We designed a system to determine whether dicentric chromosomes in Drosophila melanogaster break at random or at preferred sites. Sister chromatid exchange in a Ring-X chromosome produced dicentric chromosomes with two bridging arms connecting segregating centromeres as cells divide. This double bridge can break in mitosis. A genetic screen recovered chromosomes that were linearized by breakage in the male germline. Because the screen required viability of males with this X chromosome, the breakpoints in each arm of the double bridge must be closely matched to produce a nearly euploid chromosome. We expected that most linear chromosomes would be broken in heterochromatin because there are no vital genes in heterochromatin, and breakpoint distribution would be relatively unconstrained. Surprisingly, approximately half the breakpoints are found in euchromatin, and the breakpoints are clustered in just a few regions of the chromosome that closely match regions identified as intercalary heterochromatin. The results support the Laird hypothesis that intercalary heterochromatin can explain fragile sites in mitotic chromosomes, including fragile X. Opened rings also were recovered after male larvae were exposed to X-rays. This method was much less efficient and produced chromosomes with a strikingly different array of breakpoints, with almost all located in heterochromatin. A series of circularly permuted linear X chromosomes was generated that may be useful for investigating aspects of chromosome behavior, such as crossover distribution and interference in meiosis, or questions of nuclear organization and function.KEYWORDS dicentric chromosome; intercalary heterochromatin; fragile X; FLP D ROSOPHILA is noted for a combination of facile genetics and high-resolution cytology that allows the recovery and characterization of a variety of chromosome rearrangements. Simple structural changes such as duplications, deficiencies, inversions, and translocations are readily produced through a variety of classical and modern techniques. More complex rearrangements such as balancers, compound chromosomes, and ring chromosomes also have been generated. Ring chromosomes historically have been of interest for a number of unique properties, such as a propensity for loss during early embryonic mitoses and dominant zygotic lethality (Leigh 1976;Ashburner et al. 2005). Despite their "instability," ring chromosome stocks can be remarkably stable, with very few reported instances of spontaneous opening into linear chromosomes.In this work, we produced a number of linearized ring chromosomes. One goal was to determine whether there are preferred sites of breakage for dicentric chromosomes. Although we have previously shown that dicentric Y, 3, and 4 chromosomes can break and heal in the male germline and be recovered in offspring, these chromosomes allowed little opportunity to determine whether there were preferred sites of breakage (Ahmad and Golic 1998; Golic 2008, 2010;Titen et al. 2014). In the case of the Y chromosome, low r...

Tests of general and test anxiety were administered with a self-acceptance (SA)-acceptance of others (AO) questionnaire to 92 Ss to test the prediction that anxiety increased the usual SA-AO correlation. Results indicate that (a) anxiety is significantly associated with both lowered self-acceptance and lowered acceptance of others, (b) anxiety disrupts the SA-AO relationship by lowering self-acceptance at a greater rate than acceptance of others, (c) low anxiety permits the usual SA-AO correlation to exist. Conclusions were that (a) learning theory from which the prediction was derived is inadequate in self-theory areas of personality, (b) anxiety has a disruptive, yet systematic influence on the self-acceptance-acceptance of others relationship.

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Chromosome breakage plays an important role in the evolution of karyotypes, and can produce deleterious effects within a single individual, such as aneuploidy or cancer. Forces that influence how and where chromosomes break are not well understood. In humans, breakage tends to occur in conserved hotspots called common fragile sites (CFS), especially during replication stress. By following the fate of dicentric chromosomes in Drosophila melanogaster we find that breakage under tension also tends to occur in specific hotspots. Our experimental approach was to induce sister chromatid exchange in a ring chromosome to generate a dicentric chromosome with a double chromatid bridge. In the following cell division, the dicentric bridges may break. We analyzed the breakage patterns of three different ring-X chromosomes. These chromosomes differ by the amount and quality of heterochromatin they carry as well as their genealogical history. For all three chromosomes, breakage occurs preferentially in several hotspots. Surprisingly, we found that the hotspot locations are not conserved between the three chromosomes: each displays a unique array of breakage hotspots. The lack of hotspot conservation, along with a lack of response to aphidicolin, suggests that these breakage sites are not entirely analogous to CFS and may reveal new mechanisms of chromosome fragility.. Additionally, the frequency of dicentric breakage and the durability of their spindle attachment varies significantly between the three chromosomes and is correlated with the origin of the centromere and the amount of pericentric heterochromatin they carry. We suggest that different centromere strengths could account for this.

Chromosome breakage plays an important role in the evolution of karyotypes, and can produce deleterious effects within a single individual, such as aneuploidy or cancer. Forces that influence how and where chromosomes break are not fully understood. In humans, breakage tends to occur in conserved hotspots called common fragile sites (CFS), especially during replication stress. By following the fate of dicentric chromosomes in Drosophila melanogaster we find that breakage under tension also tends to occur in specific hotspots. Our experimental approach was to induce sister chromatid exchange in a ring chromosome to generate a dicentric chromosome with a double chromatid bridge. In the following cell division, the dicentric bridges may break. We analyzed the breakage patterns of three different ring-X chromosomes. These chromosomes differ by the amount and quality of heterochromatin they carry as well as their genealogical history. For all three chromosomes, breakage occurs preferentially in several hotspots. Surprisingly, we found that the hotspot locations are not conserved between the three chromosomes: each displays a unique array of breakage hotspots. The lack of hotspot conservation, along with a lack of response to aphidicolin, suggests that these breakage sites are not entirely analogous to CFS and may reveal new mechanisms of chromosome fragility. Additionally, the frequency of dicentric breakage and the durability of each chromosome’s spindle attachment varies significantly between the three chromosomes and is correlated with the origin of the centromere and the amount of pericentric heterochromatin. We suggest that different centromere strengths could account for this.