Biomedical subjects

Fumio Tajima

Publications and source records attributed to Fumio Tajima.

5 recordsLinked to original sources

PubMed2005

Effect of population structure on the amount of polymorphism and the fixation probability under overdominant selection.

Under overdominant selection, mutants substantially contribute to increase the amount of polymorphism. It is also known that under neutrality as the migration rates among demes decrease in a subdivided population, the amount of polymorphism increases along with the increase of the effective population size, N(e). In this study, under overdominant selection the effect of population subdivision on the amount of polymorphism was investigated using the diffusion approximation and the low migration approximation. It was shown that if selection is medium or strong (e.g., N(T)s > 1, where N(T) is the population size and s is the selective advantage of heterozygotes), the nucleotide diversity, pi, decreases along with the decrease of Nm against the increase of N(e), where N is the size of demes and m is the migration rate per deme. In addition, the ratio of the nucleotide diversity to the evolutionary rate also decreases along with the decrease of Nm. In some cases the ratio becomes smaller than that expected under neutrality as Nm decreases.

Genetics, Population↗

PubMed2005

Evolution of coadaptation in a two-locus epistatic system.

Although recent advances in genome biology have dramatically increased our understanding of the contribution of gene interactions to the development of complex phenotypes, we still lack general agreement on the process and mechanisms responsible for the evolution of epistatic systems. Even if genes in a species are indeed integrated into coadapted complexes of interacting components, simple additive evolution may eventually result in epistatic differentiation of populations. Consequently, the prevalence of epistatic gene action does not tell us anything about the role of epistatic selection in the history of population divergence. To elucidate the contribution of epistatic selection in the evolution of coadaptation, we investigate the fixation process of two mutations that interact synergistically to enhance fitness. We show by diffusion analysis and simulations that epistatic selection on cosegregating variants does not by itself promote the evolution of epistatic systems; rather, accumulation of neutral mutations may play a crucial role, creating an appropriate genetic milieu for adaptive evolution in the future generations.

Biological Evolution↗

PubMed2004

Effect of dominance on heterozygosity and the fixation probability in a subdivided population.

Under the assumptions of a subdivided population and the presence of dominance for fitness, the expected sum of heterozygosity in the total population during the lifetime of mutant was investigated. It was shown analytically and by computer simulations that in the island model the effect of dominance on the expected sum of heterozygosity decreases as the migration rate decreases and is lost almost completely when the migration rate is very low. In addition to the expected sum of heterozygosity, the fixation probability of mutant was also investigated. The effect of dominance on the fixation probability also decreases as the migration rate decreases but is not completely lost when the migration rate is very low.

Computer Simulation↗

PubMed2002

The effect of migration during the divergence.

The mean and variance of the number of nucleotide differences were obtained when the ancestral population diverged with migration. The number of nucleotide differences obtained indicates that not only the migration rate but also the period of migration has influence on a population structure. According to the migration rate and the period of migration, populations behave approximately as a single unit, diverged and isolated populations, two populations under equilibrium, or none of them. When sigma m(t) is about one, the variance of the number of nucleotide differences becomes large, where sigma m(t) is the sum of the migration rate for the period of migration. The distribution of the estimated divergence time was also obtained using computer simulations. It was found that the divergence time can be explained by sigma m(t). That is, the divergence time is mostly estimated as the time when sigma m(t) is less than 1.

Animal Migration↗

PubMed2002

A statistical test for the difference in the amounts of DNA variation between two populations.

A statistical test for the difference in the amounts of DNA variation between two populations is developed. The test statistic involves the covariance of the amount of variation between two populations, which is given by a function of their divergence time, T0. Accordingly, the power (rejection probability) of the test depends on T0. In this article, T0 is treated as unknown because it is very difficult to estimate. The test is most conservative when T0 = infinity is assumed because the covariance is zero. If T0 = 0 is assumed, the largest value of the rejection probability is obtained. Thus, the test provides a range of rejection probability unless we have a reliable estimate of T0. The test is applied to the PgiC region in three mustard species: Leavenworthia stylosa, L. crassa and L. uniflora. The results of our test show that the level of variation in L. stylosa is significantly higher than those in the other species.

DNA, Plant↗