Radiobiological studies of plants orbited in Biosatellite II.

The Biosatellite II Tradescantia experiment probed the effects of the space environment on spontaneous and radiation-induced mutation rates and on cytological changes in Tradescantia clone 02. Thirty two young flowering plants arranged in a plastic housing with the roots immersed in nutrient solution were exposed to gamma radiation from an on-board 85 Strontium source during the two-day orbital flight. Unirradiated plants were flown in a package in the spacecraft behind a tungsten radiation shield and identical non-flight control packages (with and without irradiation) were maintained at the launch site. After retrieval of the spacecraft near Hawaii, samples of root tip, ovary and stamen tissues were collected. These and the intact plants were flown to the Brookhaven National Laboratory for observations on the following end points: somatic mutation, cell size, loss of reproductive integrity resulting in stunted stamen hairs, pollen grain mortality, frequency of micronuclei in pollen, disturbed mitotic spindle function and chromosome aberrations. Analysis of data on somatic mutation, cell size and chromosome aberration end points showed no significant differences between flight and non-flight samples. However, pollen abortion, frequency of micronuclei in pollen and loss of reproductive integrity (stamen hair stunting) showed increases associated with weightlessness in irradiated material. Root tip and microspore cells showed effects of disturbed mitotic spindle function in orbited plants both with and without irradiation. Clearly differences exist between flight and non-flight material and the significance and possible mechanisms for these effects are being studied in continuing non-flight tests.

Comparison of X-ray and gamma-ray dose-response curves for pink somatic mutations in Tradescantia clone 02.

Microdosimetric data indicate that the mean specific energy, eta, produced by individual charged particles from X-rays and gamma rays is different for the two radiation qualities by nearly a factor of two. In order to test whether this influences the initial, linear component in the dose-effect relations, a comparison was made between dose-response curves for pink somatic mutations in Tradescantia clone 02 stamen hairs following X and gamma irradiations. Absorbed doses ranged from 2.66 to 300 rad.The results are in agreement with predictions made on the basis of microdosimetric data. At low doses gamma rays are substantially less effective than X-rays. The RBE of gamma rays vs. X-rays at low doses was approximately 0.6, a value lower than those usually reported in other experimental systems.