Dragoslav Marinkovic

Correlation between recombination frequency and fitness in Drosophila melanogaster

The origin and maintenance of genetic recombination are unsettled evolutionary issues. Genetic variation affecting recombination frequency appears to be pervasive in nature, suggesting that natural selection must increase recombination frequency under some circumstances. However, theoretical arguments and experimental evidence indicate that the frequency of recombination should be reduced by natural selection.A hypothesis not previously explored is that recombination modifiers may directly affect the fitness of their carriers; rather than only indirectly (through the production of recombinant progeny) as generally assumed. We have tested this hypothesis by examining three fitness components (viability, male fertility, and female fecundity) in Drosophila melanogaster homozygous for second chromosomes isolated from a natural population. Then, we have measured the frequency of recombination in flies heterozygous for each wild second chromosome and a chromosome carrying five recessive alleles.The results indicate that genes modulating the frequency of recombination have direct effects on fitness as proposed by the hypothesis. However, the correlation between frequency of recombination and fitness is negative. Thus, the riddle of recombination remains unexplained and, in fact, more puzzling that ever.

Genetic Variability in the Group of Patients with Congenital Hip Dislocation

Our study of genetic homozygosity degree includes an analysis of the presence, distribution, and individual combination of 20 selected genetically controlled morphophysiological traits in the group of patients (N = 93) with congenital hip dislocation (CDH) and in control sample consisting of schoolchildren from Belgrade (N = 200). Assuming that CDH is a genetically controlled disease, we made a hypothesis that an increased homozygosity level, as well as the changed variability among the patients, could be a population-genetic parameter for the prediction of the illness. Taking into consideration our experience, as well as the experience of numerous scientists who studied the nature of the inheritance of mono- and oligogenically controlled qualitative traits, we applied a methodology to estimate the proportion of such homozygously recessive characters (HRC-TEST). This population-genetic study did not only show statistically significant difference of the middle values of genetic homozygosity (CDH: 7.1 ± 0.2; control: 5.2 ± 0.1), but of the differences in the type of distribution too, as well as the differences in the presence of certain individual combinations of such traits. The described methodology can be used in further analyses, with hope that it can be applied as an early prognosis for decreased resistance to different diseases. The frequencies of ABO blood types in the sample of CDH patients were similar to the average value of the Serbian population, while the percentage of blood group A is slightly increased. Comparing frequencies of Rh blood groups, there is no difference between tested samples.

Multiple-choice selection for light preference inDrosophila subobscura

Drosophila subobscura flies were selected for the ability to choose one of five light intensities (i.e., 30, 300, 1300, 3200, or 6500 lux), with the aid of an apparatus which enables the flies to choose freely. The original distribution of wild flies was as follows: about 60% repeatedly chose the space lighted by 6500 lux, about 30% 1300–3200 lux, and about 10% 30–300 lux. By mating the flies within each of the three categories for 19 generations, their proportion increased from 8 to 30% at 30–300 lux, from 32 to 55% at 1300–3200 lux, and from 60 to 78% at 6500 lux. The selective response was greatest at the beginning of the selection, and declined later. Using micronized dusts to mark the flies, it was determined that on the average about 33% of the flies chose the same light intensity in both of two 24-h runs, and about 31% more chose for the second time one of the neighboring light intensities. These experiments suggest that phototactic response is a relative property, so that a fly can be “photopositive” or “photonegative” to a dimmer light (including complete darkness) and to a light of much higher intensity.

Micronuclei and ageing in a sample of Yugoslavian population

Objective: instability in the organization and expression of the genetic material has been hypothesized as the basic mechanism of ageing. Aim of this study was to quantify the effect of ageing on spontaneous micronuclei (MN) frequency in peripheral blood lymphocytes. Method: analysis of Yugoslavian population sample (164 tested individuals, age 0–62 years) has performed by application of cytokinesis-block technique (CB). Results: there was an increase of MN frequency with age, from newborns to 40-year-old persons. The total average of MN frequency per 1000 analyzed binuclear cells amounts to 8.03 ± 0.42, with variation from 0 to 26 MNs. In a sample of 29 newborns the recorded average MN frequency was 6.91 ± 0.81, while among 69 persons 25–39 years old, the MN frequency was 9.16 ± 1.00. The lowest average MN frequency, however, was noted in the sample of 34 tested individuals 40 to 62 years of age. Conclusion: an increase with age in MN frequency has been observed in samples of studied individuals from newborns to 40-year-old persons. A decrease of MN frequency in older groups could be explained by a gradual decrease of proliferative cell capacities.

Genetic variation for rate of development in natural populations of Drosophila melanogaster

We have sampled wild chromosomes from two natural populations of Drosophila melanogaster and obtained flies fully homozygous for the second chromosome, the third chromosome, or both, as well as flies heterozygous for one or both wild chromosomes and balancer chromosomes. Rate of embryogenesis (egg laying to larval hatching) and rate of development from egg to adult are measured, by classifying the individuals into fast, intermediate, and slow developmental classes. The experiments indicate that variation for rate of embryogenesis and for rate of egg-to-adult development is plentiful in the natural populations. Various hypotheses are enunciated to account for the small range of phenotypic variation observed in wild-type individuals with respect to the two parameters (embryogenesis and egg-to-adult development) and for the difficulty in changing the mean rates by artificial selection. Appropriate experiments may decide among the hypotheses, helping us to understand the genetic control of rate of ontogenesis, which is an important fitness component.

Parental age dependent changes as a source of genetic variation in Drosophila melanogaster

It has been shown repeatedly that numerous cumulative changes occur in chromosomes of D. melanogaster, as an effect of ageing which, especially in the homozygous state, significantly affect different fitness components of their carriers. It appears that the observed age-affected events are produced by systematic and ontogenetically programmed changes in genetic loads at specific chromosomes, which are transferable to following generations. It has been suggested that such changes could be of mutational origin, and that they cold be more frequent at gene loci which are epigenetically active during ontogenesis.It was demonstrated that a large sample of identical chromosomes behave quite differently in the homozygous state when obtained from aged compared to non-aged parents, producing a significant decrease in relative viability, length of preadult development, and longevity of their carriers, as well as in the frequency of recombinations of corresponding chromosomes. A specfic treatment by streptomycin resulted in remarkably milder effects of ageing, which is in accordance with the statement of some authors that such a treatment may diminish the frequency of spontaneous recessive mutations in their carriers. Thus the observed age-affected changes could be an important source of developmental and evolutionary variation of living organisms.

Selection for copulation ability of Drosophila subobscura in the absence of light.

Both under minimal light intensity (<2 lux) and in the absence of light, it was possible, by stepwise selection, to obtain a laboratory strain ofDrosophila subobscura whose individuals copulate under conditions of absolute darkness. This reproductive behavior is primarily under recessive genetic control and has a relatively high heritability, from 0.643 to 0.810. There is no significant influence of sex on heritability and copulation ability.

Age-Affected Changes in Viability and Longevity Loads of Drosophila melanogaster

Numerous authors (Szilard 1959; Burnet 1959; Failla 1960; Clark et al. 1961; Strehler 1962; Maynard Smith 1962; Comfort 1964; Medvedev 1962, 1964; Curtis 1966; Sinex 1974; Wilson 1974) propose that changes in the genomes of cells may be directly connected with the aging of organisms. Some theories maintain that the basic cause of aging is somatic mutations, but there are no clear arguments that distinguish these changes from those produced by inhibitory effects on particular genes. Beardmore (1976) emphasized that designating mutations as the cause of aging assumes that genic changes occur at least ten times more frequently in somatic than in germ cells of an organism. Possibly biochemical errors occur at transcriptional and/or translational levels. Marinkovic et al. (1973, 1975) offered experimental proof that aging of Drosophila inelanogaster results in an increase in viability genetic loads, which were expressed in second-chromosome homozygotes but not in heterozygous combinations of the same chromosomes. This result is probably due to recessive mutations, but could also be a result of the late expressivity of homozygously deleterious genes (Medawar 1952), or of genes which change their effects during the aging of an individual (Williams 1957). This study attempts to determine whether or not age-related changes in viability and longevity genetic loads are inherited, i.e., are they transmittable through young progenies of old parental individuals. If this were the case, mutations might occur in germ cells as a consequence of aging, and differential survivals of the progeny of aged and nonaged individuals might not be due to different expressivity of genes or to a kind of inhibitory effect on particular parts of their genome.

Fitness of allozyme variants in Drosophila pseudoobscura III. Possible factors contributing to the maintenance of polymorphisms in nature

Experiments have been performed to investigate the mechanisms maintaining enzyme polymorphisms in natural populations. We have measured effects on fitness of genotypic variants at three loci, Est-5, Odh, and Mdh-2, in D. pseudoobscura. Significant differences exist among the genotypes in the rate of development from egg-to-adult; there is also indication of differences in larval survival. In a population segregating for allelic variants at all three loci, there is indication that segregation distortion at meiosis or some form of gametic selection might be involved. The relative fitnesses of alternative genotypes are reversed when either different fitness components are considered, or the genotypic frequencies are changed, or the larval density is increased. These fitness reversals may contribute to the maintenance of the polymorphisms, and may account for cyclical oscillations of allozyme frequencies observed in natural populations.

Light dependence in matings ofDrosophila pseudoobscura

Mating inDrosophila pseudoobscura is known to be light independent. However, differences in the ability to mate in the presence of light or in the dark exist in lines selected for positive or for negative phototaxis.