Heterochromatic Knobs Research Articles

MORPHOLOGY OF THE PACHYTENE CHROMOSOMES INPRUNUS PERSICA

Morphology of the pachytene chromosomes was studied in three genotypes of Prunus persica (L.) Batsch. Each of the eight bivalents has been identified by its distinctive morphological features such as length, position of the centromere, and distributional pattern of the heterochromatic chromomeres. On the basis of total length the chromosomes were numbered from one to eight, the last being the shortest of the complement. In all bivalents the heterochromatin is clustered around the centromere. The chromosomes of the three clones are alike. A heterochromatic knob is present in the distal part of the shorter arm of chromosome five from 'Raritan Rose'. This knob was not present in the other two genotypes studied.Two pairs of chromosomes participate in the organization of a single nucleolus. At pachytene and later stages of prophase, small nucleoli-like bodies associated with or close to the bivalents were frequently observed.

ON THE MECHANISM OF CHROMATIN LOSS INDUCED BY THE B CHROMOSOME OF MAIZE

Knobbed regions of the regular maize complement frequently are eliminated at the second microspore division in spores which have two or more B chromosomes. Evidence is presented that no or little loss occurs in spores with one B and that the rate is not increased in spores with more than two B's.-The B chromosomes from an unrelated strain proved as effective in inducing loss as did the B's of the original high loss stock.-Chromatin loss induced by B's is restricted to knobbed A chromosomes and occurs only at the second microspore division. Knobbed chromosomes 3, 5, and 9 have been tested and all interact with B's to give loss. Chromosomes with large knobs are more frequently broken than are those with smaller knobs and knobless chromosomes show negligible loss.-Although knobs and B's are essential for chromatin elimination, modifying genes can markedly affect the rate of loss.--Two knobbed heterologous chromosomes undergo simultaneous loss more frequently than expected from independent events. The data indicate that joint loss occurs in competent cells and that preferential assortment of the two deficient chromosomes to specific poles is unlikely.-B chromosomes and deficient chromosomes assort independently at the second microspore anaphase.-Genetic data from crosses with marker genes in both arms of chromosome 3 show that breakage of the postulated dicentric bridge does not occur solely at the centric region since a variety of deficient chromosomes were recovered.-Nondisjunction of B chromosomes and elimination of knobbed chromatin take place during the second microspore mitosis. The argument is advanced that the two phenomena result from faulty replication of heterochromatic segments. The position of the nonreplicating segment in the two kinds of chromosomes determines whether nondisjunction or breakage takes place.-Finally, it is suggested that all of the reported effects of the B chromosome can be accounted for if the B is a parasitic entity having no genetic function other than controlling the replication of its proximal heterochromatic knob and increasing the ability of B-containing sperm cells to compete successfully for fertilization of the egg.

Chromosome patterns and nuclear phenomena in the cycad families Stangeriaceae and Zamiaceae

The chromosome karyotypes are described and scale diagrams prepared of 35 species representing 8 genera of the “cycad” families Stangeriaceae and Zamiaceae. The karyotype patterns, chromosome types and characteristics of the nucleolar organisers and heterochromatin are discussed in relation to their evolution in this ancient group of plants. A possible path of chromosome evolution is suggested by the concurrence in some genera of terminal heterochromatic knobs on chromosome arms and a single pair of telocentric homologues which in the remaining genera are replaced by complex partially heterochromatic nucleolar-organising chromosomes. — It is suggested that telocentric chromosomes may be of ancient origin in the cycads and that there may have been a progression from telocentric to mesocentric karyotypes. The paucity of genera today and the taxonomic isolation of the cycads from the remainder of the present-day flora renders impossible the corroboration of this theory by direct cytological comparisons with other groups.

Heterochromatic Knobs in Italian Maize Population and the Evolution of Maize in Italy

Maize samples of plants from siblings of open pollinated populations from all parts of Italy have been cytologically investigated. It has been found that the amount of heterochromatin is on the increase in all parts of Italy but more so in the central regions. New knob forming segments have been located and their origin is thought to be from de novo formation of knob substance.In the course of meiotic studies several chromosome aberrations have been observed. The rates of spontaneous mutation has also been investigated. These rates are higher in the south than north while in the center it is medium. Contrariwise the amount of heterochromatin is higher in the north and lower in the south. In view of this the frequency of spontanous mutations observed in supposed to be partly due to heterochromatin differences and chromosome interaction and partly due to chromosomal aberrations.The possible role of heterochromatin in the structure and function of maize chromosomes has been discussed.The influence of spontaneous mutations and the changing pattern of heterochromatin have been mentioned in relation to evolution of maize in Italy.

The cytology of achiasmatic meiosis in the female Tigriopus (copepoda)

1. In the harpacticoid copepods T. californicus, T. brevicornis, and T. japonicus the chromosome number is established at 2n = 24 for both sexes. Both sexes appear to be homogametic chromosomally. 2. Meiosis in the female follows the same general pattern in all three species. It is achiasmatic. 3. Distant conjugation takes place in the first meiotic prophase and continues throughout the first meiotic division. 4. MI orientation is regular and is brought about by the telosyndetic association of non-homologous pairs into a circular arrangement of eight pairs in T. californicus and T. japonicus; of nine pairs in T. brevicornis. The remaining pairs form side associations to the circular arrangement. These arrangements appear to be characteristic for each of the three species. 5. Occurrence of terminal heterochromatic knobs on the chromosomes reflects the pattern of MI orientation. 6. It is suggested that the attraction properties of these knobs in late prophase and up to a stage shortly before MI accounts for the orderly arrangement of the metaphase chromosomes, resulting in normal distributional products inspite of the lack of chiasmata. 7. Meiosis in the male is chiasmatio as much as revealed by the formation of regular bivalents in MI.

EFFECT OF AN ABNORMAL KNOB-CARRYING CHROMOSOME 10 ON PARAMUTATION OF Rr IN MAIZE

THE R' factor in maize, conditioning anthocyanin formation in seed and plant, is highly sensitive to alteration in pigment-producing action in heterozygotes with the stippled (R) or the marbled (Rmb) allele, when present in a structurally normal chromosome 10 (BRINK 1956; BRINK and WEYERS 1957). The R'rV kernels resulting from testcrosses of standard R'R', RrrT, or R'rg plants on rgfl females (colorless aleurone, green plant) are darkly mottled, whereas the corresponding class of seeds from r+OO x RrRst8 and TOflO x R'Rmb 6 matings are weakly colored. The reduction in pigment-producing action that R' invariably undergoes in RTRst and R'Rmb plants is heritable and has been attributed to a kind of genetic change at, or near, the R' locus termed paramutation (BRINK 1958). Recent studies, the detailed results of which are unpublished, show that the sensitivity of R' to paramutation in heterozygotes with stippled is greatly altered by reciprocal translocations involving the long arm of chromosome 10, in which the R locus resides. The present report is concerned with a related investigation, namely, the effects on R' action following insertion of the factor into a chromosome 10 differing from the normal in possessing a unique terminal segment on the long arm that contains a large heterochromatic knob. Abnormal chromosome 10: The knob-bearing, abnormal chromosome 10 in question, designated K10, has been found in several races of maize from Latin America and the southwestern United States and in a teosinte strain from Chapingo, Mexico (LONGLEY 1937,1938; RHOADES 1952). RHOADES (1942) observed that the proximal portion and a small distal portion of the distinctive terminal segment on the long arm of K10 are euchromatic. A conspicuous heterochromatic knob, about equal in length at pachytene to the sum of these euchromatic parts, lies between the latter. RHOADES found also that the proximal euchromatic part of K10 which is associated at pachytene with the distal one sixth of the normal chromosome 10, in K10/10 heterozygotes, differed in chromomere pattern. EMMERLING (1959) has identified three prominent chromomeres in this region that are not present in normal 10 and has observed that another, less conspicuous, chromomere lies at the base of the knob. Pairing between K10 and normal 10, in K10/10 heterozygotes, ends in the region between

Observations on the so-called sex chromatin in man.

SUMMARYThe difference in nuclear structure between male and female tissues in man has been examined in liver tissue from three embryos of each sex. The so-called sex chromatin consists of a large heterochromatic segment of the X chromosome, thus existing in duplicate in female diploid cells. The two segments have a tendency of juxtaposition resulting in a larger heterochromatic body in female than in male cells. Beside the large heterochromatic segment the X chromosome has, in the tissues studied, a euchromatic segment attached through a small terminal heterochromatic knob to a nucleolus. In male cells the euchromatic segment of the X chromosome is often joined terminally to a small heterochromatic segment believed to belong to the Y chromosome.

A CYTOLOGICAL STUDY OF ASCUS DEVELOPMENT IN PYRONEMA CONFLUENS TUL.

A cytological study of Pyronema confluens has revealed that only one meiosis occurs in the ascus, and that the haploid chromosome complement is probably 12. Differences in size and staining properties were noted between nuclei of the ascogenous hyphae and paraphyses. Diplotene chromosomes are linearly differentiated into heterochromatic knobs and achromatic intercalary segments that could not be differentiated from the cytoplasm with either basic fuchsin or propiono-carmine.