FINE STRUCTURE OF CHROMOSOMES

Electron micrographs of staminate hair cells of Tradescantia reflexa indicate that early prophase chromosomes are composed of a number of helically arranged chromonemata. Favorable preparations reveal as many as 64 identifiable subsidiary strands, assumedly arranged as intertwined pairs to form a hierarchy of pairs of pairs. The helices of the smallest discernible units have a diameter of about 125 A, with highly electron-scattering material disposed peripherally around a less dense "core." The wall of this peripheral ring has a thickness of about 40 A, and apparently represents another pair of coiled threads surrounding a 40 A central axis. The implications of the findings are discussed briefly.     

604. HYMENOXYS INTEGRIFOLIA

Summary.  Hymenoxys integrifolia (Kunth) Bierner, an unusual Composite collected in Mexico in 1996 and grown since then in the Cambridge University Botanic Garden is described and the plant is illustrated.     

THE CHROMOSOMES OF SPLNACIA OLERACEA

Ellis , J. R., and Jules Janick . (Purdue U., Lafayette, Ind.) The chromosomes of Spinaeia oleracea. Amer. Jour. Bot. 47(3) : 210—214. Illus. 1960.—The somatic chromosomes of S. oleracea are described and each has been associated with one of the 6 morphological trisomics derived from triploid-diploid crosses. Of these 6 primary trisomies, reflex had been shown by genetic studies to be trisomic for the chromosomes carrying the sex-determining factors. This chromosome is the longest of the somatic complement and has a sub-median centromere. No obvious heteromorphism of this chromosome pair was observed in staminate plants. Heteromorphism involving this chromosome pair has been reported recently in 2 varieties of cultivated spinach by Zoschke (1956) and Dressier (1958) and was earlier reported by Araratjan (1939) for the wild species, S. tetandra. However, their accounts differ markedly from each other and with the present results in respect to the morphology of this chromosome pair. This study suggests the existence of races which differ with respect to the morphology of the chromosome pair containing the X Y factors.     

罗汉果染色体组型的研究

本文采用根尖和组织培养材料,观察了罗汉果四个品种染色体数日和形态。结果表明,罗汉果体细胞染色体数目为2n=28染色体小,长度在1．4μm以下而又不易分辩着丝点,不同品种的染色体形态有所不同,染色体长度比为2．00-2.46之间。对罗汉果的归属问题进行了讨论。     

DYSPLOIDY IN HYMENOXYS TEXANA (COMPOSITAE)

The two chromosome numbers recorded for Hymenoxys texana, 2n = 16 and 2n = 6, appear to represent dysploid reductions from the base number, x = 15, for the genus.     

THE EVOLUTION OF HETEROMORPHIC SEX CHROMOSOMES

The facts and ideas which have been discussed lead to the following synthesis and model. 1. Heteromorphic sex chromosomes evolved from a pair of homomorphic chromosomes which had an allelic difference at the sex-determining locus. 2. The first step in the evolution of sex-chromosome heteromorphism involved either a conformational or a structural difference between the homologues. A structural difference could have arisen through a rearrangement such as an inversion or a translocation. A conformational difference could have occurred if the sex-determining locus was located in a chromosomal domain which behaved as a single control unit and involved a substantial segment of the chromosome. It is assumed that any conformational difference present in somatic cells would have been maintained in meiotic prophase. 3. Lack of conformational or structural homology between the sex chromosomes led to meiotic pairing failure. Since pairing failure reduced fertility, mechanisms preventing it had a selective advantage. Meiotic inactivation (heterochromatinization) of the differential region of the X chromosome in species with heterogametic males and euchromatinization of the W in species with heterogametic females are such mechanisms, and through them the pairing problems are avoided. 4. Structural and conformational differences between the sex chromosomes in the heterogametic sex reduced recombination. In heterogametic males recombination was reduced still further by the heterochromatinization of the X chromosome, which evolved in response to selection against meiotic pairing failure. 5. Suppression of recombination resulted in an increase in the mutation rate and an increased rate of fixation of deleterious mutations in the recombination-free chromosome regions. Functional degeneration of the genetically isolated regions of the Y and W was the result. In XY males this often led to further meiotic inactivation of the differential region of the X chromosome, and in this way an evolutionary positive-feedback loop may have been established. 6. Structural degeneration (loss of material) followed functional degeneration of Y or W chromosomes either because the functionally degenerate genes had deleterious effects which made their loss a selective advantage, or because shorter chromosomes were selectively neutral and became fixed by chance. 7. The evolutionary routes to sex-chromosome heteromorphism in groups with female heterogamety are more limited than in those with male heterogamety. Oocytes are usually large and long-lived, and are likely to need the products of X- or Z-linked genes. Meiotic inactivation of these chromosomes is therefore unlikely. In the oocytes of ZW females, meiotic pairing failure is avoided through euchromatinization of the W rather than heterochromatinization of the Z chromosome.(ABSTRACT TRUNCATED AT 400 WORDS)     

TELOPHASE SEGREGATION OF CHROMOSOMES AND AMITOSIS

Markedly improved fixation of leaf tissues is obtained by means of a glutaraldehyde (or acrolein)-osmium tetroxide procedure, as compared with the results of potassium permanganate or osmium tetroxide fixation methods. The procedure has proved useful in all species so far examined. Chloroplasts are particularly well preserved. In this paper details of components of the ground-substance of Avena sativa plastids are presented. They include the following:—(i) The "tromacentre" is an area of aggregated fibrils, each 85 A in diameter, and of uncertain length. Individual fibrils may be composed of subunits. The whole aggregate is usually up to 1 µ in diameter, and is visible in thin sections in the light microcope. It is present at all stages of plastid development, and, under conditions of rapid synthesis in the plastid, it may be up to 2 µ in diameter. Evidence that it is proteinaceous is presented. Osmiophilic globules are often associated with it. (ii) Areas which resemble bacterial and blue-green algal nucleoplasms, containing fibrils approximately 30 A wide. These regions are smaller than the stromacentre and, like that structure, they occur in all stages of plastid development. Unlike it, there are several such areas per chloroplast. (iii) Particles which have some of the morphological and staining characteristics of ribosomes. Present at all stages of development, they are approximately two-thirds the size of the cytoplasmic ribosomes. They can occur in groups, thus resembling polyribosomes. (iv) The remaining material is granular, and may include dissociated portions of stromacentre material. The validity of the observations and their significance is discussed.     

CHROMOSOMES OF OREITHALES (RANUNCULACEAE)

A chromosome count of 2n =48 is reported for Oreithales integrifolia from Peru. This represents the first chromosome report for this genus. This chromosome report is discussed in relation to other genera in the Ranunculaceae.     

CHROMOSOMES OF CORSIA (CORSIACEAE)

A chromosome number of 2n = 18 is reported for Corsia cornuta (plus karyotype) and Corsia clypeata of the Corsiaceae from Papua New Guinea.     

THE CHEMICAL COMPOSITION OF ISOLATED CHROMOSOMES

By means of 1 M NaCl isolated lymphocyte chromosomes can be separated into two fractions, each of which contains nucleoprotein. The fraction soluble in M NaCl consists largely of desoxyribose nucleohistone, and constitutes 90 to 92 per cent of the mass of the chromosome. The insoluble residue (the residual chromosome is a coiled thread containing some 12 to 14 per cent of ribose nucleic and about one-fifth as much desoxyribose nucleic acid; the residual chromosome accounts for 8 to 10 per cent of the mass of the chromosome. The staining of chromosomes—whether by the Feulgen procedure, by hematoxylin, orcein, or by basic dyes such as crystal violet—is due to the nucleohistone fraction which contains about 96 per cent of the nucleic acid of the chromosome. The form of the chromosome is due primarily to the protein thread of the residual chromosome. This thread is the only linear structure of microscopic dimensions in the chromosome that is not readily dispersed. When chromosomes are broken, it must be supposed that a break is made in the protein thread of the residual chromosome. The foregoing provides evidence for considering the residual chromosome to be the basis of the linear order of the genes. This would mean either that the residual chromosome is a structure around which the genes are organized or that the genes form part of its substance.     

穗花杉染色体的研究

穗花杉为国家三级重点保护的珍稀濒危植物,雌雄异株。根尖细胞染色体分析表明：雌株和雄株的染色体数目为２ｎ＝４０,其中第１对和第２对为中部着丝粒染色体,第３－２０对为端部着丝粒染色体。核型为２ｎ＝４０＝４ｍ＋３６Ｔ。雌雄株除第２对的长度稍有差异外,其余各对的相对长度和臂比都较近似,可能尚无性染色体分化。Ｇｉｅｍｓａ Ｃ带显示,间期核有３个较大的染色中心,最长的３条染色体的中央缢痕有深染色带纹,可能是着