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The polytene and mitotic chromosomes of D. melanogaster, D. simulans, D. ananassae, and D. virilis were stained with the fluorescent dye, quinacrine mustard (QM). In all these species except D. ananassae, we have detected species-specific chromosomal loci which exhibit an extremely brilliant fluorescence. Most, but not all, of the brilliantly fluorescing areas are located in heterochromatic chromosome regions. Cytochemical and chemical methods have been employed to demonstrate that the brilliant fluorescence represents regions of acid labile non-covalent binding between DNA and QM whereas the moderate overall fluorescence is primarily due to covalent bonding (by alkylation) of the QM to DNA. The exact mode of binding of QM in the brilliant areas and the nature of the DNA in these areas are not known. The possible biological significance of the DNA in the brilliant regions is discussed.

Two-wavelength Feulgen microspectrophotometry was used to determine the DNA content of mitotically-active ganglionic cells of first-and thirdinstar larvae of Drosophila melanogaster. The measurements revealed that the DNA values differ, on the average, by a factor of approximately two, with the metaphase cells of the first-instar larvae having about four times the haploid amount of the spermatozoon, and the metaphase cells of the third-instar larvae having about eight times the haploid amount. The increase from 4C to 8C in the course of development without any pronounced modification of the heterochromatic???euchromatic ratio is interpreted as evidence of an increase in the number of chromosomal strands. It is suggested, accordingly, that these mitotically-active chromosomes are multistranded or polynemie.

Analysis of reconstructions, prepared from electron micrographs of successive longitudinal serial sections, has led to the conclusion that the somatic telophase chromosome of Tradescantia paludosa contains four cytologically separable chromonemata. The four represent a pair of pairs, that is, two diplospiremes??? one with its two chromonemata arranged helically in dextrorse relationship, and the other with its two in sinistrorse relationship??? which are associated to form a tetraspireme. During anaphase and telophase the tetraspireme constitutes the chromosome; during prophase and metaphase the tetraspireme represents one of the two chromatids of the chromosome, which is accordingly an octospireme in terms of the number of cytologically identifiable chromonemata. Loose intertwining of the two tetraspiremes during late prophase accounts for the so-called relational coiling.

When the fourth chromosome of Drosophila melanogaster is attached, as the result of an induced translocation, to 21A in 2L or 60E in 2R, its tip exhibits a marked capacity to pair end-to-end with the tips of the other chromosomes. In each of the translocations, about 59 per cent of the contacts involving the tip of 4 were with the tip of X. If this pairing preference reflects structural similarity, the tip of 4 is much more like the tip of X than that of any other chromosome. The significance of this phenomenon is discussed with respect to the standard pattern of end-to-end association in the Oregon-R wild-type stock that provided the control preparations. In the 4-2L rearrangement, the interaction of chromosome 4 with the tip of 2L (101E with 21A) led to pronounced puffing in subdivisions 21A and B, as was most strikingly manifested when the distal segments of 2L failed to synapse and the homologue of paternal origin showed a large puff whereas that of maternal origin (not carrying the fourth chromosome) remained unpuffed.

Several variants resistant to 1.8 x 10(-4) M DL-methotrexate (MTX) have been isolated from the human cell lines HeLa BU25 and VA2-B by exposing them to progressively increasing concentrations of the drug. A striking variability of phenotype and chromosome constitution was observed among the different variants. All resistant cell lines exhibited a greatly increased dihydrofolic acid reductase (DHFR) activity and DHFR content; however, the DHFR activity levels varied considerably among the variants, ranging between about 35 and 275 times the parental level. In the absence of selective pressure, the increased DHFR activity was unstable, and in all cell lines but one was completely lost over a period ranging in different variants between 25 and 200 days. The MTX-resistant cells lines showed anomalies in their chromosome constitution, which involved the occurrence of a duplicated set of chromosomes in most cells of some of the variants and the presence of double minute chromosomes in all cell lines. An analysis of the correlation of loss of double minute chromosomes and loss of DHFR activity in the absence of MTX has given results consistent with the idea that the double-minute chromosomes contain amplified DHFR genes. However, the most significant finding is that, in contrast to what has been reported in the mouse system, the recognizable double-minute chromosomes varied greatly in number in different variants without any relationship to either the level of DHFR activity or the degree of instability of MTX resistance in the absence of selective pressure. These and other observations point to the occurrence in the human MTX-resistant variants of another set of DHFR genes, representing a varied proportion of the total, which is associated with the regular chromosomes, and which may be unstable in the absence of selective pressure.