3h-actinomycin-D binding to mitotic chromosomes of Drosophila melanogaster

The trithorax group and Pc group proteins are differentially involved in heterochromatin formation in Drosophila

In Drosophila, the Polycomb group and trithorax group proteins play a critical role in controlling the expression states of homeotic gene complexes during development. The common view is that these two classes of proteins bind to the homeotic complexes and regulate transcription at the level of chromatin. In the present work, we tested the involvement of both groups in mitotic heterochromatin formation in Drosophila. Using specific antibodies, we show that some of the tested Pc-G proteins are present in heterochromatin, while all the tested trx-G proteins localize to specific regions of heterochromatin in both mitotic chromosomes and interphase nuclei. We also observed that mutations in trx-G genes are recessive enhancers of position-effect variegation and are able to repress the transcription of heterochromatic genes. These results strongly suggest that trx-G proteins, along with some Pc-G proteins, play an active role in heterochromatin formation in Drosophila.

Bonus, a Drosophila TIF1 homolog, is a chromatin-associated protein that acts as a modifier of position-effect variegation

Bonus, a Drosophila TIF1 homolog, is a nuclear receptor cofactor required for viability, molting, and numerous morphological events. Here we establish a role for Bonus in the modulation of chromatin structure. We show that weak loss-of-function alleles of bonus have a more deleterious effect on males than on females. This male-enhanced lethality is not due to a defect in dosage compensation or somatic sex differentiation, but to the presence of the Y chromosome. Additionally, we show that bonus acts as both an enhancer and a suppressor of position-effect variegation. By immunostaining, we demonstrate that Bonus is associated with both interphase and prophase chromosomes and through chromatin immunoprecipitation show that two of these sites correspond to the histone gene cluster and the Stellate locus.

Y-chromosome effects on Drosophila geotaxis interact with genetic or cytoplasmic background

Previously, all of the major fruit fly, Drosophila melanogaster, chromosomes (I, II and III) have been shown to be associated with geotaxis, but the Y chromosome has not. Using two methods (back-crossing and chromosome substitution), Y chromosomes from lines that have evolved stable, extreme expressions of geotaxis were placed into different genetic and cytoplasmic backgrounds to test the resulting males for geotaxis. The results of the back-crossing do not support the interpretation of Y-chromosome effects on geotaxis. These tests do not have sufficient statistical power, however, to detect small genetic effects. In the chromosome substitution experiment, the geotaxis-line Y chromosomes were placed into high- and low-selected lines, Canton-S and Champaign wild-type backgrounds. The results of the chromosome substitution experiment provide evidence for a Y-chromosome effect on geotaxis in selected geotaxis lines, but not in wild-type stock, backgrounds. These results suggest that the Y chromosome has a small effect on geotaxis, whose detection depends on genetic and/or cytoplasmic background. The implications of these results are discussed for behaviour genetic analysis of D. melanogaster and for issues of statistical power in detecting small genetic effects.

Interaction systems between heterochromatin and euchromatin in Drosophila melanogaster

The constitutive heterochromatin is still one of the major unsolved problems in genetics. In Drosophila melanogaster three genetic systems involving specific interactions between heterochromatic and euchromatic genetic elements are known: the Segregation Distortion, the crystal-Stellate and the abo-ABO systems. The genetic and molecular analysis of each system will allow the identification of all the components and the elucidation of the mechanisms underlying their interactions. The results of this analysis should provide insights into the biological significance of heterochromatin and into the evolutionary forces that result in the maintainance and stability of this enigmatic genetic material.

Looking at Drosophila mitotic chromosomes

The repertoire of cytological procedures described in the present paper permits full analysis of brain neuroblast chromosomes. Moreover, if brains are cultured for 13 hr in the presence of 5-bromo-2'-deoxy-uridine, our fixation and Hoechst staining protocols allow visualization of sister chromatid differentiation and the scoring of sister chromatid exchanges (Gatti et al., 1979). Finally, we note that our cytological procedures can be successfully employed for preparation and staining of gonial cells of both sexes and male meiotic chromosomes (Ripoll et al., 1985; our unpublished results). Good chromosome preparations of female meiosis are obtained with the procedure described by Davring and Sunner (1977, 1979), Nokkala and Puro (1976), and Puro and Nokkala (1977). In this chapter, we have focused on the organization and behavior of Drosophila mitotic chromosomes, describing a repertoire of cytological techniques for neuroblast chromosome preparations. We have not considered the numerous excellent cytological procedures for embryonic chromosome preparations (for an example, see Foe and Alberts, 1985; Foe, 1989), because these chromosomes are usually less clearly defined than those of larval neuroblasts. In addition, we have not included the whole-mount and squashing techniques that allow chromosome visualization and spindle immunostaining of neuroblast cells (Axton et al., 1990; Gonzalez et al., 1990), male meiotic cells (Casal et al.. 1990; Cenci et al., 1994), and female meiotic cells (Theurkauf and Hawley. 1992), because the fixation methods used in these procedures alter chromosome morphology. Fixation methods for antibody staining result in poorly defined chromosomes, whereas the methanol/acetic acid fixation techniques, such as those described here, preserve very well chromosome morphology but remove a substantial fraction of chromosomal proteins. Thus, one of the major technical breakthroughs in Drosophila mitotic cytology will be the development of fixation procedures that maximize chromosomal quality with minimal removal of proteins. This will be particularly useful for precise immunolocalization of heterochromatic proteins, including those associated with the centromere.

Studies on corneal endothelial growth and repair. II. increased transcription as detected by incorporation of 3H-uridine and 3H-actinomycin D

Endothelial cells from injured frog corneas undergo increased 3H-uridine and 3H-actinomycin D (3H-AMD) incorporation as judged by autoradiography. The increase in 3H-AMD binding occurs when living endothelium is labeled in vitro or when fixed preparations are exposed to the drug. The changes in 3H-AMD incorporation detected by the two methods are comparable (55 and 62% for living and pre-fixed tissue respectively). However, when fixed endothelium is also de-histonized with 2 N HCl, differential binding of 3H-AMD is eliminated. This result suggests that the enhanced incorporation of 3H-AMD into nuclei is at least partly due to a modification in the association of chromosomal proteins with DNA and not entirely to cell permeability changes that may accompany wound repair. This contrasts with observations of cells that are killed outright by the injury. Such cells bind very large amounts of 3H-AMD compared with their living neighbors. Here the difference in incorporation is eliminated by prefixation. Thus, in the dead cells increased binding may be due to a reduction of cell surface permeability barriers which accompanies cell morbidity.