R. M. Speed

Oocyte development in XO foetuses of man and mouse: the possible role of heterologous X-chromosome pairing in germ cell survival.

The pairing characteristics of the X axis in XO human and mouse oocytes were studied by the spreading technique throughout meiotic prophase. In three human XO foetuses, germ cell development was seen to be largely blocked at the preleptotene stage. In XO mice on the other hand, oocytes surviving through pachytene increasingly show the X axis making a non-homologous association with itself or with an autosome. Such associations take the form hairpins or rings when self pairing occurs or triradial structures when involvement is with an autosome. Pairing initiation in the autosomes involved is disturbed by the X axis suggesting that the heterologous pairing seen is taking place at the earliest stage of synaptonemal complex formation, namely zygotene. It is suggested, that in the XO mouse, and perhaps also in rare fertile XO humans, survival, of a population of oocytes into the adult is ensured by the ability of the X axis to pair non-homologously at meiotic prophase, thus satisfying pairing requirements.

Meiosis in the foetal mouse ovary

A systematic search for chromosome pairing defects in foetal mouse oocytes has been carried out in two different strains (Swiss and CBA/Ca) over days 15–19 of gestation and on day 1 post-partum. The aim was to seek direct cytological evidence for a “production line” of oocyte development, or the occurrence of pairing anomalies at meiotic prophase that might lead, in the adult female, to nondisjunction at anaphase I. No evidence for either was found. The data argue against the “production line” hypothesis as the basis for maternal age-related increases in aneuploidy in the mouse. Attempts to analyse chiasmata in oocytes at diplotene were unsuccessful.

The effects of ageing on the meiotic chromosomes of male and female mice.

The effects of age on the chiasma frequencies, chiasma position and numbers of univalents at MI in males and females of three strains of mouse were examined. Males showed a slight but non significant rise in chiasma frequency in age due to an increase in bivalents with two chiasmata at the expense of single chiasmata bivalents. In contrast, females exhibited a significant decrease in chiasma frequency with age due to the loss of two chiasma bivalents with a corresponding increase in single terminal chiasmata bivalents. In both males and females there was no significant increase in univalents with age in the strains studied. Of interest was the finding of a greater degree of contraction of the MI chromosomes in the oocytes of old relative to young females, a differential contraction that was independent of culture time. This finding is discussed with regard to the “production line theory” and non disjunction at Anaphase in other strains of mice.

A human 9;20 reciprocal translocation associated with male infertility analyzed at prophase and metaphase I of meiosis

Details are given of a meiotic prophase analysis, carried out by spreading, of a human 9;20 reciprocal translocation ascertained in a subfertile, oligospermic male. Air-dried meiotic preparations revealed the presence of translocation quadrivalents at metaphase I. Germ-cell degeneration was evident from the early prophase of meiosis onward. Associations between the translocation quadrivalent and XY bivalent at pachytene were seen in only 20% of the cells and seemed not to be the prime cause of germ-cell failure. Pairing disruption around the breakpoints of the translocation at pachytene and/or pairing failure in one arm of the pachytene cross was observed in a total of 87% of all cells analyzed. This could have contributed significantly to germ-cell atresia.

Meiotic analysis of two human reciprocal X-autosome translocations

Two cases of human reciprocal X-autosome translocation, t(X;12) and t(X;2), are described in sterile males, along with meiotic findings. Each carrier had inherited the translocation from his mother. Both showed azoospermia and germ-cell maturation arrest at the primary spermatocyte level, with most cells being arrested at the pachytene stage. A few metaphase I (MI) divisions were found, with occasional metaphase II cells being seen in the t(X;2) carrier. MI air-dried preparations gave clear evidence of chain quadrivalent formation. In the t(X;2) heterozygote, the pairing characteristics of the quadrivalent at pachytene were also analyzed in electron microscopic spreads. Disturbance of pairing around the breakpoints characterized most quadrivalents, and there was evidence in about 20% of the cells that nonhomologous pairing had taken place between the translocated chromosomes and the normal chromosome 2. Comparisons are made with similar nonhomologous pairing configurations seen at pachytene in quadrivalents of male reciprocal X-autosome translocations of the mouse.

A fertile mule and hinny in China

Anecdotal reports of fertility in female mules (jack donkey x mare) and hinnies (stallion x jenny donkey) have appeared in the literature over the years, but scientists have generally regarded them with scepticism. The fact that some of these hybrids can come into estrous and ovulate makes fertility conceivable, given that opportunity for mating arises. In China, where mules are bred extensively for work on the farms, a fertile female mule and a fertile female hinny have now been verified by chromosomal investigation. Each had mated with a donkey and produced a filly foal. The foals show unique hybrid karyotypes different from the mules or hinnys and different from each others. The studies make it clear that mule and hinny fertility, at least for the female hybrid, is a real possibility.

Infertility in human males with autosomal translocations

The meiotic prophase behavior of three human reciprocal autosomal translocations is presented. Each translocation was ascertained among men attending an infertility clinic. Two involved chromosomes 3

Abnormal RNA synthesis in sex vesicles of tertiary trisomie male mice

The XY bivalent contained within the sex vesicle of chromosomally normal male mice shows virtually no RNA transcriptional activity as measured by autoradiography during the prophase of meiosis. Tertiary trisomie males, derived from the T 70 H and T 31 H reciprocal translocations, include the extra small marker chromosome to varying degrees within the sex vesicle. It has been found that RNA transcription occurs within the sex vesicles of such trisomics. An attempt has been made to correlate such activity either to the extra marker chromosome itself or to a possible reactivation of the normally quiescent sex bivalent. The relationship of the abnormal RNA synthesis to the breakdown of meiosis in tertiary trisomics and to their subsequent reduced fertility is discussed.

Delayed meiotic development and correlated death of spermatocytes in male mice with chromosome abnormalities

The kinetics of germ-cell development were examined in random-bred adult Swiss mice by means of tritium autoradiography. Comparisons were made between males with normal (+/+) and abnormal karyotypes: Rb(11.13)4Bnr/+ and T(1;13)70H/+ heterozygotes, T70H tertiary trisomics, and T70H translocation trisomics. The time taken for the first wave of labelled cells to progress from premeiotic S-phase to diplotene and then on to the second meiotic metaphase was estimated in each stock, and rates of increase of labelled meiotic figures were measured. The S-phase to diplotene interval did not differ significantly among the different genotypes, taking from 10 days, 16 h to 10 days, 19 h. In Rb4Bnr/+, T70H translocation trisomic, and T70H tertiary trisomic males, however, labelled meiotic figures accumulated at a lower rate, particularly in the tertiary trisomics. A cell delay for some meiocytes was thus indicated during meiotic prophase. A correlation was seen between the degree of meiotic delay and severity of reduction in sperm count. The period from late diplotene to metaphase II was also found to be longer in T70H tertiary trisomics than in controls (+/+) or other chromosomally abnormal males.

Meiosis of T70H translocation trisomic male mice

Meiosis of T70H/+,Ts(113)70H translocation trisomic male mice has been studied using C-banded preparations of multivalents at the first meiotic division, marker chromosomes at the second meiotic division, and sucrose-spread pachytene spermatocytes for the observation of synaptonemal complexes. During zygotene and pachytene the three marker chromosomes, 131 (long) and 113 (2 x, small) associate with the chromosomes 1 and 13 to form either a Chain of three plus bivalent (CIII+II), a Chain of four plus univalent (CIV+I) or a Chain of five (CV). During pachytene, the 113 univalent of the CIV+I configuration shows association with the sex chromosomes in the sex vesicle while for CV, the unpaired segment of chromosome 1 can do so. — The frequencies of the multivalent configurations during late diplotenemetaphase I were CIII+II, 47.6%, CIV+I, 34.5% and CV, 14.4%. However, among late diplotene-early diakinesis cells, the frequency of CIV+I was 12.9% while in (late) spermatocytes with contracted bivalents it was 45.7%. — The notion that proximity of the interstitital chiasma of chromosome 13 to the centromere affects the chances of non-disjunction for these centromeres within the multivalent (i.e., adjacent 2 segregation) has been strengthened by the observed adjacent 2 frequency of 13.0% in the translocation trisomics compared to the 26.3% found in T70H/+ translocation heterozygotes with more proximal chiasmata. Thus, for reciprocal translocations between acrocentric chromosomes, a proximally located chiasma in an interstitial segment enhances the chance of adjacent 2 segregation. — Other parameters of irregular chromosome behaviour at anaphase I, such as equational separation of chromosome 113 into chromatids and non-disjunction of normal bivalents, were increased in the translocation trisomics when compared with T70H/+ translocation heterozygotes, especially among adjacent 2 segregating cells.