Interchromosomal Effects of Heterochromatic Deletions on Recombination in DROSOPHILA MELANOGASTER

It is now known that partial deletions of the satellite sequences in X-chromosome heterochromatin result in a significant decrease in intrachromosomal recombination in the proximal region of the X chromosome of D. melanogaster (YAMAMOTO and MIKLOS 1978). It is important to ask then if the loss or gain of heterochromatin on the X also alters recombination in other chromosomes of the genome (interchromosomal effects). I have looked for such alterations by measuring recombination in chromosome 3. The results clearly indicate that the partial loss of X-chromosome heterochromatin not only decreases crossing over in the proximal region of the X chromosome itself, but also increases the frequency in chromosome 3, especially in the euchromatic regions around the centromere. Furthermore, the greater the deficiency of X heterochromatin, the higher is recombination in chromosome 3. This finding not only provides further evidence in support of the hypothesis that heterochromatin, in this case mainly composed of satellite DNA, regulates the recombination system, but it demonstrates that when the satellite content of one chromosome of the D. melanogaster genome is altered, there is an alteration in the crossover characteristics of other chromosomes in the same complement. If the amount of satellite DNA in a genome is being continuously altered, then one can predict that the recombination system is also being continually perturbed. Thus, the changing gene combinations produced indirectly by increases or decreases of heterochromatin are among the components available to organisms to break up or form new gene combinations upon which selection can act.     

Heterozygous Effects on Fitness of Ems-Treated Chromosomes in DROSOPHILA MELANOGASTER

The heterozygous effects on fitness of second chromosomes carrying mutants induced with different doses of EMS were ascertained by monitoring changes in chromosome frequencies over time. These changes were observed in populations in which the treated chromosomes, as well as untreated competitors, remained heterozygous in males generation after generation. This situation was achieved by using a translocation which links the second chromosome to the X chromosome; however, only untranslocated second chromosomes were mutagenized. Chromosomes were classified according to their effects on viability in homozygous condition. A preliminary homozygosis identified completely lethal chromosomes; secondary tests distinguished between drastic (viability index < 0.1) and nondrastic chromosomes. Chromosomes that were nondrastic after treatment were found to reduce the fitness of their heterozygous carriers by 3-5%. The data show that flies homozygous for these chromosomes were about 2.7% less viable per treatment with 1 mm EMS than flies homozygous for untreated chromosomes. By comparing the fitness-depressing effects of nondrastic EMS-induced mutants in heterozygous condition with the corresponding viability-depressing effects measured by Temin, it is apparent that the total fitness effects are several times larger than the viability effects alone. Completely lethal chromosomes derived from the most heavily treated material reduced fitness by 11% in heterozygous condition; approximately half of this reduction was due to the lethal mutations themselves.     

Heterozygous Effects of Irradiated Chromosomes on Viability in DROSOPHILA MELANOGASTER

Two large experiments were conducted in order to evaluate the heterozygous effects of irradiated chromosomes on viability. Mutations were accumulated on several hundred second chromosomes by delivering doses of 2,500r over either two or four generations for total X-ray exposures of 5,000r or 10,000r. Chromosomes treated with 5,000r were screened for lethals after the first treatment, and surviving nonlethals were used to generate families of fully treated chromosomes. The members of these families shared the effects of the first irradiation, but differed with respect to those of the second. The chromosomes treated with 10,000r were not grouped into families since mutations were accumulated independently on each chromosome in that experiment. Heterozygous effects on viability of the irradiated chromosomes were tested in both isogenic (homozygous) and nonisogenic (heterozygous) genetic backgrounds. In conjunction with these tests, homozygous viabilities were determined by the marked-inversion technique. This permitted a separation of the irradiated chromosomes into those which were drastic when made homozygous and those which were not. The results indicate that drastic chromosomes have deleterious effects in heterozygous condition, since viability was reduced by 2–4% in tests performed with the 10,000r chromosomes, and by 1% in those involving the 5,000r material. Within a series of tests, the effects were more pronounced when the genetic background was homozygous. Nondrastic irradiated chromosomes did not show detectable heterozygous effects. They also showed no homozygous effects when compared to a sample of untreated controls. In addition, there was no evidence for an induced genetic component of variance with respect to viability in these chromosomes. These results suggest that the mutants induced by high doses of X-rays are principally drastic ones which show deleterious effects on viability in heterozygous condition.     

Heterochromatic Recombination in Germ Cells of DROSOPHILA MELANOGASTER Females

Heterochromatic recombination in germ cells was found to occur in females of Drosophila melanogaster having a specific genotype. Results of the present study can be summarized as follows: (1) The frequency of heterochromatic recombination descreases consistently and markedly as the female ages. (2) The female that induces heterochromatic recombination is associated with reduced number of progeny when she is young, but as she gets older, the number of progeny increases, approaching that of the normal female. The reduction in the number of progeny is due to unhatchability of eggs produced, not to reduced egg laying. (3) Cytoplasmic factors affect the above two traits. These traits seem to be due to interaction between chromosomal and cytoplasmic elements. (4) These traits are not expressed in males. (5) The increase in recombination frequency seems to be limited to the centric heterochromatin.—It is suggested that heterochromatic recombination is one of the traits associated with the I-R system of hybrid dysgenesis in D. melanogaster.     

An Effect of Centromere Function on the Behavior of Ring-X Chromosomes in DROSOPHILA MELANOGASTER

It is shown that under the influence of an autosomal meiotic mutant that causes abnormalities in meiotic centromere function (mei-S332), ring-X chromosomes are frequently nonrecoverable. Evidence is presented that this nonrecoverability is caused by a failure of sister ring-chromatids to successfully effect an equational separation with resultant dominant lethality. Because mei-S332 results in meiotic abnormalities only after replication has been completed, and because ring chromosomes are normally transmitted with approximately the same efficiency as rod chromosomes, it is suggested that during replication in normal meioses, sister ring-chromatids form mutually interlocked ring complexes that are resolved without genetic consequences at anaphase II, with the resolution owing at least in part to normal centromere function.     

The Genetic Identification of a Heterochromatic Segment on the X Chromosome of DROSOPHILA MELANOGASTER

An autosomal euchromatic maternal-effect mutant, abo (= abnormal oocyte), interacts with, or regulates the activity of, the heterochromatin of the sex chromosomes of Drosophila melanogaster. It is shown that this interaction or regulation with the X chromosome involves a specific heterochromatic locus or small region that maps to the distal penultimate one-eighth of the basal X-chromosome heterochromatic segment.     

Mitotic Recombination in the Heterochromatin of the Sex Chromosomes of DROSOPHILA MELANOGASTER

The frequency of spontaneous and X-ray-induced mitotic recombination involving the Y chromosome has been studied in individuals with a marked Y chromosome arm and different XY compound chromosomes. The genotypes used include X chromosomes with different amounts of X heterochromatin and either or both arms of the Y chromosome attached to either side of the centromere. Individuals with two Y chromosomes have also been studied. The results show that the bulk of mitotic recombination takes place between homologous regions.