Dot Chromosome Research Articles

Characterization of Heterochromatin in Three Species of the Montium Subgroup of Drosophila.

The heterochromatin of 3 species of the montium subgroup of Drosophila, namely D. nagarholensis, D. jambulina, and D. kikkawai was analyzed. Within the basic karyotypic configuration of 2n = 8, interspecies differences in the pattern and the intensity of C- and Q-band heterochromatin particularly of the Y and dot chromosomes were recorded. [on SciFinder(R)]

Wrinkling of the eye in hybrids between Drosophila virilis and Drosophila lummei is caused by interaction of maternal and zygotic genes

Hybrids of Drosophila virilis females and Drosophila lummei males have visible developmental anomalies in external adult structures. Reciprocal hybrids are normal, and the anomalies are not found in other interspecific F1 hybrids of the virilis subgroup. Antibiotic treatments with ampicillin, streptomycin and tetracycline did not cure the syndrome. The genetic basis of the wrinkling of the compound eye was analysed in detail. Both male and female F1 progeny of D. virilis mothers were affected, indicating that wrinkling of eyes is maternally determined. F1 hybrid females produced healthy progeny. Backcross hybrid females (D. virilis x F1) with 16 different combinations of autosomes, either heterozygous or homozygous for D. virilis, were crossed with D. lummei males. It was found that homozygous D. virilis chromosomes 2 and 5 together were necessary to induce the maternal effect. The hybrid zygote responded to the maternal effect when the chromosomes 2, 4 and/or 5 were heterozygous. Elimination of the small sixth (dot) chromosome of D. lummei from the hybrids was correlated with the wrinkling of eyes, but was not the cause, as the two phenomena had a different genetic basis and temperature response. Furthermore, the eyes were wrinkled in flies which presumably had had no D. lummei dot chromosome to lose.

The genetic basis of a species-specific character in the Drosophila virilis species group.

The genetic basis of the species-specific dorsal abdominal stripe of Drosophila novamexicana was examined. The dorsal stripe is present in D. novamexicana and absent in all other members of the Drosophila virilis species group. Interspecific crosses between D. novamexicana and genetically marked D. virilis revealed that all four of the autosomes (except the tiny dot chromosome, which was not marked) and the sex chromosomes (the X and Y chromosome effects could not be disentangled) showed a significant effect on the width of the dorsal stripe. All the autosomes act approximately additively; only minor interactions were detected among them. No significant maternal effects were found. This means that a minimum of five loci are involved in the character difference between the two species, and this is the maximum number that this technique could discern. These results suggest that, based on the number of factors involved in the character difference, the inheritance of this character should be considered polygenic, but because chromosome 2 (the largest chromosome in the species) contributed over half of the variance toward the character difference, it is best to consider the inheritance oligogenic based on effect. The implications of these findings are discussed in light of the importance of macromutation in speciation and the sex chromosome theory of speciation.

Inversion Polymorphism in Drosophila obscura

The inversion polymorphism of Drosophila obscura Fallen, a European species of the obscura group of the subgenus Sophophora, is described. A total of 21 inversions have been recorded; they are located in the five large chromosomes of the species (a dot chromosome is also present) and form 25 gene arrangements present in the species' natural populations. Strains from five different countries were studied. Two of these inversions were found to be pericentric, and the remaining were paracentric. The presence of "hot" points (multibreaks) was noticed. The distribution of the relative lengths of inversions conformed to the Van Valen-Levins distribution, contrary to what happens in D. subobscura. Observations also showed that there is no crossover inhibition between nonoverlapping inversions. The phylogeny of chromosome C was reconstructed; the existence of several arrangements not found was postulated; and the primitive gene arrangement, linking D. obscura to its closely related species was identified, as well as the primitive gene arrangements of the other chromosomes. Photographic maps of the chromosomes of D. obscura are provided here.

Chromosome variation in callus cells of Luzula elegans Lowe with nonlocalized kinetochores.

Variation of chromosome number and constitution in the callus cells of Luzula elegans Lowe (syn. L. purpurea Link.) (2n=6) was investigated. The chromosomes of the plant have nonlocalized kinetochores. Callus was induced and subcultured on the basal medium of Linsmaier and Skoog (1965) with 3.0mg/l of 2, 4-D and 300mg/l of casein hydrolysate, and with 3.0mg/l of 2, 4-D and 1g/l of yeast extract. The ratio of increase in fresh callus weight was about three and a half times after two months of subculture on the medium with casein hydrolysate. The ratio of increase on the medium with yeast extract was larger than that on the medium with casein hydrolysate. Aneuploid cells and new type chromosomes were observed. Most new type chromosomes were medium-sized fragment and dot chromosome.

Localization and characterization of rDNA in Drosophila tumiditarsus.

Using in situ nucleic acid hybridizations, the genes that code for 28, 18 and 5S rRNA have been localized in the polytene chromosomes of Drosophila tumiditarsus. The 5S genes are found at a single site near the centromere of the second chromosome, whereas the 28 and 18S genes are found at the nucleolar organizer region of the dot chromosome. The dot chromosome has been previously described as alpha-heterochromatic. However, our cytochemical and autoradiographic results do not support such a conclusion. The autoradiographic results reveal that the dot chromosome is transcriptionally active and is not late-replicating, as is expected of alpha-heterochromatin. Further, the dot chromosomes possess none of the usual staining characteristics of heterochromatin except for its lack of polytene bands. Using rRNA-DNA filter hybridizations, we find that the rDNA of D. tumiditarsus salivary glands is under-replicated. This is the first species of Drosophila where the rDNA in not found on the sex chromosomes, and is the first report of an under-replicated autosomal locus which is not located in heterochromatic blocks.

The effect of dexamethasone on the karyotype of A9HT cells.

In the majority of glucocorticoid-resistant receptor containing variants of A9HT cells, the numbers of dot chromosomes was found to be significantly higher than in the parent cells. The higher number of dot chromosomes, however, did not always correlate with the resistance of L cells to dexamethasone. The formation of dot chromosomes was not the consequence of dexamethasone-induced chromosome breaks. Dot-chromosome formation could not be induced by dexamethasone in human diploid cells. It seems likely that dexamethasone can select for A9HT cells with higher number of dot chromosomes.

Biologie Évolutive du Genre Zaprionus Recherches sur le Complexe Spécifique de Z. Tuberculatus [Dipt. Drosophilidae

SummaryZaprionus sepsoides, which has been synonymized with Z. tuberculatus since 1948, is actually a separate species. Both species and the newly discovered Z. mascariensis form a very homogeneous complex in external morphology. The two former species are widespread on the African continent, while the third one is endemic in Mauritius and La Réunion.In four kinds of savannah biotopes investigated in Ivory Coast, tuberculatus and sepsoides were found in rather constant relative frequencies, sepsoides being more abundant. On the other hand, observed relative frequencies were strongly dependent on the fruit species which was used as a trap bait.Important morphological differences are found in internal organs and eggs. Development time is considerably shorter, and stature is smaller, in tuberculatus than in sepsoides; also, male forceps and female ovipositor bristles numbers are significantly different. A scanning electron microscope study has shown differences in fine structures of the phallus and integument and has given information on the nature of Zaprionus dorsal longitudinal stripes.Both tuberculatus and sepsoides have 2N = 12 chromosomes. There is a heterochromatic Y. Dot chromosomes appear banded in tuberculatus and unbanded in sepsoides larval salivary gland smears.From the various results obtained we conclude that the Zaprionus tuberculatus complex is of real interest for studies on both speciation processes and population dynamics.

Localization of constitutive heterochromatin in the chromosomes of Drosophila virilis.

The proximal regions of the rod-shaped autosomes and the X-chromosome of Drosophila virilis were shown to be heterochromatic. The Y-chromosome appeared to be entirely heterochromatic while the tiny dot chromosomes showed no detectable heterochromatic regions.

Studies on the satellite DNAs of Drosophila nasutoides: Their buoyant densities, melting temperatures, reassociation rates and localizations in polytene chromosomes

The karyotype of Drosophila nasutoides reveals a very large autosome pair at the metaphase plate. The application of the C-banding technique shows that this chromosome is almost entirely heterochromatic and an isochromosome (Cordeiro et al., 1975). Examination of the DNA isolated from purified nuclei of D. nasutoides in neutral CsCl gradients reveals four major satellites. As much as 60% of the total DNA appears as satellites in the DNA from larval brains. The buoyant densities of the four satellites, designated as I through IV in the order of descending density, are 1.687, 1.682, 1.669 and 1.665 g/cm 3, respectively. All four satellites show strand separations in alkaline CsCl gradients with the least separation in satellite III. Thermal denaturation studies with purified native satellites show that satellites I and IV consist of repeats of identical sequences, whereas satellites II and III show a large sequence variation between repeating units. As much as 10 to 24% base-pair mis-matching is observed in the reassociated satellite II. The sequence complexities obtained from DNA reassociation kinetics data are 5, 103, 2.3 × 10 6 and 46 nucleotide pairs for the satellites I, II, III and IV, respectively. The complexity of satellite III is almost as large as that of Escherichia coli, when the reassociation rate is corrected according to the amount of mis-matching in this satellite. All four satellite sequences are localized in one chromosome (dot chromosome) according to in situ hybridizations to polytene chromosomes. The large heterochromatic chromosome seen at the metaphase plate appears as the dot chromosome after polytenization. Therefore, the large heterochromatic chromosome contains all four satellite DNA components.

Heterochromatic chromosomes and satellite DNAs of Drosophila nasutoides

Drosophila nasutoides is distinguished from other Drosophila species in that the metaphase karyotype shows a pair of very large V-shaped chromosomes. With Giemsa, a distinctive C-banding pattern is revealed along the arms of this large chromosome, indicating a largely heterochromatic nature. Furthermore, the banding patterns of the arms are symmetrical, indicating that it is an iso-chromosome. A comparison between the metaphase karyotype and polytene chromosomes suggests that the large V chromosome appears as the dot chromosome in polytene squash. One autosome has twice the arm length of typical Drosophila polytene chromosomes and arose either by centric fusion and a pericentric inversion, or by translocation connecting distal ends with a subsequent loss of one centromere. This chromosome appears to have a short arm which ectopically pairs with the proximal region of the long arm, representing a duplication of about ten bands. When the nuclear DNA is examined by neutral CsCl gradient, four satellites are observed. As much as sixty percent of the total DNA appears as satellites in the lysate of larval brains. No satellite was detectable in the lysate of salivary glands. These observations led us to suggest that the heterochromatic nature of the large V chromosome is due to the presence of all four satellites in this chromosome and that this large chromosome appears as the dot because of the under-reduplication of the satellites during polytenization.

Karyotypes and Chromosome Polymorphism in Brazilian Rodents

SUMMARYKaryotypes of ten species of Brazilian rodents, collected in the State of Sao Paulo are presented: Akodon (Thaptomys) nigrita nigrita (2 n = 52); Zygodontomys lasiurus (2 n = 34); Thomasomys (Delomys) dorsalis collinus (2 n = 82); Oxymycterus sp1. and Oxymycterus sp2. (2 n = 54); Calomys sp. (2 n = 66); Clyomys laticeps laticeps (2 n = 34); Euryzygomatomys guiara (2 n = 46); Proechimys iheringi iheringi (2 n = 62, 64) and Echimys sp. (2 n = 90).The analysis of 30 specimens of Zygodontomys lasiurus disclosed a diploid number of 34 chromosomes. In three female specimens the length of one X chromosome was half the length of the other, due to a deletion on the long arm. In this case, the probably normal X chromosome presents a subterminal centromere and is morphologically identified.A heteromorphic autosomal pair was found in Oxymycterus sp1. and Oxymycterus sp2.In Proechimys iheringi iheringi the male had a pair of dot chromosomes less than the female; such chromosomes might be centric fragments resulting from reciprocal translocations which behave as supernumeraries.

THE NUCLEAR CYTOLOGY OF SIROGONIUM1,2

SUMMARYThe nuclear cytology of 9 strains of Sirogonium is described. The interphase nucleus contains 1–3 nucleoli, a nucleolar‐organizing track, and many (>100) chromocenters. During the division cycle the nucleoli are transformed into a nucleolar substance which becomes associated with the chromosome and is transported through mitosis on the chromosomes. All strains possess minute dot chromosomes varying in length at metaphase from 0.5 to 1.5 μ; satellite chromosomes are 2.5–3.5 μ long. The number of chromosomes varies from 48 ± 2 to 100 ± 2. No evidence of centromeric activity was observed.

Comparative morphology of nucleolar DNA in Drosophila. II. Drosophila fulvimaculoides and Drosophila tumiditarsus.

AbstractThis paper reports the demonstration, using fluorescence microscopy, of nucleolar DNA in two species of Drosophila. In Drosophila fulvimaculoides, the nucleolar DNA presents a variable morphology, suggestive of puffing activity. This material, which sometimes shows a banded structure like that of the polytene chromosomes, is shown not to be coextensive with the Y chromosome. Nucleolar DNA is demonstrated in Drosophila tumiditarsus also, and previous reports of an association of the dot chromosome with the nucleolus in this species are confirmed. The special usefulness of these two species for various sorts of investigation in pointed out.

Centric pairing and crossing-over in Drosophila melanogaster.

This cyto-genetic disproportionality has fostered the widespread inference that the centric and medial regions of the visible chromosome have a differential organization that influences their ability to undergo exchange. Such a view is in harmony with the concept that the centric regions are genetically “inert,” which has developed out of their heterochromatic properties. MATHER, however, has demonstrated (MATHER 1939) that centric heterochromatin undergoes crossingover more frequently when attached medially, and that proximity to the centromere itself is a deterrent to exchange. This is in agreement with BEADLE’S earlier observation (BEADLE 1932) that distal material attached near a centromere by translocation undergoes a reduction in exchanges, and with BEADLE’S interpretation of a “centromere effect” on crossing-over. A special case of the disparity of physical and genetic length is chromosome 4 of D. melanogaster. This is a small chromosome, often called a “dot chromosome” because of its appearance in somatic metaphase. Nonetheless, its cytological length is such that compared with regions of equal size in other chromosomes it should produce in the neighborhood of one to five percent crossovers. Actually, however, crossing-over in this pair of chromosomes almost never occurs in diploids.

Genetic studies in <i>Drosophila ananassae</i>

This lecture* is a comprehensive summary of all the genetic studies concerned with Drosophila ananassae carried out by the writer and his co-workers since 1931 when the flies were found by chance in Tokyo. It was presented under three major headings, (a) mutants and gene analysis, (b) experimental studies, and (c) studies on population genetics.Genetic characteristics of this species are as follows:(1) The fourth chromosome is considered to correspond to the dot chromosome of D. melanogaster or other species of the same group, composed of a bit of euchromatin and a large proportion of heterochromatin, the latter of which occupies major part of the V-shaped body. Further, it is partly identical to some parts of X- and Y-chromosomes, causing such a special allelism of bobbed locus that +bb in Y is allelic with bb in IV and not in X.(2) In both the second and third chromosomes, except a few dominant genes, all other mutants found belong to the left arms of the chromosomes. The interpretation that the mutability of genes differs in the two arms is preferable.(3) Power to enhance crossing over was found in the right arm of the second chromosome. The enhancer is able not only to increase the female crossing over somewhat, but also to induce male crossing over. The same kind of enhancer has been known in the third chromosome too.It is added that this species is a cosmopolitan and domestic species and that the flies frequently carry various types of chromosomal rearrangements in the natural population.

ON THE DOT CHROMOSOMES OF DROSOPHILA REPLETA AND D. HYDEI

ON THE DOT CHROMOSOMES OF DROSOPHILA REPLETA AND D. HYDEI