The evolution of sex chromosomes

Structurally distinct sex chromosomes (X and Y) are the most familiar mode of genetic sex determination and have evolved independently in many different taxa. The evolutionary paths by which their characteristic properties may have evolved are reviewed. These properties include the failure of X and Y to recombine through much or all of their length, the genetic inertness of much of the Y chromosome, dosage compensation of the activity of X chromosomal loci, and the accumulation of repeated DNA sequences on the Y chromosome.     

Sex chromosomes in lizards

Karyotypes of many species of the genus Sceloporus support the generalization that there are no morphologically recognizable sex chromosomes in lizards; however, there is a marked sexual dimorphism in the karyotypes of Sceloporus jarrovi and Sceloporus poinsetti. During meiosis in males, whose diploid number of chromosomes is 31, preferential segregation of chromosomes from a trivalent results in heterogamety.     

Prokaryotic chromosomes and disease

Recent insights into bacterial genome organization and function have improved our understanding of the nature of pathogenic bacteria and their ability to cause disease. It is becoming increasingly clear that the bacterial chromosome constantly undergoes structural changes due to gene acquisition and loss, recombination, and mutational events that have an impact on the pathogenic potential of the bacterium. Even though the bacterial genome includes additional genetic elements, the chromosome represents the most important entity in this context. Here, we will show that various processes of genomic instability have an influence on the many manifestations of infectious disease.     

On the mitotic movements of chromosomes

My conclusions, which, I confess, are tentative and based mainly on studies of one kind of cell, the grasshopper neuroblast, may be summarized as follows. The late prophase orientation of the chromosomes is a carry-over from the late telophase orientation. It is apparently maintained by means of the centromeres, which appear to be attached within a limited region of the nucleus throughout telophase, interphase, and prophase. Metaphase orientation of the chromosomes may be explained as the resultant of two forces: a force involving the centromere and spindle, which is responsible for keeping the centromeres in the equatorial plane of the spindle, and a repulsion force involving the noncentromeric portion of the chromosomes, which results in a tendency toward uniform spacing of the chromosomes outside the spindle. Anaphase separation of sister chromatids and their subsequent movement toward the poles of the spindle involves at least four distinct phases: (i) the initial poleward movement of the centromeres, which may be due to intrinsic repulsion or to a force acting between spindle and centromeres that produces an angle of almost 90 degrees between the separated and unseparated portions of the chromatids; (ii) the autonomous separation of the noncentromeric part of the chromosome; (iii) elongation of the spindle, beginning just after the sister chromatids are separated proximally and ending when the longer chromatids are about to lose contact distally; and (iv) the later movement apart of the daughter chromosomes, probably resulting from a pushing force exerted by elongation of the interzonal fibers.     

欧洲黑莓的染色体观察

以组培苗根尖和茎尖为材料,明确了适合欧洲黑莓优良品种‘冬福瑞’(Rubus fruiticosusvar.‘Thorn-free’)染色体观察的去壁低渗法主要条件参数,即:根尖在0.02%秋水仙素和2 mmol/L的8-羟基喹啉混合液(1∶1)中预处理2 h,用2.5%的纤维素酶和果胶酶混合液(1∶1)酶解去壁2 h,后低渗30 min,以确定冬福瑞染色体数。确定了欧洲黑莓‘冬福瑞’为四倍体,染色体数目为28。     

Histone acetylation in insect chromosomes

Acetylation of histones takes place along the salivary gland chromosomes of Chironomus thummi when RNA synthesis is active. It can be observed but not measured quantitatively by autoradiography of chromosome squashes. The "fixatives" commonly used in preparing squashes of insect chromosomes preferentially extract the highly acetylated "arginine-rich" histone fractions; the use of such fixatives may explain the reported absence of histone acetylation in Drosophila melanogaster.     

Centromeres in human meiotic chromosomes

Two heterochromnatic spots were observed in most bivalents of human spermnatocytes at the late diplotene stage from six individuals. It is our belief that these dark staining bodies are the centromeres of the meiotic chromosomes.     

Evidence of paleozoic chromosomes from lycopod microgametophytes

A Pennsylvanian arborescent lycopod cone, Lepidostrobus schopfii, has microspores that have been found to have intracellular features that are interpreted as nuclei and mitotic chromosomes. The cellularized gametophytes conform to the early stages of growth that occur in modern Selaginella microgametophytes. Since the megagametophyte of L. schopfii is similar in development to extant species of Isoetes, the fossil now is known to have portions of its life cycle in common with both Selaginella and Isoetes.     

文蛤染色体的研究

采用PHA和秋水仙素体内注射法，以鳃组织为材料，低渗-空气干燥法制片，对文蛤的染色体进行了分析研究。结果表明，文蛤染色体绝对长度较小，为0.91-1.96μm；其二倍体数目为2n=38，核型公式为N=18m 14sm 6t，染色体总臂数NF=70。同时，本文还对帘蛤科种类的核型演化关系进行了初步探讨。     

文蛤染色体的研究

采用PHA和秋水仙素体内注射法，以鳃组织为材料，低渗-空气干燥法制片，对文蛤的染色体进行了分析研究。结果表明，文蛤染色体绝对长度较小，为0.91-1.96μm；其二倍体数目为2n=38，核型公式为N=18m 14sm 6t，染色体总臂数NF=70。同时，本文还对帘蛤科种类的核型演化关系进行了初步探讨。     

Nullisomic tetrahymena: eliminating germinal chromosomes

Germinal and somatic functions in Tetrahymena are performed separately by the micro- and macronuclei, respectively. Cells with haploid micronuclei were mated with diploids to yield monosomic progeny. These were induced to undergo a form of self-fertilization, generating cells lacking both copies of one or more of the five chromosomes in the micronucleus while still possessing a complete macronuclear genome.     

淮南猪染色体的研究

采用微量外周血淋巴细胞培养以及核仁组成区银染色(Ag—NORs)技术,对淮南猪体细胞核型以及核仁组成区进行了研究,结果表明,淮南猪二倍体细胞染色体组成为38xy(公)或38xx(母),NF=64;细胞分裂中期银染阳性核仁组成区为1、2、3和4的频率分别为0.3659、0.2683、0.1951和0.1707,平均有2.17条染色体的核仁组成区被正染。