Autosomal Univalents Research Articles

Meiotic chromosome analysis of the giant water bug, Lethocerus indicus.

The giant water bug, Lethocerus indicus (Lepeletier and Serville) (Heteroptera: Belostomatidae), a native species of Southeast Asia, is one of the largest insects belonging to suborder Heteroptera. In this study, the meiotic chromosome of L. indicus was studied in insect samples collected from Thailand, Myanmar, Loas, and Cambodia. Testicular cells stained with lacto-acetic orcein, Giemsa, DAPI, and silver nitrate were analyzed. The results revealed that the chromosome complement of L. indicus was 2n = 22A + neo-XY + 2m, which differed from that of previous reports. Each individual male contained testicular cells with three univalent patterns. The frequency of cells containing neo-XY chromosome univalent (∼5%) was a bit higher than that of cells with autosomal univalents (∼3%). Some cells (∼0.5%) had both sex chromosome univalents and a pair of autosomal univalents. None of the m-chromosome univalents were observed during prophase I. In addition, this report presents clear evidence about the existence of m-chromosomes in Belostomatidae.

Chromosomal effects of sodium selenite in vivo

Sodium selenite (Na2SeO3) was injected intraperitoneally (0.8 mg Se/kg body weight) into 12 male NMRI mice, while 6 control animals received sterile water. After 24 h, cytogenetic preparations of bone marrow and testis were made. Bone marrow of the Na2SeO3-treated mice contained no more cells with structural chromosomal aberrations (0.5 % in 666 cells) or gaps (1.4 %) than that of the controls (1.1 % aberrations, 0.8 % gaps, in 264 cells). There was also no difference between the two groups in the numbers of metaphase-diakinesis stages of primary spermatocytes with chromosomal fragments (0.2 % in 982 cells in Na2SeO3-group, 0.4% in 225 cells in controls), autosomal univalents (0.1% in Na2SeO3-group, 0.0 % in controls) or XY-univalents (0.1 % in Na2SeO3-group, 0.9 % in controls).

The mechanisms behind the regular segregation of autosomal univalents in Calocoris quadripunctatus (Vil.) (Miridae, Hemiptera)

One autosomal chromosome pair was found to appear as univalents in about 13.5% of late prophase I and metaphase I nuclei in Calocoris quadripunctatus males, providing a possibility to analyze the segregational behaviour of univalent chromosomes. It was found, that either syntelically or amphitelically orientated at metaphase I, the univalents segregated regularly from each other at the first meiotic division. On the basis of the behaviour of univalents, it is Suggested that syntelically orientated univalents utilized a mechanism operating during meiotic prophase, and amphitelically orientated, a mechanism operating during anaphase I for their regular segregation. Details involved in these mechanisms are present in all normal meioses. Further, it is suggested, that although the existence of such mechanisms can be considered beneficial, their disadvantage lies in the fact that they reduce selection for the existence of at least one chiasma per bivalent, thus leading to the occurrence of a small amount of nondisjunction in meiosis.

Cytogenetic studies on Mepraia gajardoi (Heteroptera: Reduviidae). Chromosome behaviour in a spontaneous translocation mutant

We analysed the chromosome complement and male meiosis in a natural population of Mepraia gajardoi (Hemiptera: Reduviidae: Triatominae). The normal karyotype of this species is 20 autosomes plus X1X2Y in males and X1X1X2X2 in females. We confirmed that M. gajardoi is cytogenetically strikingly different from M. spinolai (with which it was previously included), which supports the taxonomic separation of these two species. For the first time in the holocentric chromosomes of the subfamily Triatomi- nae, a structural rearrangement was detected. Observations made during meiosis and mitosis indicated that one of eleven individuals had undergone a fusion between two non-homologous chromosomes, probably before germinal cell development. This autosomal translocation resulted in chromosomal irregularities such as an autosomal trivalent, autosomal univalents and chromosomal frag- ments, which altered the normal segregation of both autosomes and sex chromosomes. The chromosomal fragments resemble super- numerary or B chromosomes, which supports the idea that these chromosomes are remnants of structural rearrangements. The genetic consequences of translocations in holokinetic chromosomes and their role in the evolution of chromosomes in triatomines are also discussed.

Cytogenetic Studies of Some Cephalosporines Antibiotics on Mouse Germinal Cells.

The ability of the 2 cephalosporins antibiotics “Cefadroxil” and “Cefaclor” to induce chromosomal abnormalities in mouse spermatocytes was investigated. Male Swiss mice were treated orally by gavage once with the doses 40, 80, 160 mg kg-1 b.wt. of “Cefadroxil” and 20, 40, 80 mg kg-1 b.wt. of “Cefaclor” and samples were taken 24 h after the treatment. A repeated daily treatment with therapeutic does of 40 mg “Cefadroxil” kg-1 b.wt. and 20 mg “Cefaclor” kg-1 b.wt. were given for 14 successive days samples were taken 24 h post treatment. The percentage of chromosomal aberrations in diakinesis-metaphase I spermatocytes increased in a dose dependent manner and was found to be statistically significant after high and repeated doses. It reached 6.0±0.5% and 5.6±0.7% (P<0.05) after treatment with 160 mg “Cefadroxil” kg-1 b.wt. and 80 mg “Cefaclor” kg-1 b.wt. respectively compared with 21.6±0.9% after treatment with “Mitomycin C”. The repeated doses of 14×40 mg “Cefadroxil” kg-1 b.wt. and 14×20 mg “Cefaclor” kg-1 b.wt. induced 7.4±0.6% and 6.0±0.7% (P<0.01) respectively. The 2 cephalosporins induced abnormal chromosomal associations including univalents (X-Y) and autosomal univalents) as well as structural chromosome aberrations. “Cefadroxil” and “Cefaclor” induced a dose-dependent increase in the percentage of abnormal sperm heads. Its highest value was 5.05±0.70% and 4.39±0.79% (P<0.05) after oral treatment with “Cefadroxil” and “Cefacolr” respectively, compared with 17.65±0.44 (P<0.01) after treatment with “Mitomycin C”. The results indicate that cephalosporines antibiotics “Cefadroxil” and “Cefaclor” have a remarkable cytogenetic effect on spermatocytes and sperm heads of the mouse when used as high and repeated doses.

Autosomal univalents as a common meiotic feature in Jadera haematoloma (Herrich-Schaeffer, 1847) and Jadera sanguinolenta (Fabricius, 1775) (Heteroptera: Rhopalidae: Serinethinae)

So far, available cytogenetic data on 24 species of Rhopalidae reveal a male diploid chromosome number of 13, with a pair of m chromosomes and an X0/XX (male/female) sex chromosome determining system. As a rule Heteroptera have holokinetic chromosomes and a pre-reductional type of meiosis: the autosomal bivalents and the m pseudobivalent segregate reductionally at first meiotic division, while the X chromosome segregates equationally. In the present study, the meiotic chromosome behaviour was analyzed in males from different Argentinean populations of Jadera haematoloma and J. sanguinolenta. Our results corroborate the diploid chromosome number and general patterns of male meiosis previously reported by other authors in samples from Brazil and Texas (USA). Among bivalents, one is remarkably larger and may present one or two terminal chiasmata. Comparison of mean chi- asma frequency between Jadera haematoloma (5.63) and J. sanguinolenta (5.14) revealed that differences are significant. In most individuals of both species the largest pair appears as univalents in a variable number of cells and shows a regular meiotic segrega- tion. Autosomal univalents orientate axially at metaphase I (with their long axis parallel to the spindle axis) and segregate equation- ally at anaphase I. At metaphase II they associate end-to-end forming a pseudobivalent that segregates reductionally at anaphase II. An hypothesis is suggested to explain the appearance of the largest pair, either as a ring/rod bivalent or as univalents within the same individual, although an asynaptic or desynaptic origin of the univalents cannot be ascertained. The highly regular meiotic behaviour of this autosomal pair could ensure a high fertility of the individuals, and could be considered a selectively neutral condition or, at least, not detrimental.

Sex-dependent frequency and type of autosomal univalency at the first meiotic metaphase in mouse germ cells

Univalents at the first meiotic metaphase in mouse spermatocytes occur mainly in the XY pair, making it difficult to compare the amounts of univalency in males and females. In this study, the amounts of autosomal univalency in male and female meiosis were compared using the model strain CBA-T6, in which univalency of the small marker autosome pair T6 has been shown to occur very frequently in spermatocytes. Mice from inbred CBA and DBA strains were also analysed. The total frequencies of univalency (sex chromosomes plus autosomes) in metaphase I spermatocytes were 45.6% in CBA, 36.9% in CBA-T6, and 37.3% in DBA males. The aneuploidy in metaphase II spermatocytes ranged from 1.4 to 3% in these strains, which was in agreement with previous findings that most primary spermatocytes with abnormal chromosome configurations are arrested in their development before metaphase II. In the CBA-T6 strain, autosomal univalency at metaphase I mostly involved chromosome pair T6; however, its frequency differed significantly between the sexes, amounting to 18.9% in spermatocytes and 4.3% in oocytes. In the CBA strain, autosomal univalents at metaphase I were seen in 7.7% of the spermatocytes and 1.4% of the oocytes and, in DBA mice, in 4.9% of the spermatocytes and 3.8% of the oocytes. However, in DBA oocytes, when univalency occurred it usually concerned a greater number of bivalents in one cell (range: 2-19 disjoined bivalents), a phenomenon very rare in males of this strain. This study shows that univalent formation differs between the male and female types of meiosis.

Potential Transmission of the Cytogenetic Effects of Cisplatin in the Male Germline Cells of Swiss Mice

Cisplatin (CP) (in Oncoplatin), a widely used drug in cancer chemotherapy, and cyclophosphamide (CY) (in Endoxan), another anticancer drug, were investigated as the test chemical and positive control, respectively, for their cytogenetic effects on spermatogonia of mice at 24 hours post-treatment after a single exposure. The different doses of the chemicals tested were CP 2, 3, 5 mg/kg and CY 40 mg/kg b.w. of mice. Each of the doses of CP induced a significant number of chromosomal aberrations, mostly chromatid breaks and fragments. The potential transmission of such cytogenetic effects of the chemicals from spermatogonia to spermatocytes was assessed at week 4 post-treatment from the primary spermatocytes, which showed a significant number of aberrant spermatocytes with atypical bivalents viz. spermatocytes with autoso-mal and/or XY univalents, tetravalents and with extra elements. The probable causes of the formation of univalents and tetravalents are discussed. The transmission of the cytogenetic effects of the chemicals from spermatogonia up to sperm was assessed at week 8 post-treatment from the morphology of sperm collected from vas. Quantitatively the transmission of such effects was found decreased substantially by the time the exposed spermatogonia became sperm. Still there was the occurrence of a few abnormal sperm at week 8 post-treatment. The probable causes of the quantitative decrease in the transmission of the effects from spermtogonia to sperm are discussed.

Colchicine effects on meiosis in the male mouse: II. Inhibition of synapsis and induction of nondisjunction

This report follows from our earlier study using synaptonemal complex (SC) analysis in which colchicine administered to mouse spermatocytes specifically at leptotene/zygotene blocks synapsis, resulting in univalents at early pachytene. Despite loss of severely damaged cells from the prophase population, substantial numbers of cells with lesser damage progress to late pachytene on schedule. The present study tests whether the surviving cells would continue through meiotic divisions and if so, whether the univalents at MI result in hyperploidy at MII. At 7 days after treatment (late pachytene) 5.9% of the surviving population contains at most four autosomal axial univalents. In whole chromosome preparations 10 days post-colchicine the highest frequency of MIs with univalents is 5.2%. The maximum number of autosomal “chromosomal” univalents per cell is four. The percentage of cells with autosomal univalents at late pachytene, is not significantly different from the percentage of cells with chromosomal univalents at MI. We infer from these observations that the two kinds of univalents are equivalent. At days 11–12 post-colchicine, hyper (and hypo) ploidy at AI–MII is observed. We conclude that univalents produced by colchicine-induced asynapsis at leptotene/zygotene persist and lead to nondisjunction at division I and hyperploidy at division II. If the hyperploid spermatids mature, they would give rise to aneuploid sperm, thus constituting a mechanism for inducing aneuploid (e.g., trisomic) zygotes after fertilization. It is also observed that chiasma frequency (number of chiasmata per bivalent, univalents excluded) is reduced by about 15% of the control. Nondisjunction is known to be the endpoint of colchicine action when administered at prometaphase-MI, interfering with the segregation of homologues through effects on the MI–AI spindle. We show that nondisjunction is also the endpoint of colchicine's effect at early pachytene, in this case causing synaptic inhibition that creates univalents which are then distributed randomly at first division. These conclusions draw special attention to predivision meiotic events, particularly those affecting synapsis, and their sensitivity to induced and/or inherent effects that may have consequences later at meiotic divisions, creating risk to the chromosomal constitution of the gametes.

The role of sister chromatid cohesiveness and structure in meiotic behaviour.

Sister chromatid cores, kinetochores and the connecting strand between sister kinetochores were differentially silver stained to analyse the behaviour of these structures during meiosis in normal and two spontaneous desynaptic individuals of Chorthippus jucundus (Orthoptera). In these desynaptic individuals most of the chromosomes appear as univalents and orient equationally in the first meiotic division. Despite this abnormal segregation pattern, the changes in chromosome structure follow the same timing as in normal individuals and seem to be strictly phase dependent. Chromosomes in the first prometaphase have associated sister kinetochores and sister chromatid cores that lie in the chromosome midline; we propose that this promotes the initial monopolar orientation of chromosomes. However, the requirements of tension for stable attachment to the spindle force the autosomal univalents to acquire amphitelic orientation. Sister kinetochores behave in a chromosome orientation-dependent manner and, in the first metaphase, they appear to be interconnected by a strand that can be detected by silver impregnation, as seen in the second metaphase of wild-type individuals. The disappearance of the sister kinetochore-connecting strand, needed for equational chromatid segregation, however, can only take place in the second meiotic division. This connecting strand is ultimately responsible for the inability of chromosomes to segregate sister chromatids in the first anaphase.

Colchicine effects on meiosis in the male mouse. I. Meiotic prophase: synaptic arrest, univalents, loss of damaged spermatocytes and a possible checkpoint at pachytene.

Antimitotic agents administered at the time of synapsis (leptotene/zygotene) have been shown to induce synaptic abnormalities visible during pachytene in the male mouse. The object of this study was to test the hypothesis that cells with relatively large amounts of colchicine-induced damage to the synaptonemal complex (SC) are eliminated from prophase whereas cells with relatively small amounts of SC damage proceed through to the end of prophase. Male mice were injected with tritiated thymidine to mark a cohort of spermatocytes at premeiotic S-phase for tracking through pachytene. Forty-eight hours later, when those cells were at leptotene/zygotene, colchicine was administered intratesticularly. Whole-mount SC spreads were made from animals sacrificed at various times following colchicine administration, and prepared for autoradiography. The marked cells were examined by light and electron microscopy and the kind and number of synaptic abnormalities were scored throughout pachytene. Colchicine-induced SC damage included single axial elements (univalents), together with partially synapsed and nonhomologously synapsed SCs. The amount of SC damage (amount and type per cell and frequency of cells with damage) scored at early pachytene exceeded by three- to fivefold the amount at late pachytene. This is consistent with spermatogenic cell loss from the seminiferous tubule via colchicine-induced destruction of Sertoli cell microtubules. The presence of spermatocytes with no more than four autosomal univalents at late pachytene indicates that some cells with low amounts of synaptic damage progress to the end of pachytene. The loss of the most severely damaged cells may represent a meiotic checkpoint at early pachytene in the male mouse.

A comparative study of orientation at behavior of univalent in living grasshopper spermatocytes.

Orientational movements and modes of segregation at anaphase I were analyzed in three different types of univalents in living spermatocytes of the grasshopper species Eyprepocnemis plorans, namely the sex univalent, three types of accessory chromosomes and spontaneous and induced autosomal univalents. When two or more univalents were present in the same spindle, their dynamics were directly compared. Chromosomes may show variable velocity and number of reorientations: the X and the most common B types (B1 and B2) are slow and rarely reorient, a more geographically restricted B (B5) is faster and reorients more often, and autosomal univalents are the fastest and show the highest frequency of reorientations. Nonetheless, the X and the accessories are rigorously reductional at anaphase I whereas autosomal univalents often fail to migrate or divide equationally. This indicates that orientational and segregational behavior are controlled mainly by chromosomal rather than cellular characteristics and that chromosomes may display a great variety of strategies to achieve regular segregation.

X-Y chromosome dissociation in mice and rats exposed to increased testicular or environmental temperatures.

Heating the testes, scrota and tails of mice and rats by immersion in a water bath at 42 degrees C for 20 min caused an increased percentage of X-Y univalents in meiotic preparations made after 6 and 12 days respectively. It was also confirmed that exposing mice of a cool-adapted strain to an environment at 33 degrees C for 5 days resulted in an increase in the percentage of X-Y and autosomal univalents in meiotic preparations made after a recovery period of 2 days. Mice of a strain adapted to living at 33 degrees C also showed a higher rate of X-Y dissociation than control cool-adapted mice, but a lower frequency of autosomal univalents than cool-adapted mice exposed to the hot environment. The testes of the heat-adapted mice were even more sensitive than the testes of cool-adapted mice to the effects of local heating, as judged by the fall in testis weight 21 days afterwards.

Deficiency of X and Y chromosomal pairing at meiotic prophase in spermatocytes of sterile interspecific hybrids between laboratory mice (Mus domesticus) and Mus spretus.

The normal association between the X and Y chromosomes at metaphase I of meiosis, as seen in air-dried light microscope preparations of mouse spermatocytes, is frequently lacking in the spermatocytes of the sterile interspecific hybrid between the laboratory mouse strains C57BL/6 and Mus spretus. The purpose of this work is to determine whether the separate X and Y chromosomes in the hybrid are asynaptic, caused by failure to pair, or desynaptic, caused by precocious dissociation. Unpaired X-Y chromosomes were observed in air-dried preparations at diakinesis, just prior to metaphase I. Furthermore, immunocytology and electron microscopy studies of surface-spread pachytene spermatocytes indicate that the X and Y chromosomes frequently fail to initiate synapsis as judged by the failure to form a synaptonemal complex between the pairing regions of the X and Y chromosomes. Several additional chromosomal abnormalities were observed in the hybrid. These include fold-backs of the unpaired X or Y cores, associations between the autosome and sex chromosome cores, and autosomal univalents. The occurrence of abnormal autosomal and XY-autosomal associations was also correlated with cell degeneration during meiotic prophase. The primary breakdown in hybrid spermatogenesis occurs at metaphase I (MI), with the appearance of degenerated cells at late MI. In those cells, the X and Y are decondensed rather than condensed as they are in normal mouse MI spermatocytes. These results, in combination with the previous genetic analysis of spermatogenesis in hybrids and backcrosses with fertile female hybrids, suggest that the spermatogenic breakdown in the interspecific hybrid is primarily correlated with the failure of XY pairing at meiotic prophase, asynapsis, followed by the degeneration of spermatocytes at metaphase I. Secondarily, the failure of XY pairing can be accompanied by failure of autosomal pairing, which appears to involve an abnormal sex vesicle and degeneration at pachytene or diplotene.

Involvement of chromatid cohesiveness at the centromere and chromosome arms in meiotic chromosome segregation: A cytological approach

Kinetochores and chromatid cores of meiotic chromosomes of the grasshopper species Arcyptera fusca and Eyprepocnemis plorans were differentially silver stained to analyse the possible involvement of both structures in chromatid cohesiveness and meiotic chromosome segregation. Special attention was paid to the behaviour of these structures in the univalent sex chromosome, and in B univalents with different orientations during the first meiotic division. It was observed that while sister chromatids of univalents are associated at metaphase I, chromatid cores are individualised independently of their orientation. We think that cohesive proteins on the inner surface of sister chromatids, and not the chromatid cores, are involved in the chromatid cohesiveness that maintains associated sister chromatids of bivalents and univalents until anaphase I. At anaphase I sister chromatids of amphitelically oriented B univalents or spontaneous autosomal univalents separate but do not reach the poles because they remain connected at the centromere by a long strand which can be visualized by silver staining, that joins stretched sister kinetochores. This strand is normally observed between sister kinetochores of half-bivalents at metaphase II and early anaphase II. We suggest that certain centromere proteins that form the silver-stainable strand assure chromosome integrity until metaphase II. These cohesive centromere proteins would be released or modified during anaphase II to allow normal chromatid segregation. Failure of this process during the first meiotic division could lead to the lagging of amphitelically oriented univalents. Based on our results we propose a model of meiotic chromosome segregation. During mitosis the cohesive proteins located at the centromere and chromosome arms are released during the same cellular division.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytological effects of khat (Catha edulis) in somatic and male germ cells of mice.

Cytological effects of khat (Catha edulis), a popular drug of abuse from Southern Arabia and Eastern Africa, have been studied in Swiss albino mice. The studies on the somatic system involved the use of micronucleus test and the cytological analysis of the mitotic index in the femoral cells of mice. In the micronucleus test, the mice were treated with different doses of khat extract (125, 250 and 500 mg/kg, p.o.) 30 and 6 hours before sacrificing the animals. The polychromatic erythrocytes were screened for the induction of micronuclei. For the analysis of bone marrow cytotoxicity, the mice were treated with the dose of 125, 250 and 500 mg/kg, body weight, p.o. daily for 5 consecutive days. The animals were sacrificed and the femoral cells were microscopically examined for the mitoses. Following the same schedule of treatment, studies on the cytogenetic analysis of meiotic chromosomal aberrations and the sperm head abnormality were undertaken. Khat extract significantly increased the frequency of micronucleated polychromatic erythrocytes, induced bone marrow depression and reduced the mitotic index of the somatic cells. It induced significant chromosomal aberrations viz., aneuploids, autosomal univalents, univalents of the sex chromosomes and polyploids. The frequency of abnormal sperms was also increased.

Relationship between the arrangement of microtubules and chromosome behaviour of syntelic autosomal univalents during prometaphase in crane fly spermatocytes.

During meiotic prometaphase of crane fly spermatocytes, syntelic autosomal univalents are able to move between the spindle poles several times. The relationship between the behaviour of chromosomes and the arrangement of microtubules during this stage was studied using a fixation technique (Nicklas et al. 1979) which makes it possible to examine a certain cell in an electron microscope after live observation. After reorientation, when a syntelic univalent had started moving towards the opposite spindle pole, a short chromosome fibre was found. When a univalent had reached the equator, a chromosome fibre could be traced up to the spindle apex. During the movement towards the opposite spindle pole the degree of disorder in the chromosome fibre was high, whereas it was low in the fibre of a motionless univalent. The degree of disorder was determined by the relative proportion of skew fibre microtubules. At the beginning of reorientation a chromosome fibre was still present, but later, it was no longer possible to recognize such a fibre. Instead of a chromosome fibre, a bundle of microtubules laterally associated with the kinetochore was observed. Some microtubules of this bundle had a direct contact with the kinetochore. These observations strongly hint that the laterally associated microtubules have an important function in the reorientation of syntelic autosomal univalents.

Three-dimensional architecture of chromosome fibres in the crane fly: co-oriented autosomal bivalents and amphitelic sex univalents during prometaphase

A technique for the fixation of cells during live observation (Nicklas et al. 1979) was used to investigate chromosomes which were moving at the time of fixation. Chromosome fibres were reconstructed by tracking their microtubules in longitudinal serial sections. A considerable proportion of non-kinetochoric microtubules (free microtubules, fMTs) is skewed with respect to the fibre axis. These skew fMTs contribute to the degree of disorder. It was found that the difference in the relative proportion of skew fMTs between “active” fibres (oriented in the direction of movement) and “passive” fibres (oriented backwards) is significantly correlated with the chromosome velocity (correlation coefficient r=0.796, P=0.01). It can be concluded that the pulling force generated in the chromosome fibre is a function of skew fMTs.

In VivoCytogenetic Effects of Amidopyrine + Allobarbitone on Meiotic Chromosomes of Mice

SUMMARYCytogenetic effects of an analgesic and antipyretic drug Amidopyrine + Allobarbitone were investigated in meiotic cells of Swiss albino male mice. Two sets of experiments were conducted. In one set animals received 0.65, 1.3 and 2.6 mg. doses of the drug in 0.5 ml quantities for only once (single doses series) and in other the same doses were given for thrice at 24 hr. intervals (cumulative dose series) to simulate human therapeutic conditions. Adequate controls were maintained. Meiotic chromosome preparations were made from testes after 24 hr. and at weekly intervals for five weeks. Drug exhibited a very mild clastogenic and poor c-mitotic action. It induced autosomal and sex chromosomal univalents to a significant level, the former being more. Multiple doses were found to be relatively more effective in bringing about the anomalies.

Unipolar orientation stability of the sex univalent in the grasshopper (Melanoplus sanguinipes)

The behavior of the X chromosome in living Melanoplus sanguinipes primary spermatocytes was examined utilizing phase contrast microscopy and micromanipulation. During early prometaphase I, the X univalent establishes a unipolar orientation which is normally stable. In 28 cells, the X chromosome did not reorient once during a total of 122 h of observation. Though normally stable, the X chromosome can be induced to reorient by micromanipulation. An anomalous increase in X reorientation was observed in one testicular preparation. Stable unipolar orientation is an intrinsic characteristic of the X chromosome and was not displayed by two small autosomal univalents discovered in one cell. The two autosomal univalents, resulting from asynapsis or desynapsis, oscillated back and forth within the spindle, averaging 26.0 and 17.6 min respectively between reorientation events. The behavior of the M. sanguinipes X chromosome reveals an orientation stability mechanism that does not involve tension created by forces from opposing kinetochoric spindle fibers.