Familial incidence of twinning

The frequency of twinning among women who have already borne twins, the 'repeat frequency', is significantly higher than in the general population. Individual propensity is not necessarily genetic in origin, but pedigree studies (for reviews see refs 1, 2) confirm that twinning is a family trait. Studies based on archives restrict this conclusion to dizygotic (DZ) twinning and the maternal side, while studies based on interviews of relatives of twins find monozygotic (MZ) twinning and the paternal side also to be involved. However, interview studies can overestimate, while archive studies can underestimate, the real frequency of twinning. We have now analysed the incidence of twinning in the families of 950 zygosity-determined, unselected twin pairs under complete ascertainment. Our results indicate that a propensity to MZ twinning, as well as one to DZ twinning, can be inherited through the maternal line, and that the two mechanisms of twinning might be related. We have also found a paternal role in DZ, but not in MZ twinning.

A gene controlling condensation of heterochromatin in Drosophila melanogaster

A temperature-sensitive lethal mutant of Drosophila melanogaster was used to identify an essential cell cycle function that is necessary for the mitotic condensation of heterochromatic but not of euchromatic portions of the genome. This mutant is an allele at a locus (mus-101) identified earlier by the use of mutagen-sensitive mutants. The data suggest that the mutagen-sensitive and repair-defective phenotypes of viable mus-101 mutants result from a disruption in chromosome organization.

Renal dysfunction as a possible cause of essential hypertension in predisposed subjects

In 65 young normotensive subjects with two hypertensive parents (HP), and in 55 matched subjects with two normotensive parents (NP), the following factors were measured: renal plasma flow (RPF), glomerular filtration rate (GFR) both as Inutest and creatinine clearances; 24-hr urinary output; plasma renin activity (PRA); Na and K in plasma and in 24-hr urine and 24-hr urinary excretion of aldosterone. In 30 HP and in 34 NP, the cardiac output and plasma concentrations of noradrenaline, adrenaline, and dopamine were also measured in the supine position and after 10 min of standing. The HP have greater RPF (P less than 0.01), faster GFR (P less than 0.02), greater 24-hr urinary output (P less than 0.05), and lower PRA (P less than 0.01) than the NP. All the other factors were similar in the two groups of patients. It is proposed that the differences in renal function in the HP and the NP may be due to an abnormality in tubular handling of ions and water in the HP, which may be responsible for the increase in blood pressure in a proportion of patients with essential hypertension.

[Evaluation of functional periodicity of the pituitary gland by harmonic analysis.--III Circadian prolactin secretion in subjects with prolactin-secreting adenoma]

The 24-hour periodicity of PRL secretion in 7 prolactinoma subjects of both sexes has been studied by means of the Fourier's harmonic analysis. In all cases elevated PRL titres have been found, that did not show the physiological changes due to sleep and wake. In most of the cases could not be demonstrated either a significant secretory periodicity or secretory episodes occurred with a frequency higher than in the control groups. Such secretory features may depend on changes in the neoplastic cells' receptors for regulating neurotransmitters.

Region-specific effects on chromosome integrity of mutations at essential loci in Drosophila melanogaster

Two mutagen-sensitive loci of Drosophila melanogaster, mus-105 and mus-109, previously identified by viable alleles, are shown to specify essential functions. Lethal alleles at the loci produce larvae that have degenerate imaginal discs and die at the larva-pupa boundary. Our data suggest that the causes of lethality are intolerable levels of cell death produced by high frequencies of chromosome aberrations (in excess of 0.5 aberration per cell per cycle). The pattern of aberrations is a locus-specific character. In mus-105 mutants the most common aberrations are breaks and exchanges in euchromatic portions of the genome whereas in mus-109 mutants the most common aberrations are breaks at heterochromatin-euchromatin junctions. The sensitivity of these junctions to breakage in mus-109 mutants is a property of all such junctions whether natural or produced by a rearrangement that juxtaposes heterochromatin and euchromatin. Larvae carrying the combination of two viable mutants, mus-105(A1) mus-109(D1), die at the larva-pupa boundary and display a high frequency of aberrations (0.7 per cell vs. 0.075 for either mutant alone) clustered at euchromatin-heterochromatin junctions. This synergistic interaction suggests there is a class of lesions that can be repaired by both mus-105(+) and mus-109(+). Thus, the apparent euchromatic specificity of mus-105(+), which was inferred from the pattern of predominantly euchromatic breakage observed in mus-105 mus-109(+) flies, is in fact generated by the wild-type function of mus-109(+) masking an effect of mus-105 in the heterochromatin. The fact that lethal mutants at the mus-105 and mus-109 loci have small imaginal discs coupled with the observation of a maternal effect of mus-105 suggests a paradigm for the control of cell division during the life cycle of Drosophila.

Cytological dissection of sex chromosome heterochromatin of Drosophila hydei

Prophase chromosomes of Drosophila hydei were stained with 0.5 microgram/ml Hoechst 33258 and examined under a fluorescence microscope. While autosomal and X chromosome heterochromatin are homogeneously fluorescent, the entirely heterochromatic Y chromosome exhibits an extremely fine longitudinal differentiation, being subdivided into 18 different regions defined by the degree of fluorescence and the presence of constrictions. Thus high resolution Hoechst banding of prophase chromosomes provides a tool comparable to polytene chromosomes for the cytogenetic analysis of the Y chromosome of D. hydei. - D. hydei heterochromatin was further characterized by Hoechst staining of chromosomes exposed to 5-bromodeoxyuridine for one round of DNA replication. After this treatment the pericentromeric autosomal heterochromatin, the X heterochromatin and the Y chromosome exhibit numerous regions of lateral asymmetry. Moreover, while the heterochromatic short arms of the major autosomes show simple lateral asymmetry, the X and the Y heterochromatin exhibit complex patterns of contralateral asymmetry. These observations, coupled with the data on the molecular content of D. hydei heterochromatin, give some insight into the chromosomal organization of highly and moderately repetitive heterochromatic DNA.

Relationships among chromatid interchanges, sister chromatid exchanges, and meiotic recombination in Drosophila melanogaster

Repair- and recombination-defective mutations at two loci (mei-9 and mei-41) of Drosophila melanogaster have been examined for their effects on the induction of chromosome aberrations by x-rays and the formation of sister chromatid exchanges (SCEs). Irradiation of larval neuroblast cells during the S phase with x-rays showed that mutants at both of these loci are about 10 times more sensitive than wild type to the induction of chromosome aberrations. The pattern of induced aberrations was characteristic for each mutant locus: in cells bearing mei-9 mutations most breaks were chromatid deletions, whereas in the presence of mei-41 mutations similar frequencies of chromatid and isochromatid deletions were observed. Furthermore, chromatid interchanges could not be induced in cells carrying mei-9 alleles; therefore these mutations define a step necessary for chromatid rejoining. mei-41 alleles also define a function involved in the formation of chromatid interchanges; total exchanges were less frequent than expected from nonmutant controls; and the proportion of exchanges arising by symmetrical rejoining was markedly reduced. These data indicate that chromatid and isochromatid deletions have different molecular steps in their formation, and that different molecular mechanisms are also involved in the symmetrical and unsymmetrical rejoining in chromatid interchanges. Neuroblast cells of larvae bearing mei-9 and mei-41 alleles were also treated for 13 hr with 5-bromodeoxyuridine at 9 mug/ml in order to differentiate sister chromatids for the scoring of SCEs. Whereas mei-41 had a normal level of SCEs, mei-9 exhibited a frequency of SCEs that was about 70% that of the control. Because both mei-9 and mei-41 mutations result in defective meiotic recombination, these data suggest that they define steps shared by symmetrical interchange formation and meiotic recombination that do not participate in the formation of most SCEs.

Intraspecific polymorphism of sex chromosome heterochromatin in two species of the Anopheles gambiae complex

The Hoechst 33258 banding pattern of the mitotic chromosomes of several laboratory and natural populations of the sibling species A. gambiae and A. arabiensis has been analyzed. A clear intraspecific polymorphism of sex chromosome heterochromatin has been observed. Nevertheless in each species heterochromatic variations fall within a characteristic species-specific pattern. Moreover, while laboratory polulations tend to be monomorphic for a given heterchromatic variant, natural populations exhibit a high degree of intrapopulation polymorphism. The possible role of sex chromosome heterochromatin in controlling fertility and mating behaviour of Anopheles mosquitoes is discussed.

Effects of recombination-deficient and repair-deficient loci on meiotic and mitotic chromosome behavior in Drosophila melanogaster

The results of recent genetic and cytological studies on recombination-defective and repair-defective mutants of Drosophila melanogaster are summarized. These studies show that there is substantial overlap between the functions used in various aspects of DNA metabolism in Drosophila. Most loci first identified by either recombination-defective or mutagen-sensitive mutants have been shown also to function in nonmutagenized mitotic cells where their action is necessary to maintain the integrity of the genome: mutants at particular loci produce elevated frequencies of chromosome breakage, mitotic exchange, mutation, and/or chromosome loss. Genetic studies of meiotic recombination show that many of the loci identified by recombination-defective mutants restrict where along the chromosome arms exchange may occur. Recent EM studies suggest that the products of at least some of these loci are components of recombination nodules. Region-specific control of DNA metabolism is also indicated by the finding of nonrandom patterns of chromosome breakage in some mutagen-sensitive mutants. Recombination-defective mutants at two loci have been studied for their effects on sister chromatid exchanges (SCEs) and x-ray induced aberrations. Mutants at both loci are defective in steps necessary for the production of symmetrical chromatid interchanges but have little effect on SCEs.

Genetic control of chromosome breakage and rejoining in Drosophila melanogaster: spontaneous chromosome aberrations in X-linked mutants defective in DNA metabolism

Eight X-linked recombination-defective meiotic mutants (representing five loci) and 12 X-linked mutagen-sensitive mutants (representing seven loci) of Drosophila melanogaster have been examined cytologically in neuroblast metaphases for their effects on the frequencies and types of spontaneous chromosome aberrations. Twelve mutants, representing five loci, significantly increase the frequency of chromosomal aberrations. The mutants at these five loci, however, differ markedly both in the types of aberrations produced and the localization of their effects along the chromosome. According to these criteria, the mutants can be assigned to four groups: (i) mutants producing almost exclusively chromatid breaks in both euchromatin and heterochromatin; (ii) mutants producing chromatid and isochromatid breaks in both euchromatin and heterochromatin; (iii) mutants producing chromatid mutants producing chromatid and isochromatid breaks clustered in the heterochromatin.

Lack of spontaneous sister chromatid exchanges in somatic cells of Drosophila melanogaster

Neural ganglia of wild type third-instar larvae of Drosophila melanogaster were incubated for 13 hours at various concentrations of BUdR (1, 3, 9, 27 micrograms/ml). Metaphases were collected with colchicine, stained with Hoechst 33258, and scored under a fluorescence microscope. Metaphases in which the sister chromatids were clearly differentiated were scored for the presence of sister-chromatid exchanges (SCEs). At the lowest concentration of BUdR (1 microgram/ml), no SCEs were observed in either male or female neuroblasts. The SCEs were found at the higher concentrations of BUdR (3, 9, And 27 micrograms/ml) and with a greater frequency in females than in males. Therefore SCEs are not a spontaneous phenomenon in D. melanogaster, but are induced by BUdR incorporated in the DNA. A striking nonrandomness was found in the distribution of SCEs along the chromosomes. More than a third of the SCEs were clustered in the junctions between euchromatin and heterochromatin. The remaining SCEs were preferentially localized within the heterochromatic regions of the X chromosome and the autosomes and primarily on the entirely heterochromatic Y chromosome.--In order to find an alternative way of measuring the frequency of SCEs in the Drosophila neuroblasts, the occurrence of double dicentric rings was studied in two stocks carrying monocentric ring-X chromosomes. One ring chromosome, C(1)TR94--2, shows a rate of dicentric ring formation corresponding to the frequency of SCEs observed in the BUdR-labelled rod chromosomes. The other ring studied, R(1)2, exhibits a frequency of SCEs higher than that observed with both C(1) TR94--2 and rod chromosomes.

A renal abnormality as a possible cause of "essential" hypertension

The renal abnormality which causes hypertension in the Milan hypertensive strain of rats disappears as hypertension develops. Because of the many analogies between the condition in these rats and "essential" hypertension in man, the same pattern of change may occur if a renal abnormality is the cause of essential hypertension in man. This hypothesis was tested in two groups of young normotensive subjects matched for age, sex, and body-surface area; in the first group both parents were hypertensive, and in the second group both parents were normotensive. Renal plasma-flow, glomerular filtration-rate, plasma-volume, plasma-renin activity, plasma-concentrations of Na+, K+, and catecholamines, 24 h urinary excretion of Na+, K+, and aldosterone, and the cardiac index were measured so that renal function and the role of factors affecting blood-pressure regulation could be assessed. Renal plasma-flow was significantly higher (p less than 0.01) in the first group, whereas results of tests for all the other factors were almost the same in both groups. The hypothesis that a primary kidney abnormality causes hypertension in a proportion of patients with essential hypertension is proposed.

Familial hypertension and hormonal profile, renal haemodynamics and body fluids of young normotensive subjects

1. Almost all the factors that may cause a rise in blood pressure are, in turn, influenced by the increase in blood pressure per se. Thus any primary involvement of one or more of these factors in the pathogenesis of essential hypertension must be evaluated before or during the development of hypertension.

2. Young normotensive subjects both of whose parents are hypertensive have a much higher probability of developing hypertension than those whose parents are both normotensive.

3. The following measurements were made in 56 subjects of the first group (both parents hypertensive) and 35 of the second group (both parents normotensive), matched for age, sex and body surface area: renal plasma flow and glomerular filtration rate, using p-aminohippurate and inulin clearance; 24 h urinary excretion of aldosterone, protein and electrolytes; plasma renin activity; plasma volume. Plasma catecholamines and cardiac index were also measured in 26 subjects of the first group and 25 subjects of the second group using a radioenzymic method and echocardiography.

4. All these factors were similar in the two groups except that renal plasma flow was higher in the first group (767·2 ± 30 versus 650·7 ± 17 ml/min, P < 0·01). Plasma renin activity tended to be lower in subjects with a higher renal plasma flow, but there was no significant negative correlation between the two factors.

5. The possibility that the higher renal plasma flow in subjects with a high probability of developing hypertension is a compensatory mechanism for a primary intrarenal defect is discussed.