The association of acrocentric chromosomes in 1000 normal human male metaphase cells

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65834/1/j.1469-1809.1967.tb00544.x.pd

Late Replication Domains in Polytene and Non-Polytene Cells of Drosophila melanogaster

In D. melanogaster polytene chromosomes, intercalary heterochromatin (IH) appears as large dense bands scattered in euchromatin and comprises clusters of repressed genes. IH displays distinctly low gene density, indicative of their particular regulation. Genes embedded in IH replicate late in the S phase and become underreplicated. We asked whether localization and organization of these late-replicating domains is conserved in a distinct cell type. Using published comprehensive genome-wide chromatin annotation datasets (modENCODE and others), we compared IH organization in salivary gland cells and in a Kc cell line. We first established the borders of 60 IH regions on a molecular map, these regions containing underreplicated material and encompassing ∼12% of Drosophila genome. We showed that in Kc cells repressed chromatin constituted 97% of the sequences that corresponded to IH bands. This chromatin is depleted for ORC-2 binding and largely replicates late. Differences in replication timing between the cell types analyzed are local and affect only sub-regions but never whole IH bands. As a rule such differentially replicating sub-regions display open chromatin organization, which apparently results from cell-type specific gene expression of underlying genes. We conclude that repressed chromatin organization of IH is generally conserved in polytene and non-polytene cells. Yet, IH domains do not function as transcription- and replication-regulatory units, because differences in transcription and replication between cell types are not domain-wide, rather they are restricted to small “islands” embedded in these domains. IH regions can thus be defined as a special class of domains with low gene density, which have narrow temporal expression patterns, and so displaying relatively conserved organization

XXII.—Deficiency Effects of Ultra-violet Light in<i>Drosophila melanogaster</i>

Stadler and Sprague in a series of papers (1936) succeeded in demonstrating by a genetical method the effect of ultra-violet radiation in maize pollen, and found that in their experiments in the X-ray series the chromosomal changes are very common, while in the ultra-violet series they are rare.</jats:p

A NOTE ON CHROMOSOME LENGTH AND TEMPERATURE ILLUSTRATED IN TWO SPECIES OF RODENTS

Comparison of the karyotypes of Praomys natalensis and of Aethomys namaquensis, both described by Matthey, and statistical analysis of Dr. B. Woolf of their body temperature from data supplied by Dr. Davis, suggests that chromosome length may depend on the temperature at which cell divisions occur. The possible mechanism of this dependence is tentatively discussed. </jats:p

Oocyte pachytene analysis of Cattanach's <i>fd</i> translocation

Oocyte pachytene analysis of embryos heterozygous for the fd translocation confirmed the facts of this translocation as described by Ohno and Cattanach. Besides that it revealed that a small piece of the X-chromosome has been transferred to the autosome where in about 25% of cases it shows less stainable expansion.</jats:p

Sexual dimorphism in mouse gametogenesis

Diplotene and diakinesis chiasma frequency in oöcytes of the mouse cannot be studied successfully with the present technique. Metaphase chiasmata have been examined in thirty-nine oöcytes. It is deduced that the total diplotene map length in females is about 2300 cM. compared with 1950 cM. in males. There is sexual dimorphism in the frequency of chiasmata, which is paralleled by similar dimorphism in frequencies of crossing-over, measured genetically.The two sexes differ in the duration of various stages of meiosis. In adult males the pachytene stage, lasting for about 7 days, is directly followed by diplotene and diakinesis, after which the metaphase stage sets in. The sex bivalent in males develops visible chiasmata much earlier than do the autosomes and it precedes them in anaphase separation. Quick terminalization of chiasmata in it leads in a fair proportion of cases to precocious separation and in less than 1% of cases to cytologically detectable non-disjunction of sex chromosomes.In females the pachytene stage appears in oöcytes of the embryo and is followed by the dictyotene stage, which last still ovulation, i.e. between 35–40 days and several months. Since in the oöcyte chiasmata are formed and move during the dictyotene stage, it follows that stainable materials of the chromosomes are not necessary for the formation and movement of chiasmata and are concomitant with pairing and anaphase separation. It follows also that the time for chiasma formation and movement is in females at least five to six times longer than in males. In old oöcytes in which time is available for maximum terminalization of chiasmata, non-disjunction may appear with detectable frequency. This mechanism may also operate in cases of Mongolism in man, where non-disjunction of an autosome has been recently cytologically established and higher frequency of incidence of the condition for old mothers has been known for some time.It is possible that the differences in duration of various stages of gametogenesis are connected with the period at which gametic selection is operating: in spermatogenesis after the second meiotic division, in oögenesis prior to first meiotic metaphase.</jats:p

VIII.—The Structural Differentiation of Chromosome IV of<i>Drosophila simulans</i>and its Behaviour in<i>melanogaster</i>Genotype

The problem of viability and fertility of interspecific hybrids inDrosophilahas been attacked most recently by Muller and Pontecorvo (1941). They crossed diploidDrosophila simulansmales with triploidmelanogasterfemales, and, by using a specially designed technique, succeeded in introducing into the marked genotype ofmelanogastersome chromosomes ofsimulans. Amongst the many interesting results obtained, it was found that not all interspecific combinations of chromosomes were viable; the large chromosomes of one species show a definite “need” for some genes or group of genes in other chromosomes of the same species, and these authors have drawn the conclusion that there are a number of complementary “lethals” in both species located in the major chromosomes (X, II, and III).</jats:p