Heterochromatin Content Research Articles

MACROPHAGE-MELANOCYTE HETEROKARYONS

Mouse peritoneal macrophages, which do not synthesize DNA in vitro, were fused with melanocytes, a mouse cell strain which proliferates rapidly in vitro. DNA synthesis was induced in macrophage nuclei 2-3 hr after fusion and occurred irrespective of the number of macrophage nuclei present per melanocyte nucleus in each heterokaryon. 50-80% of macrophage nuclei initiated DNA synthesis in the 3-7 hr period after fusion. The activation of most 11-12-day chick red cell nuclei in melanocyte cytoplasm took longer than 10 hr. The lag before DNA synthesis may reflect the heterochromatin content of each nucleus. Studies with actinomycin showed that heterokaryon RNA synthesis was essential for subsequent macrophage DNA synthesis. This RNA was synthesized 1-4 hr before the DNA and was unlikely to be ribosomal RNA, since it was insensitive to <0.1 microg/ml actinomycin. Melanocytes and macrophages were treated before fusion with actinomycin and bromotubercidin to bring about a more selective inhibition of RNA synthesis. Macrophages pretreated for 1 hr with 5 microg/ml of actinomycin showed less than 20% of control RNA synthesis in the first 4 hr after fusion, but a normal activation of macrophage DNA synthesis. Pretreatment of melanocytes for 3-7 hr with 5 microg/ml bromotubercidin, a reversible inhibitor of RNA synthesis, prevented macrophage DNA synthesis without affecting macrophage RNA synthesis in the heterokaryons (81% of control). These studies showed that only melanocyte RNA synthesis was essential for the production of macrophage DNA. The exposure of one cell partner to actinomycin before fusion caused cross-toxicity of the untreated nucleus after fusion. Bromotubercidin, an adenosine analogue which is incorporated into RNA, did not give rise to such cross-toxicity after fusion. Once the macrophage nucleus becomes activated in the heterokaryon it becomes less sensitive to the action of actinomycin.

A terminal association of two pericentric inversions in first metaphase cells of the Australian grasshopper Austroicetes interioris (Acrididae).

Austroicetes interioris is polymorphic for pericentric inversions. The three autosomal polymorphisms are considered to be heterotic. In each case the chromosome with the inversion differs from its standard counterpart in the amount of heterochromatin present. Consequently the various karyotypes have appreciable diversity in heterochromatin content. Two of the inversion chromosomes form a terminal association considered to be chiasmate in nature. The resulting quadrivalents favour one particular first metaphase orientation and this causes segregation distortion. The terminal associations and the heterochromatin disparity between the members of each polymorphism are considered to be due to translocations with break points situated in regions of little genetic activity near chromosome tips and causing interchange of telomeres but not of euchromatic segments. Evolutionary implications of such rearrangements are discussed.

Genetic and environmental control of melanotic tumors indrosophila

These investigations are concerned with the expression of the genemt which is responsible for the development of pseudotumors depending on different internal and external conditions. Chromosomal rearrangements such as inversions are powerful modifiers ofmt. The Y chromosome has a strongly suppressing effect which is evidently due to its heterochromatin content and particularly to its nucleolus organizer. The view is presented that inversions inDrosophila populations can be considered as “master mutations” which by themselves change the expression of a large number of genes affecting quantitative characters of ecological importance. High temperature decreases, low temperature increases the percentage of tumorous flies. There is also a profound effect of nutrition on the percentage of tumorous individuals, some substances, especially certain amino acids, furthering the development of tumors, other amino acids suppressing the development of tumors markedly. There are parental effects on the expression ofmt produced by genetic as well as environmental conditions; they may be more strongly paternal than maternal. There are also grandparental effects due to genetic as well as environmental influences. It was not tested if the extrachromosomal transmission of the changedmt expression lasts beyond the second generation. In a discussion of the results it is argued that a slight change in the entire system of message transmission, such as the RNA content of the cell, is more important in terms of differentiation and evolution than the profound change in an individual message due to the mutation of a single gene or DNA particle.

Spontaneous chromosome breakage in hybrid endosperms.

1. The behaviour of the chromosomes ofTrillium grandiflorum, T. cernuum, T. erectum andT. luteum (all 2n=10) was studied in cold-treated endosperms obtained from intergeneric fertilizations betweenParis quadrifolia (2n=20), where they are readily distinguishable fromParis chromosomes by their heterochromatin content. 2. Spontaneous breakage ofTrilliumchromosomes occurs in endosperms of the crossT. grandiflorum ×Paris quadrifolia which is only a little higher in frequency than that found in pureTrillium endosperms. 3. InParis ×Trillium endosperms the following abnormalities appear: a) Spontaneous chromosome-breakage inTrillium chromosomes, b) Loss ofTrillium chromosomes through delayed arrangement on the spindle. c) Delayed anaphase separation ofTrillium chromosomes, d) Cessation of the cold effect in heterochromatic segments ofTrillium chromosomes in metaphases of older endosperms. (e) Upset in spindle formation. 4. The breakage ofTrillium chromosomes in endosperms fromParis-Trillium crosses varies between 18.1 and 35.5%. TheParis chromosomes are only slightly broken if at all. 5. Breakage is distributed unevenly over the five types ofTrillium chromosomes and is concentrated in particular regions of chromosomes. 6. The frequency of breakage in corresponding chromosomes is higher where more heterochromatic segments occur. From this it is concluded that there is a genetic control of spontaneous chromosome breakage by the heterochromatin. 7. Both endosperm and seed development collapses in all crosses. Degeneration is earlier in theTrillium-Paris crosses where spontaneous chromosome breakage is relatively low. Spontaneus chromosome breakage therefore may not necessarily always be the cause of endosperm degeneration.