Genetically unstable DNA sequences of 16.1 kb in length were isolated from the white locus of the w ^mky strain of Drosophila simulans. This insertional DNA has some unique characteristics as a transposon. It is found in high numbers in this strain and its revertant strains w ^psm1 and w^cho, but not elsewhere, and the sequence is a tandem triplication of a basic repeating unit. In order to determine the structure of the insert as whole and the functional unit as a transposon, we analyzed nine clones isolated from genomic libraries of w^psm1 and w ^cho. The repeating unit of the 16.1 kb insertion was the retrotransposon ninja and the DNA sequence of the entire element was determined. The ninja transposon is 6644 bp in length, with a 316 bp long terminal repeat (LTR) on each end. It contains two openreading frames (ORFs), and the pol region is divided between the two ORFs in contrast the organization of other retrotransposons. An alignment analysis of the reverse transcriptase sequences suggested that the ninja element is the first Drosophila retrotransposon belonging to the Pao subgroup.

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The temporal genetic structure of four pairs of juvenile (2-3 years old) and adult (ca. 50-100 years old) populations of Camellia japonica was examined using allozyme markers. No significant differences in genetic variability between two groups were found in the populations examined. About 52 and 37% of positive fixation indices calculated in the four juvenile and adult populations were significantly larger than zero with mean F_IS values of 0.254 and 0.194, respectively. This indicates a considerable deficiency of heterozygotes in the juvenile and adult populations as a possible result of family structuring. Values of F_IS, F_IT, estimates of migration rate per generation (Nm), and F_ST were not significantly different between juvenile and adult populations, though slightly higher mean values of F_IS, F_IT and Nm, and a lower F_ST were found in the juvenile populations in comparison to the adult populations. However, previous studies on other woody plants showed that the seed, seedling, and juvenile stages generally have significantly higher levels of inbreeding than found in adults. The high evolutionary potential maintained in C. japonica may in part result in homogeneous levels of genetic diversity and genetic structure in time.

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Three sorts of chromosomal variations, loss of satellite, cytomixis and translocation, were observed in hybrids between different natural populations of Avena canariensis. A heteromorphic rod bivalent is frequently found in the hybrids between early and late flowering ecotypes of A. canariensis and one satellite chromosome has been lost in the early flowering ecotype of Lanzarote Island. Cytomixis was observed in a number of inter-population hybrids, and one genotype was shown to induce cytomixis causing a great loss of chromatin. The plants showing cytomixis were 70-80% fertile but cytomixis was considered to be an isolating mechanism. A quadrivalent (or trivalent) caused by a single reciprocal translocation was observed in the populations which originated from a distinct geographic region i.e. the northern part of Lanzarote.

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To elucidate possible mechanisms for maintaining B-chromosomes (Bs) in natural populations of Aegilops mutica, cytological behavior of Bs during first pollen grain mitosis was examined in five plants with 1 to 3Bs. Male and female transmission rates of Bs were also studied in the progeny obtained from reciprocal crosses between the plants with and without Bs. In 1B plants, 44% of the PMCs lost the B during meiosis. In 2B and 3B plants, only 10% and 20% of the Bs were lost during meiosis, respectively. No difference was observed in the frequency of B-carrying progeny between the reciprocal crosses, suggesting that the rates of B-chromosome loss during meiosis were similar in EMCs and PMCs. At the anaphase and telophase of first pollen grain mitosis, two sister chromatids of Bs underwent nondisjunction and preferential distribution into the generative nucleus in almost all the B-carrying pollen grains. As a result, most of the progeny obtained from the crosses using B-carrying plants as male parents had even numbers of Bs. No such accumulation of Bs occurred in the course of megasporogenesis, judging from the female transmission rate of Bs. Further, it was suggested that B-carrying gametes took part in fertilization without any advantages or disadvantages. It was, thus, concluded that nondisjunction and preferential distribution of B-chromatids into the generative nucleus during first pollen grain mitosis in 2B and 3B plants were the most effective accumulation mechanisms of Bs in Ae. mutica. The frequency of Bs in natural populations of Ae. mutica might be determined by their accumulation during the first pollen grain mitosis in 2B and 3B plants, the loss during meiosis in 1B plants, and the loss caused by the reduced viability and/or fertility in individuals with a larger number of Bs.

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A 15 year-old girl with Prader-Willi syndrome has been studied by cytogenetie and fluorescence "in situ" hibridization analysis, using centromeric probes. These studies have shown that the underlying mechanism could be an only copy of chromosome 15 (maternally derived) in some cells.

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To examine whether parental alleles were randomly transmitted from heterozygous donor plants to anther culture (AC)-derived plants, two AC-derived populations and one F₂ population in rice were compared for 50 RFLP markers distributed on the rice chromosomes. Two populations which were developed through different anther culture methods (the ordinary method and the direct regeneration method) and the F₂ population were produced from an F₁ hybrid between distantly related cultivars of Nipponbare (japonica type) and Milyang 23 (indica type). RFLP analysis revealed that ten and eleven of the 50 markers in the two AC-derived populations showed distorted segregation ratios from the theoretical ratio of 1:1. Parental alleles were not randomly transmitted from the F₁ plant to the AC-derived plants. Additionally, the segregation ratios of seven and six RFLP markers, respectively, were distorted both from the 1:1 ratios and from the observed ratios in the F₂ population. The chromosomal regions involving these markers were on chromosomes 1, 3, 7, 10, 11 and 12. Some of the regions were different in the two AC-derived populations. Non-random assortment of parental alleles might be influenced by anther culture methods.

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Since the process of amino acid substitution is stochastic, it is of great importance to consider the variance and covariance of the number of amino acid substitutions. Formulae for the variance and covariance of the number of amino acid substitutions estimated by Kimura's method are presented. The bootstrap method shows that the formulae are accurate. The formulae are useful for studies of molecular evolution, e.g., reconstruction of a minimum-evolution tree and the relative-rate test.

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From Escherichia coli cells with a deletion in the ssrA gene that encodes 10Sa RNA after treatment with a mutagen, we isolated two temperature-sensitive mutants, which we designated TS15 and TS101. The temperature-sensitive (ts) phenotype of the mutants could be overcome by introduction of the wild-type ssrA gene but not by the mutants of ssrA. By a complementation test using Kohara's mini-set of clones and by subcloning of a fragment from the phage clone 246, we found that both mutations were in the prs gene that encodes phosphoribosyl pyrophosphate synthetase. Sequencing of the mutant prs gene of TS101 showed that residues 215, cysteine, in the encoded protein had been changed to tyrosine. That such a mutant exists suggests that 10Sa RNA associate with the prs gene product in a functional way.

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