Position Of Chiasmata Research Articles

Centromere localization at meiosis and the position of chiasmata in the male and female mouse

Techniques for obtaining differential Giemsa staining of the paracentromeric (p.c.) regions of male and female mouse meiotic chromosomes (centromeric heterochromatin) were explored and standard procedures developed for the different meiotic cells in the two sexes. The best result followed the use of heat at controlled pH in Sorensen's phosphate buffer or in Standard Saline Citrate (SSC) solutions. With these techniques, morphological features of the p.c. regions and their variation were studied in normal animals (CFLP strain) and in a strain (AKR) homozygous for a centric fusion [T(11; ?)-1 Ald] between chromosomes No. 6 and No. 15 (Miller et al., 1971). The Y chromosome was often found to show distinct p. c. staining at first and apparently at second meiotic metaphase, and the X and Y chromosomes were found to associate as bivalents by their long arms. Autosomal p.c. regions showed variation in size which might indicate differences between non-homologous chromosomes but a tendency to similarity between homologues. Differences were found between males and females in respect to proportions and variation of bivalents with single and double chiasmata. The relative positions of chiasmata were different in the two sexes. The presence of the centric fusion in the males did not seem to affect the pairing behaviour of the remaining autosomes or of those taking part in the centric fusion. The possibility is discussed that the p.c. regions, to which also other functions would seem to appertain, may be important for chromosome recognition and pairing, possibly on a quantitative basis.

The maps of the lampbrush chromosomes of Triturus (Amphibia Urodela). IV. Triturus vulgaris meridionalis.

The lampbrush chromosomes of Triturus vulgaris meridionalis were isolated from the germinal vesicle of medium and large-sized oocytes and studied with phase-contrast microscope. The maps were constructed on the basis of the lengths and major morphological features of the chromosomes. The length of each map is equal to the mean of the relative lengths of the corresponding chromosome from different oocytes (the relative length of each chromosome is represented by the ratio between its absolute length and that of chromosome XII from the same complement, conventionally considered as 100 units long). The maps arranged in decreasing length order, were oriented according to the most frequent position of chiasmata, as centromeres were not always evident. — Chromosomes VI and XI bear a sphere in subterminal position. Landmarks typical for T. vulgaris meridionalis are the loops inserted on chromosomes VIII (47 units), X (23 units), XI (34 units) and XII (34 units) frequently presenting themselves under the form of double loop bridges of considerable extension. On chromosomes I (4 units), VI (13 units), X (4 units) and XI (36 units) giant bodies were found that are sometimes comparable to dense-matrix loops. Chromosome XI includes a nucleolus-organizing region, sometimes identifiable by the presence of an inserted nucleolus. Normal and granular loops (much extended at times), axial granules, globules, and loopless bars supplement the morphology of the lampbrush chromosomes of this species.

Chiasma distribution in Truxaline grasshoppers

Similar patterns of chiasma distribution are found within the individual arms of the chromosome complement in four species of Truxaline grasshopper. There is a linear relationship between chiasma frequency and chromosome arm length although the telocentric elements have a consistently higher mean number of chiasmata per unit of arm length. The positions of successive chiasmata can be defined in terms of residual (‘r.c.’ and ‘r.t.’) and interference (T) distances which vary in value according to both arm length and chiasma frequency. There is a tendency for one chiasma to lie in a distal position which is accentuated when additional chiasmata form. Supernumerary B chromosomes do not appear to influence the overall control mechanism of chiasma distribution. There is no indication that bivalents within a nucleus compete for chiasmata nor does the chiasma distribution in one arm of the metacentric members influence that in the other. It is suggested that the control of chiasma formation is determined mainly by interference factors.

THE ORIENTATION OF MULTIPLE ASSOCIATIONS RESULTING FROM INTERCHANGE HETEROZYGOSITY

Among the many factors thought to influence the orientation associations in interchange heterozygotes, previous evidence suggests that there is a genetically controlled mechanism (or mechanisms) which is distinct from that varying the position and number of chiasmata. In rye, variation between 50% and 90% disjunction frequency has been ascribed to this mechanism. Nevertheless, in this species, directed segregation has been found in newly arising interchanges, suggesting that the genetic constitution has been selected to give a high disjunction frequency. Two alternative explanations for these findings were considered. First, the influence on orientation may be a secondary effect of factors whose primary effect is of selective importance in normal, non- interchange individuals. Second, direct selection for a high disjunction frequency may have occurred in a manner analogous to Fisher's theory of the evolution of dominance. (auth)

SUB‐CHROMATID REARRANGEMENTS IN TRILLIUM ERECTUM. I. ORIGIN AND NATURE OF CONFIGURATIONS INDUCED BY IONIZING RADIATION

Wilson, G. B. (Michigan State U., East Lansing.), A. H. Sparrow, and Virginia Pond. Subchromatid rearrangements in Trillium erectum. I. Origin and nature of configurations induced by ionizing radiation. Amer. Jour. Bot. 46(4): 309–316. Illus. 1959.—Microsporocytes of Trillium erectum were x‐irradiated with 25 r at various stages of meiotic prophase and at first metaphase. Analysis of these cells at the following first and second anaphase revealed that post‐pachytene irradiation produces 2‐side‐arm bridges which are indicative of half‐chromatid exchanges. The occurrence of these bridges and knowledge of the structure and spatial relationship of chromatid strands in T. erectum have led to certain conclusions regarding the target and the number of strands broken by a single event: (1) the most likely target for primary effects is the 4 associated half‐chromatids of a half‐bivalent. The results of irradiation experiments suggest that the half‐bivalent is effectively as well as structurally quadripartite at stages following pachytene. (2) Consideration of the configurations which would result from breakage and rejoining of 2, 3 or all 4 strands of the half‐bivalent indicates that only 2 of the 4 half‐chromatids are broken by a single event. Exchanges between 2 half‐chromatids of sister chromatids will produce two recognizable types of 2‐side‐arm bridges: one with a true dicentric half‐chromatid and one in which the bridge results merely from an interlocking of coils. Whether a 2‐side‐arm bridge appears at first or second meiotic anaphase is determined by the position and number of chiasmata between the point of breakage and the kinetochore. No 2‐side‐arm bridges have been detected at microspore anaphase following meiotic prophase irradiation. The types of configurations which might be expected at microspore metaphase as a result of broken 2‐side‐arm bridges are noted.

Sub-Chromatid Rearrangements in Trillium erectum. I. Origin and Nature of Configurations Induced by Ionizing Radiation

Wilson, G. B. (Michigan State U., East Lansing.), A. H. Sparrow, and Virginia Pond. Subchromatid rearrangements in Trillium erectum. I. Origin and nature of configurations induced by ionizing radiation. Amer. Jour. Bot. 46(4): 309–316. Illus. 1959.—Microsporocytes of Trillium erectum were x-irradiated with 25 r at various stages of meiotic prophase and at first metaphase. Analysis of these cells at the following first and second anaphase revealed that post-pachytene irradiation produces 2-side-arm bridges which are indicative of half-chromatid exchanges. The occurrence of these bridges and knowledge of the structure and spatial relationship of chromatid strands in T. erectum have led to certain conclusions regarding the target and the number of strands broken by a single event: (1) the most likely target for primary effects is the 4 associated half-chromatids of a half-bivalent. The results of irradiation experiments suggest that the half-bivalent is effectively as well as structurally quadripartite at stages following pachytene. (2) Consideration of the configurations which would result from breakage and rejoining of 2, 3 or all 4 strands of the half-bivalent indicates that only 2 of the 4 half-chromatids are broken by a single event. Exchanges between 2 half-chromatids of sister chromatids will produce two recognizable types of 2-side-arm bridges: one with a true dicentric half-chromatid and one in which the bridge results merely from an interlocking of coils. Whether a 2-side-arm bridge appears at first or second meiotic anaphase is determined by the position and number of chiasmata between the point of breakage and the kinetochore. No 2-side-arm bridges have been detected at microspore anaphase following meiotic prophase irradiation. The types of configurations which might be expected at microspore metaphase as a result of broken 2-side-arm bridges are noted.

The determination of position in crossing-over

The positions of chiasmata at metaphase are dependent on (a) their positions at the time of formation, and (b) subsequent movement. These two factors may now be separated analytically.