Genetic and environmental components of chiasma control

Variation in chiasma frequency within and between individuals has been investigated in Schistocerca gregaria and Stethophyma grossum. By taking sequential samples of the same testis in S. gregaria it has been demonstrated that there is considerable variation in chiasma frequency between times, within individuals, and that this variation is not evenly distributed between individuals of the same population. The response and recovery patterns after exposure to successive temperature treatments also indicate a differential sensitivity of individuals within the experimental population. — An analysis of interfollicular variation in chiasma frequency has revealed significant differences between follicles within individuals both in S. gregaria and a Spanish population of S. grossum. A comparable analysis on individuals of S. grossum carrying supernumerary segments shows that the presence of such segments increases the amount of variation between follicles within individuals and also between individuals within the population. — The distribution of cell chiasma frequencies in S. gregaria is normal whereas in Stethophyma cell frequencies approximate to a Poisson distribution. The structurally different supernumerary segments, present in Austrian and Spanish populations of S. grossum, both increase mean chiasma frequency and they modify the between cell variance in different ways — the former interchromosomally and the latter intrachromosomally. — The differences in chiasma frequency between follicles and between times within individuals and the differential reaction to heat shock reflect differences in the pattern of genotype-environment interaction. Similarly it can be argued that the presence of polymorphisms involving supernumerary material may play an equivalent role by increasing the between cell or between bivalent variance within individuals, a unique form of genotype-environment interaction at the endophenotypic level. — Such variation in genotype sensitivity to environmental modification has important adaptive value especially in organisms which are subject to recurrent, often random, changes in their environment.     

Genetic and environmental components of chiasma control

A disruptive selection programme for high and low chiasma frequency over four generations has produced two lines of Schistocerca gregaria with mean values of 21.6 and 18.0, respectively. The mean of the base population was 20.4. Realised heritability estimates for both the high and low lines were 27.4% and 48.8% respectively; only the latter value is significant. The non-significant value in the high line has resulted either from unidirectional dominance or because the genes responsible for high chiasma values themselves undergo a higher recombination rate in high frequency individuals and thus reduce the response to selection. — It is suggested that the genes governing chiasma frequency can be regarded as mainly neutral due to the large additive genetic component, at least in low chiasma frequency types. Accordingly, chiasma frequency variation and its adaptive significance may not be such an important component of fitness as has been proposed in the past.     

Genetic and environmental factors affecting chiasma formation in locusts

By means of crosses with the African Migratory locust it has been demonstrated that various strains carry different segregations of genes which quantitatively condition the formation of chiasmata during meiosis. These genes occur in polygenic series, and in one low frequency strain the polygenes show an epistatic effect for low chiasma frequency, while another low frequency strain seems to have attained this property through gamma radiation. In reciprocal crosses between the lastnamed low and a medium frequency strain there appears to be a maternal effect, with heterosis and epistasis respectively in the reciprocal crosses. In one strain two lines were selected for plasticity and non-plasticity of hopper colour change in small populations, and this selection has resulted in multiple changes in that the two lines now diverge in the three main physical criteria of phase transformation, viz. hopper colour, adult morphometric ratios and chiasma frequencies; one line is now solitarious for these characteristics, the other more gregarious. The genotypes selected out could be related to the ability to utilize the gregarization pheromone.A 10° rise in temperature above 22° C has the effect of increasing chiasma frequencies significantly, but increase in the carbon dioxide content of the atmosphere has no effect. Male and female hoppers produce equivalent amounts of the chiasma-inducing pheromone, while solitarised hoppers do not lose the ability to produce this pheromone. The crucial stage for the action of this pheromone on chromosomes is during the fifth nymphal instar. The phase status of parents appears to affect chiasma frequencies in the offspring, more specifically that of the mother, so that there is further evidence of maternal effect on chiasma frequency.     

Chromosome-specific control of chiasma formation

A desynaptic mutant of Hypochoeris radicata, 2n = 8, has been found in a population from France. The mutant is remarkable in that at metaphase-I over 90% of PMC's have a pair of univalents while two pairs are found in only 1% of cells. Only chromosome IV is affected by the desynapsis which is controlled by a single recessive gene. Bivalent chiasma frequency in cells with a pair of univalents is higher than in cells with complete bivalent formation indicating partial dependence of cell chiasma number on the availability of some factor within the anther. The F₂, produced by full-sib mating, has a raised frequency of univalents in the chromosomes other than pair IV. The higher level results from enforced sib-mating of an obligate outbreeder and is not related to the action of the major gene. It is suggested that chiasma formation is controlled both polygenically and by major genes which operate on a hierarchical system. Some genes affect the behaviour of the entire complement while the action of others is specific to individual chromosomes.     

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.This recearch was partly supported by NIH Grant GM 701-05.     

Genetic and environmental components of family history in type 2 diabetes

Family history of diabetes is a major risk factor for type 2 diabetes (T2D), but whether this association derives from shared genetic or environmental factors is unclear. To address this question, we developed a statistical framework that models four components of variance, including known and unknown genetic and environmental factors, using a liability threshold model. Focusing on parental history, we simulated case–control studies with two first-degree relatives for each individual, assuming 50 % genetic similarity and a range of values of environmental similarity. By comparing the association of parental history with T2D in our simulations to case–control studies of T2D nested in the Nurses’ Health Study and Health Professionals Follow-up Study, we estimate that first-degree relatives have a correlation of 23 % (95 % CI 15–27 %) in their environmental contribution to T2D liability and that this shared environment is responsible for 32 % (95 % CI 24–36 %) of the association between parental history and T2D, with the remainder due to shared genetics. Estimates are robust to varying model parameter values and our framework can be extended to different definitions of family history. In conclusion, we find that the association between parental history and T2D derives from predominately genetic but also environmental effects.     

Chromosome-specific control of chiasma formation in Crepis capillaris

An experimental population of Crepis capillaris (2n=6) displays frequent chromosome-specific univalence affecting all three chromosome pairs of the complement independently in different plants. The frequency of univalence in the population varies from 0% in some plants to about 40% of pollen mother cells in other plants. Most commonly, affected pollen mother cells contain just one pair of univalents and wherever the frequency of cells containing univalents exceeds 10%, a chromosome-specific effect almost invariably appears. Univalence affecting the A, C and D chromosome pairs is about equally frequent in the population. The mean cell chiasma frequencies of affected plants are generally lower than those of normal plants from the same population, despite suggestions of compensating increases in the chiasma frequencies of unaffected chromosomes pairs in the presence of specific univalence of a third pair. Breeding tests have been carried out, and observations made on pachytene stages which demonstrate that the specific univalence is due to recessive genes causing desynapsis following apparently normal pairing of homologues.     

Genetic and environmental components of growth in nestling blue tits (Parus caeruleus)

We investigated the effect of brood‐size mediated food availability on the genetic and environmental components of nestling growth in the blue tit (Parus caeruleus), using a cross‐fostering technique. We found genetic variation for body size at most nestling ages, and for duration of mass increase, but not of tarsus growth. Hence, nestling growth in our study population seems to have the potential to evolve further. Furthermore, significant genotype–environment interactions indicated heritable variation in reaction norms of growth rates and growth periods, i.e. that our study population had a heritable plasticity in the growth response to environmental conditions. The decreasing phenotypic variance with nestling age indicated compensatory growth in all body traits. Furthermore, the period of weight increase was longer for nestlings growing up in enlarged broods, while there was no difference to reduced broods in the period of tarsus growth. At fledging, birds in enlarged broods had shorter tarsi and lower weights than birds in reduced broods, but there was no difference in wing length or body condition between the two experimental groups. The observed flexibility in nestling growth suggests that growing nestlings are able to respond adaptively to food constraint by protecting the growth of ecologically important traits.     

Genetic and environmental components of variation in eumelanin and phaeomelanin sex-traits in the barn owl

Knowledge of the mechanism underlying the expression of melanin-based sex-traits may help us to understand their signalling function. Potential sources of inter-individual variation are the total amount of melanins produced but also how biochemical precursors are allocated into the eumelanin and phaeomelanin pigments responsible for black and reddish-brown colours, respectively. In the barn owl (Tyto alba), a eumelanin trait (referred to as 'plumage spottiness') signals immunocompetence towards an artificially administrated antigen and parasite resistance in females, whereas a phaeomelanin trait ('plumage coloration') signals investment in reproduction in males. This raises the question whether plumage coloration and spottiness are expressed independent of each other. To investigate this question, we have studied the genetics of these two plumage traits. Crossfostering experiments showed that, for each trait, phenotypic variation has a strong genetic component, whereas no environmental component could be detected. Plumage coloration is autosomally inherited, as suggested by the similar paternal-to-maternal contribution to offspring coloration. In contrast, plumage spottiness may be sex-linked inherited (in birds, females are heterogametic). That proposition arises from the observation that sons resembled their mother more than their father and that daughters resembled only their father. Despite plumage coloration and spottiness signalling different qualities, these two traits are not inherited independent of each other, darker birds being spottier. This suggests that the extent to which coloration and spottiness are expressed depends on the total amount of melanin produced (with more melanin leading to a both darker and spottier plumage) rather than on differential allocation of melanin into plumage coloration and spottiness (in such a case, darker birds should have been less spotted). A gene controlling the production of melanin pigments may be located on sex-chromosomes, since the phenotypic correlation between coloration and spottiness was stronger in males than in females.     

Chromosomal control of chiasma distribution in house mice. Analysis of chiasma distribution in homo- and heterozygotes by inversion in chromosome 1

Examination of chiasma distribution in the chromosome 1 in male mice homo- and heterozygous for distal inversion In(1)12Rk and in normal mice was carried out. No differences in chiasma distribution was found between homozygotes for the inversion and homozygotes for normal chromosome 1. A drastic change in this trait was revealed in heterozygous animals. In heterozygotes, the telomeric segments of SC were asynapsed and unavailable for recombination. This leads to significant decrease in the frequency of bivalents bearing chiasmata in pretelomeric region. In turn, it produced chiasma redistribution in proximal noninverted portion of the bivalent 1. These results could be interpreted as evidence for chromosomal control of chiasma distribution pattern: the distance of certain part of the chromosome from telomere and interference (which also operates at the chromosomal level) are more important for determination of the chiasmata frequency in the given region, than its genetic content.     

A re-examination of chiasma terminalization and chiasma frequency in male mice

The distribution and frequency of chiasmata have been analyzed in male BALB/c mice. Bivalents were classified in terms of the number of interstitial chiasmata (CH) and terminal associations (TA) present as follows; 1CH, 2CH, 1TA, 2TA, 1CH·1TA, 1CH·2TA, 2CH·1TA, 2CH-2TA and 0. We provide evidence that the TA frequently dissociates during 1st meiotic prophase. Consequently six of the observed bivalents may be derived from three basic bivalent types (namely 2CH·2TA, 1CH·2TA and 2TA) by dissociation of the TA according to the following schemas: (1) 2CH·2TA2CH·1TA2CH, (2) 1CH·2TA1CH·1TA1CH, and (3) 2TA1TA0. We also provide evidence that interstitial chiasmata do not move, which implies that a TA can not be formed by chiasma terminalization. The chiasma frequency estimated by assuming that terminal associations do not result from terminalized chiasmata is 17.2±2.4 compared to a value of 25.4±2.2 calculated on the assumption of chiasma terminalization.     

Evidence for separate genetic control of crossing over and chiasma maintenance in maize

The meiotic cytological behavior of chromosomes in maize microsporocytes homozygous for the recessive mutant desynaptic was studied at various stages. It was found that following apparently normal pachytene synapsis there appears to be sporadic precocious desynapsis. By diakinesis bivalents heterozygous for a distal knob have often separated to pairs of univalents, each with a knob-carrying and a knobless chromatid. From the frequency of such events it is inferred that the crossover process is probably not affected by the mutant and that the genetic defect affects instead a distinct function concerned with chiasma maintenance following crossing over. Since precocious separation of dyads to monads at prophase II was also found in the desynaptic material, it is suggested that normal chiasma maintenance until anaphase I and normal dyad integrity maintenance between anaphase I and anaphase II may depend upon the same mechanism; it is also suggested that this may involve a special tendency for cohesiveness of sister chromatids during meiosis, beyond that which is ordinarily found at mitosis.     

Relational coiling and chiasma frequency

Conclusions The amount of relational coiling of chromatids during the prophase of mitosis and the average number of chiasmata were determined in the largest chromosome of five species of locusts. The statistical analysis of the data obtained suggests that probably there is a correlation between the amount of relational coiling per unit of chromosome length and the number of chiasmata. A similar suggestion was put forward bySax (1936) andHusted (1938), which was based upon observations made in different plant species. Though the difficulties encountered during the present cytological analysis of the chromosome structures, as well as the small number of species investigated, do not allow us to draw a definite conclusion, nevertheless it seems probable from our data that a greater degree of relational coiling of chromatids during mitosis is accompanied by a higher chiasma frequency during meiosis. The correlation furthermore indicates also that the physical basis of the tendency of chromatids, and that of paired chromosomes, to twist round one another relationally may be the same.Fellow of the Rockefeller Foundation.     

Genetic susceptibility and environmental estrogen-like compounds

Environmental chemicals with estrogenic activities have been suggested to be able to interact with the endocrine system. Endogenous estrogen is synthesized in the ovarian theca cells of premenopausal women or in the stromal adipose cells of the breast of postmenopausal women and minor quantities in peripheral tissue. These cells, as well as breast tissue, express all the necessary enzymes for this synthesis, CYP17, CYP11a, CYP19, 17-beta-hydroxysteroid hydrogenase, steroid sulfatase as well as enzymes further hydroxylating estradiol, such as CYP1A1, CYP3A4, CYP1B1, catechol-o-methyltransferase (COMT). Polymorphisms in these enzymes may have a possible role in the link between environmental estrogens and hormone-like substances and the interindividual risk of breast cancer.     

Genetic and environmental determinants of menstrual characteristics

BACKGROUND:

The impact of women''s menstrual cycle on her quality of life, health, work, and community is substantial. Menstrual disturbance is linked with general ill conditions such as migraine, asthma, and endocrinopathies. The clinical significance of medical interventions to prevent these conditions becomes clear if the role of genetic or environment is clarified.

AIMS:

To identify the genetic and environmental contribution on menstrual characteristics.

SETTING AND DESIGN:

This was a cross-sectional study in 2 Asian countries.

MATERIALS AND METHODS:

2 cohorts of monozygotic and dizygotic twins born between (1945-1988, n = 122) and (1951-1993, n = 71) were taken. A standard questionnaire was designed inclusive of socio- demographic characteristics of subjects as well as menstrual history (duration, interval, amount, irregularity). Subjects were interviewed by phone.

STATISTICAL ANALYSIS:

Quantitative variables were analyzed using Falconars’ formula as well as maximum likelihood analysis. Structural modeling was then applied to twin correlations to provide estimates of the relative genetic and/or environmental factors contribution in determining the measured trait.

RESULTS:

Menstrual characteristics were found to be under environmental influence where the best fitting model for menstrual interval and duration was common environment. CDF plotting confirmed the results for both variables. Proband-wise concordance analysis for amount of menstruation, amenorrhea, and irregular menstruation revealed no genetic influence. The best fitting model for menstrual irregularity was CE (C73%, E27%). The same model was defined for amenorrhea (C48%, E52%).

CONCLUSIONS:

Environmental factors are most likely responsible to determine the menstrual flow, its integrity, and regularity. These factors need to be studied further.     

Genetic and epigenetic effects of environmental arsenicals

Environmental arsenic compounds and their methylated metabolites do not form adducts with DNA, but do cause oxidative DNA damage. Chromosome aberrations are seen at toxic concentrations. Genetic effects that occur at non-toxic concentrations include aneuploidy, comutagenesis (resulting from indirect effects on DNA repair), and delayed mutagenesis (probably secondary to aneuploidy and/or epigenetic effects). Effects of trivalent arsenicals on poly(ADP ribose) polymerase and P53 activation may mediate effects on DNA repair and aneuploidy. A growing literature points to the epigenetic effects of arsenic compounds in cells and in vivo. A review of the current literature on DNA methylation, histone modifications and microRNA effects is presented.