Meiosis in Sxr male mice

Cytological evidence for the existence of a Y-autosomal rearrangement in meiotic cells of Sxr mice has been sought. At pachytene, in silverstained light microscope spread preparations of XY, Sxr/+ and XO, Sxr/+ spermatocytes, evidence for pairing or association between a possible Y-bearing segment of a specific autosome and a sex chromosome could not, however, be found. Such sex chromosome-autosome associations as were seen were non-specific in nature, and occurred no more often in Sxr mice than in controls.     

Meiosis in double trisomic Brassica campestris

Meiosis in a double trisomic Brassica campestris (2n=20+1+1) found among the progeny of autotriploid B. campestris (3n=30) was studied to detect homologies of duplicate types of chromosomes in the a genome of Brassica. The two extra chromosomes paired with the corresponding homologues and formed two trivalents in 10.5% of nuclei revealing the double trisomic nature. They formed a separate bivalent in 15.6% of nuclei proving the homology of these two chromosomes in the a genome. Anaphase I and II segregations revealed a normal disjunction of chromosomes.Emeritus Professor of Botany & Emeritus Scientist, Indian Council of Agricultural Research.     

Dose-effect relationship for X-ray-induced translocations in spermatogonia of normal and T70H translocation heterozygous mice

The induction of reciprocal translocations in stem cell spermatogonia was studied in normal mice and T70H translocation heterozygotes using spermatocyte analysis many cell generations after irradiation. Dose-response relationships were constructed employing acute X-ray exposures of 1-10 Gy. The obtained results confirmed our earlier observations that T70H heterozygous mice were overall less sensitive to the induction of translocations compared to normals. The shape of the dose-response curve for T70H heterozygotes was also clearly different from normal mice. No simple correlations between cell killing, measured as testis weight loss, and observed frequencies of chromosomal anomalies could be established. In general, selective elimination of translocation carrying stem cells of T70H heterozygotes seem to be mainly responsible for the differences in induction pattern between the two types of mice.     

Meiosis in trisomic female mice with Robertsonian translocations. I. Prophase pairing

The prophase oocytes of two murine Robertsonian translocation (Rb) trisomies of chromosomes 16 and 19 were investigated using electron microscopy and a whole-cell micro-spreading technique after silver staining. About 20% of fetuses of each type were trisomic. They were obtained by mating animals heterozygous for two Rb's, monobrachially homologous for either chromosome 16 or 19, to an entirely acrocentric stock. Because of the almost inevitable prenatal mortality of the trisomic embryos, their fetal ovaries were "rescued" by an in vitro method for prophase studies. Analysis of the recovered oocytes showed frequent, close pairing associations of the three trisomic axes and evidence suggesting that the closely apposed axes coincided with the side-by-side formation of parallel, complete, true synaptonemal complexes; hence, the cytogenetic dogma that pairing is always two-by-two was contradicted. The presence of two parallel complexes has implications for crossing-over recombination. Triple associations of axes were found in almost half the trisomy 19 (Ts19) and in about 70% of the trisomy 16 (Ts16) prophases. The extent of triple associations varied and was greater in Ts16 than in Ts19 oocytes. Other relevant observations concerned the proportions of univalents and of univalence of the trisomic axes (21% in Ts16 and 46% in Ts19) and the distinctive, thickened appearance of all univalent axes. The pairing behaviour observed in balanced heterozygotes confirms what appears to be nonhomologous pairing and synaptic adjustment within the short-arm axes of the Rb trivalents.     

Clonal analysis of radiation-induced translocations in stem-cell spermatogonia of normal and T70H translocation heterozygous mice

7 T(1;13)70H/+ and 13 +/+ male mice were given 2 doses of 250 rad acute X-rays separated by 24 h. The +/+ mice were analysed in 2 groups during the first meiotic division for induced translocations, on average 177 and 233 days after irradiation, and the T70H/+ mice were analysed in parallel with the second group of +/+ males. One testis was treated with normal air-drying procedures yielding a random sample of cells. The other testis was processed according to a new technique, which enabled separate analysis of the various locations along the seminiferous epithelium where groups of cells are synchronously in the diakinesis-metaphase I stage of meiosis. The number of cells in such groups was estimated. Both capita epididymes were used for a sperm count. In agreement with an earlier finding, fewer induced translocations were recovered from the T70H/+ mice than from +/+ mice (10.6 versus 19.2%, air-drying technique).Estimates of the group sizes in combination with the occurrence of induced translocations yielded the following information. A synchronously moving group of diakinesis-metaphase I cells originates from, on average, 1.25 stem cells (Appendix). We found an indication for a reduction in group size by 33% when a clone originated from a stem cell carrying an induced translocation compared with a wild-type clone (see Appendix). Both, the data on group size and the sperm counts indicate that, 7 months after the irradiation, the seminiferous epithelium has not totally recovered. Final recovery seems to be slower or absent in the T70H/+ males. The data obtained from the T70H/+ heterozygotes indicate the stem-cell spermatogonia to be responsible for the reduction of the rate of translocation induction with this karyotype, either due to a reduced formation rate or due to a diminished capacity of some of the induced translocation-carrying stem cells to proliferate into a clone reaching the meiotic divisions.     

Meiosis in four trisomic and one double trisomic males of the newt Pleurodeles waltlii

In the newt Pleurodeles waltlii, meiosis was studied in four trisomic and one double trisomic males. Study of first prophase shows that trivalent frequencies and trivalent configurations are correlated with chromosome length; moreover, trivalent configurations indicate that long chromosomes have multiple points of initiation of synapsis whereas two points might be adequate to secure synapsis of short chromosomes. From the study of metaphase II it appears that the extra chromosomes segregate in half of the spermatocytes II. Some abnormal spermatocytes, resulting from nondisjunction of chromosomes at mitosis or at first division of meiosis, or from precocious division of chromosomes at first division of meiosis were observed. In the male double trisomic meiosis fails at anaphase of second division; this accounts for the sterility of the individual.     

Reduced oocyte numbers in tertiary trisomic mice with male sterility

Oocyte counts carried out in 3- to 5-day-old tertiary trisomic Ts(5(12))31H mice revealed a mean reduction of 71% in the number of oocytes as compared with that of normal littermates. The pool of small oocytes was reduced by 75%, and the number of growing oocytes by 8%. The sperm count of the trisomic males was less than 1% of normal, with most spermatozoa being abnormal (Beechey et al., 1980). These results indicate that the presence of the extra 5(12) chromosome, which causes male sterility, also has a marked effect on oogenesis. Possible reasons for the difference in severity of the gametogenic impairment in males and females are discussed.     

Spermatogenic delay and increased chiasma frequency in T70H/+ male mice with hydroxyurea-Trenimon limited spermatocyte populations

T(1;13)70H/+ male mice were treated with hydroxyurea (HU) and Trenimon (T). This karyotype offers excellent possibilities for estimating number and position of chiasmata and segregation in meiotic anaphase I. By their cell-killing action during spermatogenesis, HU and T produce large gaps in the spermatogenic line. The surviving population between the gaps was analysed at diakinesis--metaphase I and metaphase II. We found by autoradiography a considerable retardation of the development from resting primary spermatocytes (RPS) to metaphase I and II as compared to untreated T70H/+ males. Furthermore we found increased chiasma frequencies in diakinesis--metaphase I (MI) and reduced nondisjunction frequencies at anaphse I as a result of the treatments applied. The latter effect could not be explained by the increased chiasma frequency.     

Synaptonemal complex analysis in spermatocytes and oocytes of tertiary trisomic Ts(512)31H mice with male sterility

Whole-mount preparations of silver-stained spermatocytes and oocytes from Ts(512)31H mice were examined in the electron microscope. The 5(12) chromosome was associated with the XY bivalent in the large majority of spermatocytes, whereas in about one-half of the oocytes, the 5(12) was associated with either unpaired chromosomes or heterochromatic parts of chromosomes or showed self-synapsis. There was a tendency for 5(12) chromosomes to be more fully heterochromatic in oocytes than in spermatocytes. A large proportion of oocytes (50%) and a much smaller proportion of spermatocytes exhibited various errors of chromosome pairing, but these proportions were only marginally greater than in control gametocytes from mice with normal karyotypes. It is concluded that the observed errors of pairing bear no simple relation to the almost complete breakdown of spermatogenesis and the marked impairment of oogenesis that occur in tertiary trisomic Ts(5(12))31H mice.     

Testicular functions in mice bearing an autosomal translocation (T 145H)

In the mouse, the autosomal reciprocal translocation T (7; 19) 145 H caused complete male sterility. The spermatogenesis was arrested at prophase or early metaphase I stages during the first meiotic division. The exocrine and endocrine testicular functions of azoospermic males (T 145 H/+) were compared with those of normal male littermates (+/+) at 63 days of age. Testis and epididymis weights in T 145 H/+ were significantly lower than those in +/+. By histological examination, the interstitial cells appeared preponderant but this was probably illusory due to the decrease in seminiferous tubular size and diminished testicular size. Moreover, androgen activity in T 145 H/+ seemed normal judging by weights of androgen target tissues (prostate, seminal vesicles), and plasma testosterone level.     

Meiosis in trisomic female mice with Robertsonian translocations. II. Chromosome behaviour at first and second meiotic metaphases

First and second meiotic metaphases (MI and MII, respectively) from female mice of Robertsonian translocation (Rb) stock, trisomic for chromosome 16 (Ts16) or 19 (Ts19), were studied. The mature trisomic oocytes were derived from explanted fetal ovaries that had been cultured and then transplanted so as to mature heterotopically. Multivalent configurations involving the Rb chromosomes and the additional trisomic acrocentric were analysed. Pentavalent configurations occurred in 74.5% of 98 Ts16 MI and 44.2% of 249 Ts19 MI oocytes; quadrivalents (with a univalent acrocentric) were found in 9.2% of Ts16 MI and 10.8% of Ts19 MI oocytes. In 1% of Ts16 MI and 4% of Ts19 MI oocytes, there were two Rb bivalents and a univalent trisomic acrocentric. Rb trivalents and Rb bivalents occurred together in 14.3% of Ts16 MI and 39.4% of Ts19 MI oocytes. Chiasma frequencies were similar in trisomic and chromosomally balanced MI. Chiasma position, distribution, and localization were nearly identical, whether they were found in Rb multivalents or acrocentric bivalents, but one control group (from chromosomally balanced Ts19 littermates) had significantly more terminal chiasmata. Within the triple homologous region of 8% of Rb pentavalents, two chiasmata were observed in the same relative position in the two sister chromatids of one of the three homologs, suggesting a lapse in chiasma position interference. Assortment at MI anaphase was influenced by secondary nondisjunction of the Rb. The ratio of balanced to unbalanced MII oocytes was 1:4 in both trisomies.     

Meiotic nondisjunction and translocation segregation in dwarf (dw/dw) and control T(1;13)70H heterozygous mice

The meiotic behavior of translocation heterozygous T70 (1;13)H/+ male mice with a Snell dwarf (dw/dw) genotype was compared with that of nondwarf T70H/+ controls. A four-fold increase in the nondisjunction frequency of the normal bivalents occurred as a consequence of the dwarf genotype. This increase is identical to that seen in karyologically normal dwarf males. No effect of the dwarf condition on the segregation of the translocation multivalent could be noted. Thus, translocation heterozygosity does not enhance the meiotic instability caused by the hypopituitary dwarf condition. From a small sample of oocytes from T70H/+ and chromosomally normal dwarf females it is concluded that nondisjunction in females is not increased by the dwarf condition. In general we conclude that animals with higher spontaneous nondisjunction levels are not necessarily more sensitive to factors increasing nondisjunction.     

Heritable translocation study in male mice with trimethyl phosphate

Dominant lethal and heritable translocation studies were performed in male mice receiving a single intraperitoneal injection of trimethyl phosphate (TMP). The germ cell stage investigated was the spermatid. Methyl methanesulfonate (MMS) was used as a positive control in the latter study. A dominant lethal assay gave marked dose-dependent increases in early fetal deaths. Heritable translocations were detected at 1000 or 1500 mg of TMP/kg in F1 male progeny when screening for semi-sterility and cytogenetically analyzing the meiotic or mitotic chromosomes. Translocation induction was higher at the higher TMP dose (14.3%) than at the lower dose (5.3%) and the yield from the higher dose was similar to that induced by 50 mg of MMS/kg (11.0%). Most of the translocation carriers were semi-sterile or sterile. The data confirm conclusions from other dominant lethal studies showing TMP to be capable of causing chromosomal damage in mouse spermatids and show that certain types of damage result in heritable translocations.     

Effects of post-irradiation interval on translocation frequency in male mice

Hybrid male mice were given 5 Gy + 5 Gy acute X-rays 24 h apart, with cytological examination of testes 16-19, 39-42 and 64-66 weeks later. Mean testis weights were significantly lower in the youngest group than in the other two. However, translocation frequencies in spermatocytes of the youngest group (mean of 0.57 per cell) were significantly higher than in either of the other two groups, which gave similar values averaging 0.36 translocations per cell. There was highly significant heterogeneity in translocation yields within the youngest group. The decline in translocation yield with time after irradiation is in line with that reported by Léonard and Deknudt (1970) in inbred strain C57BL males. Analysis of all available data suggests that high translocation yields are found during late stages in the process of germ-cell repopulation of the testis after high radiation doses and may be connected with changing frequencies of radiosensitive and radioresistant stem cell populations as repopulation proceeds.     

Meiosis in male PL/J mice: a genetic model for gametic aneuploidy

Sperm from mice of the PL/J strain have a high frequency of sperm-head morphology abnormalities. Fluorescence in situ hybridization (FISH) methods revealed that PL/J sperm are also characterized by a high frequency of aneuploidy. The traits of abnormal sperm head morphology and aneuploidy are associated with numerous meiotic abnormalities. Spermatocytes of PL/J mice exhibit chromosome asynapsis during meiotic prophase as well as reduced crossing over, revealed by analysis of both MLH1 foci in pachytene spermatocytes and chiasmata seen at the first meiotic metaphase. During the first meiotic division, roughly one-third of the PL/J spermatocytes exhibit aberrant spindle morphology, with abnormalities including monopolar spindles, split spindle poles, and incomplete spindle formation and centrosomal abnormalities. F1 progeny of a cross between PL/J and C57BL/6J did not exhibit a high frequency of either sperm aneuploidy or sperm head morphology aberrations, as would be expected if the PL/J traits were dominant. Among progeny of a backcross of F1 mice to PL/J, none of 16 males assessed exhibited elevated frequencies of sperm head morphology abnormalities. Four of the individuals exhibited elevated sperm aneuploidy, but not at the levels of the PL/J parents. Thus, it is likely that the aberrant PL/J traits are due to several genes and/or modifiers affecting the generation of both sperm aneuploidy and abnormal sperm head morphology.     

A new type of nonhomologous synapsis in T(X;4)1R1 translocation male mice

The synaptonemal complexes of T(X;4)1R1 (abbreviated R1) translocation heterozygotes have been examined by electron microscopy and compared with those of two X-7 translocations: R5 and R6. The X chromosome breakpoint of R1 is estimated to lie between 78 and 82% from the proximal end of the X, in the same general region as the R5 and R6 breakpoints. The position of the autosomal breakpoint of R1, like that of R6, is about 30% from the proximal end of the respective autosome. R1 is also similar to R6 in that there is extensive nonhomologous synapsis both in quadrivalents and heteromorphic bivalents. We have recently found that the location of breakpoints with respect to the position of the G-bands appears to be related to the synaptic behavior seen in translocation heterozygotes. If both breaks of a reciprocal translocation lie in G-light bands, as was the case with R5, synapsis is confined to homology. However, if one break lies in or immediately adjacent to a G-dark band, there is nonhomologous synapsis, as occurs with R1 and R6. Comparison of the synaptic behavior of R1 with R5 and R6 leads to the conclusion that this G-band-related nonhomologous synapsis is of a different type than the "synaptic adjustment" phenomenon that has been described by Moses (1977a). This G-band-related nonhomologous synapsis is not substage-specific, but competes with homologous synapsis during zygotene-early pachytene.     

Meiosis: how male flies do meiosis

In meiosis in male fruitflies, chromosome pairing events do not facilitate genetic exchange, but rather create bivalents that can be sequestered to discrete pockets of the prophase nucleus. This assignment of homologs to a common pocket likely facilitates the orientation of homologous centromeres to opposite poles.     

Meiosis in translocation heterozygotes in the mosquito Culex pipiens

Adult Culex pipiens males irradiated with both X-rays and neutrons were crossed to untreated females and F₁-egg rafts were checked for dominant lethality. F₁-progenies were outcrossed with normal individuals in order to obtain lines with inherited semisterility. From a total of 120 lines that showed a certain amount of sterility 12 lines were studied cytologically. 10 lines showed reciprocal chromosome exchanges.—At late pachytene and diplotene cross configurations with large asynaptic regions at the center of the cross are obligatory. Bivalents, chains of three, chains of four, and ring configurations are present at metaphase and anaphase I. The different frequencies of the occurrence of such multiples are dependent on the chromosomes involved in the exchange, the length of the pairing segments and the chiasma frequencies in these segments. Chiasma frequency in the interstitial segments is reduced by means of chiasma interference over the centromere and by asynapsis near the breakage points. — Alternate, adjacent-1- and adjacent-2-distributions are present to a different extent. Alternate distribution is most, adjacent-2-distribution least frequent. — The role of translocations and the probability of their becoming effective in pest eradication programs is discussed.