Genetic control of sex-chromosomal univalency in the spermatocytes of C57BL/6J and DBA/2J mice

The genetic control of sex-chromosomal univalency was examined in the primary spermatocytes of the mouse. The C57BL/6J strain expresses 3% X-Y univalency and DBA/2J expresses 37% univalency. The reciprocal F1 and the eight types of reciprocal backcross males were examined. In the C57BL/6J--DBA/2J strain pair, X--Y univalency is controlled by three genetic systems. Autosomal factors of unknown number that are dominant in DBA/2J increase the probability of univalency from 3% in C57BL/6J to 12%. The DBA/2J-Y chromosome, in place of the C57BL/6J-Y chromosome, has an additive effect to increase the probability of univalency from 12 to 37% in the DBA/2J strain. Two X-chromosome factors that differ between C57BL/6J and DBA/2J regulate the probability of univalency. The X-chromosome factors appear to be separated by sufficient distance so that, with the DBA/2J-Y chromosome and dominant DBA/2J autosomal factors, there are two recombinant classes of X--Y univalency at 20 and 60%. The genetic factors in the univalency trait may be involved in the regulation or structure of the terminal attachment sites between the X and Y chromosomes.     

Chiasma frequency,distribution and interference maps of mouse autosomes

Chiasma frequencies were analysed and chiasma positions measured in diakinesis/metaphase I autosomal bivalents from oocytes and spermatocytes of F₁ hybrid C3H/HeH×101/H mice. Twenty chromosome size ranks, including the presumptive X bivalent, could be distinguished in oocytes, and nineteen autosomal ranks plus the XY pair spermatocytes. Overall, mean cell chiasma frequencies of the two sexes did not differ significantly once the contribution of the presumptive X bivalent and the XY pair were taken into account. Sex related differences in chiasma distribution patterns were evident, however. In monochiasmate bivalents, the chiasma was most commonly located interstitially in oocytes while in spermatocytes it could be either interstitial or distal. In dichiasmate bivalents, the chiasmata tended to be more centrally located in oocytes than in spermatocytes. Minimum inter-chiasma distances did not appear to show any great variation in chromosome pairs of different sizes, however, mean inter-chiasma distances did increase with the bivalent length. The minimum-inter chiasma distance data suggest that chiasma interference is complete over a chromosomal segment equating to approximately 60 Mb. Measurement of the positions of chiasmata along chromosome arms open up the possibility of producing chiasma-based genetic maps for all the autosomes of the mouse.     

Meiotic chromosome structure. Kinetochores and chromatid cores in standard and B chromosomes of Arcyptera fusca (Orthoptera) revealed by silver staining.

The behaviour of two chromosome structures in silver-stained chromosomes was analyzed through the first meiotic division in spermatocytes of the acridoid species Arcyptera fusca. Results showed that at diakinesis kinetochores and chromatid cores are individualized while they associate in bivalents of metaphase I; only kinetochores and distal core spots associate in the sex chromosome. Metaphase I is characterized by morphological and localization changes of both kinetochores and cores which define the onset of anaphase I. These changes analyzed in both autosomes and in the sex chromosome allow us to distinguish among three different substages in metaphase I spermatocytes. B chromosomes may be present as univalents, bivalents, or trivalents. Metaphase I B univalents are characterized by separated cores except at their distal ends and individualized and flat sister kinetochores. At anaphase I sister kinetochores of lagging B chromatids remain connected through a silver-stained strand. The behaviour of cores and kinetochores of B bivalents is identical with that found in the autosomal bivalents. The differences in the morphology of kinetochores of every chromosome shown by B trivalents at metaphase I may be related to the balanced forces acting on the multivalent. The results show dramatic changes in chromosome organization of bivalents during metaphase I. These changes suggest that chromatid cores are not involved in the maintenance of bivalents. Moreover, the changes in morphology of kinetochores are independent of the stage of meiosis but correlate with the kind of division (amphitelic-syntelic) that chromosomes undergo.     

Frequency of X-Y chromosome dissociation in mouse spermatocytes from interstrain crosses,recombinant inbred strains,and chimeras: Possible involvement of paternal genome imprinting

The frequency of dissociation of the X-Y chromosome bivalent in diakinesis-metaphase I spermatocytes differs significantly between two inbred mouse strains, CBA (29%) and KE (7%), that were used to obtain reciprocal F1 hybrids, and to develop recombinant inbred (R1) strains. The level of X-Y dissociation was significantly higher in (KExCBA)F1 hybrids sired by the CBA males (24%) than in reciprocal F1 hybrids (12%), revealing the inheritance after the father. Among 14 RI strains, nine were concordant with KE, one with CBA, and four had intermediate phenotype, significantly different from both progenitor strains. This shows that at least two genes are involved, and their possible linkage with agouti and Trf loci is suggested. The linkage with agouti was confirmed by testing additional 10 CBXE incipient RI strains. There was no significant difference in the level of X-Y dissociation between EXCB RI strains derived from the original cross sired by the CBA males and CBXE RI strains derived from the reciprocal cross. The involvement of the Y chromosome-linked factors was unlikely because it was found earlier (Krzanowska, 1989: Gamete Res 23:357–365) that two congenic strains, KE and KE.CBA, differing with respect to the source of the Y chromosome, had the same level of X-Y dissociation. Thus, the difference obtained between reciprocal F1 hybrids is interpreted in terms of paternal genome imprinting imposed by CBA males and propagated only in the presence of some alleles derived from this strain. Analysis of six KE ? CBA-T6 chimeras, among them three germ line chimeras, points to the conclusion that the tendency to low or high level of X-Y chromosome dissociation is expressed rather autonomously by KE or CBA-T6 spermatocytes (as recognized by a marker chromosome pair), respectively, and was not modified by the presence of somatic cells of the opposite strain. © 1994 Wiley-Liss, Inc.     

Dynamic changes in Rad51 distribution on chromatin during meiosis in male and female vertebrates

Antibodies against human Rad51 protein were used to examine the distribution of Rad51 on meiotic chromatin in mouse spermatocytes and oocytes as well as chicken oocytes during sequential stages of meiosis. We observed the following dynamic changes in distribution of Rad51 during meiosis: (1) in early leptotene nuclei there are multiple apparently randomly distributed, foci that by late leptonema become organized into tracks of foci. (2) These foci persist into zygonema, but most foci are now localized on Rad51-positive axes that correspond to lateral elements of the synaptonemal complex. As homologs synapse foci from homologous axes fuse. The distribution and involvement of Rad51 foci as contact points between homologs suggest that they may be components to early recombination nodules. (3) As pachynema progresses the number of foci drops dramatically; the temporal occurrence (mice) and physical and numerical distribution of foci on axes (chickens) suggest that they may be a component of late recombination nodules. (4) In early pachynema there are numerous Rad51 foci on the single axis of the X (mouse spermatocytes) or the Z (chiken oocytes) chromosomes that neither pair, nor recombine. (5) In late pachynema in mouse spermatocytes, but not oocytes, the Rad51 signal is preferentially enhanced at both ends of all the bivalents. As bivalents in spermatocytes, but not oocytes, begin to desynapse at diplonema they are often held together at these Rad51-positive termini. These observations parallel observations that recombination rates are exceptionally high near chromosome ends in male but not female eutherian mammals. (6) From diakinesis through metaphase I, Rad51 protein is detected as low-intensity fluorescent doublets that localize with CREST-specific antigens (kinetochores), suggesting that Rad51 participates, at least as a structural component of the materials involved, in sister kinetochore cohesiveness. Finally, the changes in Rad51 distribution during meiosis do not appear to be species specific, but intrinsic to the meiotic process.     

Meiosis in Mesostoma ehrenbergii ehrenbergii (Turbellaria,Rhabdocoela)

At metaphase I of meiosis in spermatocytes of Mesostoma ehrenbergii ehrenbergii [2n=10] three bivalents and four univalents form. The same two chromosome pairs always form the univalents. Analysis of metaphase I, anaphase I and metaphase II configurations in fixed testis material suggested that the distribution of the four univalents is not a random process but the correct segregation of one member of each pair to each pole is actively achieved before the end of metaphase I. In live preparations of testis material univalents were observed to move between the poles of metaphase I cells, eventually reaching the correct segregation. All cells observed to enter anaphase I had the correct segregation of univalents. It is proposed that the univalent movement during metaphase I is directed towards obtaining the correct segregation of univalents before the cells enter anaphase.     

X-Y chromosome dissociation in mouse strains difering in efficiency of spermatogenesis: Elevated frequency of univalents in pubertal males

Dissociation of the X-Y chromosome bivalent in diakinesis-metaphase I spermatocytes of adult mice was significantly more frequent in the CBA strain (29%) than in C57, KP, or KE strains (7–11%). Autosome dissociatio (1–5%) involved only the smallest chromosome pairs. Eleyatedfrequency of X-Y dissociation in the CBA strain correlates with significantly lower testes weight and lower yield of spermatogenesis, which suggests that sex bivalent dissociation man be responsible for some loss of spermatogenic cells. However, sperm quality is not affected, the percentage of normal spermatozoa and their fertlizing capacity being higher in CBA thatn in the remaining strains. Two congenic strains, KE and KE. CBA (the latter with the Y chromosome introduced from CBA), had the same level of X-Y dissociatios, suggesting that the Y chromosome plays no rle in the determination of this character. In comparison with adult males pubertal (27–29 day-old) males had twice as hig a frequency of X-Y dissociation in KE an KP strains, and combined frequeicies of dissociated sex and autosome bivalents were significantly higher in pubertal males of all tested strains. Although te level of chromosome dissociation is not sufficient to explain increased mortality of germ cells observed in pubertal males, it could be one of the contributing factors.     

Segregation of the amphitelically attached univalent X chromosome in the spittlebug <Emphasis Type="Italic">Philaenus spumarius</Emphasis>

In meiosis I, homologous chromosomes combine to form bivalents, which align on the metaphase plate. Homologous chromosomes then separate in anaphase I. Univalent sex chromosomes, on the other hand, are unable to segregate in the same way as homologous chromosomes of bivalents due to their lack of a homologous pairing partner in meiosis I. Here, we studied univalent segregation in a Hemipteran insect: the spittlebug Philaenus spumarius. We determined the chromosome number and sex determination mechanism in our population of P. spumarius and showed that, in male meiosis I, there is a univalent X chromosome. We discovered that the univalent X chromosome in primary spermatocytes forms an amphitelic attachment to the spindle and aligns on the metaphase plate with the autosomes. Interestingly, the X chromosome remains at spindle midzone long after the autosomes have separated. In late anaphase I, the X chromosome initiates movement towards one spindle pole. This movement appears to be correlated with a loss of microtubule connections between the kinetochore of one chromatid and its associated spindle pole.     

The aurora B kinase AIR-2 regulates kinetochores during mitosis and is required for separation of homologous Chromosomes during meiosis

BACKGROUND: Mitotic chromosome segregation depends on bi-orientation and capture of sister kinetochores by microtubules emanating from opposite spindle poles and the near synchronous loss of sister chromatid cohesion. During meiosis I, in contrast, sister kinetochores orient to the same pole, and homologous kinetochores are captured by microtubules emanating from opposite spindle poles. Additionally, mechanisms exist that prevent complete loss of cohesion during meiosis I. These features ensure that homologs separate during meiosis I and sister chromatids remain together until meiosis II. The mechanisms responsible for orienting kinetochores in mitosis and for causing asynchronous loss of cohesion during meiosis are not well understood. RESULTS: During mitosis in C. elegans, aurora B kinase, AIR-2, is not required for sister chromatid separation, but it is required for chromosome segregation. Condensin recruitment during metaphase requires AIR-2; however, condensin functions during prometaphase, independent of AIR-2. During metaphase, AIR-2 promotes chromosome congression to the metaphase plate, perhaps by inhibiting attachment of chromatids to both spindle poles. During meiosis in AIR-2-depleted oocytes, congression of bivalents appears normal, but segregation fails. Localization of AIR-2 on meiotic bivalents suggests this kinase promotes separation of homologs by promoting the loss of cohesion distal to the single chiasma. Inactivation of the phosphatase that antagonizes AIR-2 causes premature separation of chromatids during meiosis I, in a separase-dependent reaction. CONCLUSIONS: Aurora B functions to resolve chiasmata during meiosis I and to regulate kinetochore function during mitosis. Condensin mediates chromosome condensation during prophase, and condensin-independent pathways contribute to chromosome condensation during metaphase.     

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.by C. Heyting     

Cytogenetics of chromosome rearrangements in Tribolium castaneum.

The karyotype of the red flour beetle, Tribolium castaneum, was reexamined and improved by restriction enzyme banding with HpaII. After this treatment, each of the 10 chromosomes were identified in spermatogonial metaphase cells and 3 of the 8 autosomal bivalents and the XY pair were identified in spermatocyte metaphase I nuclei. Based on centromere position, relative length, and banding pattern, probable correlations between some of the mitotic chromosomes and some of the metaphase I bivalents were ascertained. Thus improved, the karyotypes of beetles harboring genetically defined translocations were investigated. Spermatocyte metaphase I nuclei were most informative, as normal chromosome pairing was visibly disrupted by rearrangements. Bivalents associated with each rearrangement were identified. Results demonstrated that each of the five best defined T. castaneum linkage groups corresponds to a different chromosome and established correspondence between bivalents and linkage groups 1-4. The relevance of these findings is discussed with regard to Tribolium genetics and evolution.     

Effect of heterozygosity for insertions of homogeneously stained regions in chromosome 1 of the house mouse on synapsis in meiotic prophase

Electron microscope analysis of surface-spread synaptonemal complexes (SC) in oocytes and spermatocytes from double cis heterozygotes for Is(HSR; 1C5)1Icg and Is(HSR; 1E3)2Icg was carried out. Aberrant chromosomes were isolated from the feral population of Mus musculus musculus of Novosibirsk. They contain homogeneously stained regions of total length of about 30% of Chr 1 mitotic metaphase. Heteromorphic bivalents of Chr1 with different lengths of the lateral elements of SC and the loop in the intermedial position were revealed in 4.4% spermatocytes and 20% oocytes of heterozygous animals. The loop size depends on the stage of meiosis: it is maximal at late zygotene and decreases up to disappearance during pachytene.     

Differential immunolocalization of a putative Rec8p in meiotic autosomes and sex chromosomes of triatomine bugs

Hemipteran chromosomes are holocentric and show regular, special behavior at meiosis. While the autosomes pair at pachytene, have synaptonemal complexes (SCs) and recombination nodules (RNs) and segregate at anaphase I, the sex chromosomes do not form an SC or RNs, divide equationally at anaphase I, and their chromatids segregate at anaphase II. Here we show that this behavior is shared by the X and Y chromosomes of Triatoma infestans and the X(1)X(2)Y chromosomes of Triatoma pallidipennis. As Rec8p is a widely occurring component of meiotic cohesin, involved in meiotic homolog segregation, we used an antibody against Rec8p of Caenorhabditis elegans for immunolocalization in these triatomines. We show that while Rec8p is colocalized with SCs in the autosomes, no Rec8p can be found by immunolabeling in the sex chromosomes at any stage of meiosis. Furthermore, Rec8p labeling is lost from autosomal bivalents prior to metaphase I. In both triatomine species the sex chromosomes conjoin with each other during prophase I, and lack any SC, but they form "fuzzy cores", which are observed with silver staining and with light and electron microscopy during pachytene. Thin, serial sectioning and electron microscopy of spermatocytes at metaphases I and II reveals differential behavior of the sex chromosomes. At metaphase I the sex chromosomes form separate entities, each surrounded by a membranous sheath. On the other hand, at metaphase II the sex chromatids are closely tied and surrounded by a shared membranous sheath. The peculiar features of meiosis in these hemipterans suggest that they depart from the standard meiotic mechanisms proposed for other organisms.     

Histone modifications related to chromosome silencing and elimination during male meiosis in Bengalese finch

We report here that a germline-restricted chromosome (GRC) is regularly present in males and females of the Bengalese finch (Lonchura domestica). While the GRC is euchromatic in oocytes, in spermatocytes this chromosome is cytologically seen as entirely heterochromatic and presumably inactive. The GRC is observed in the cytoplasm of secondary spermatocytes, indicating that its elimination from the nucleus occurs during the first meiotic division. By immunofluorescence on microspreads, we investigated the presence of histone H3 modifications throughout male meiosis, as well as in postmeiotic stages. We found that the GRC is highly enriched in di- and trimethylated histone H3 at lysine 9 during prophase I, in agreement with the presumed inactive state of this chromosome. At metaphase I, dimethylated histone H3 is no longer detectable on the GRC and its chromatin is more faintly stained with DAPI. The condensed GRC is underphosphorylated at serine 10 compared to the regular chromosomes during metaphase I, being phosphorylated later at this site after the first meiotic division. From these results, we proposed that trimethylation of histone H3 at lysine 9 on the GRC chromatin increases during metaphase I. This hypermethylated state at lysine 9 may preclude the phosphorylation of the adjacent serine 10 residue, providing an example of cross-talk of histone H3 modifications as described in experimental systems. The differential underphosphorylation of the GRC chromatin before elimination is interpreted as a cytologically detectable byproduct of deficient activity of Aurora B kinase, which is responsible for the phosphorylation of H3 at serine 10 during mitosis and meiosis.     

Nonrandom chromosome segregation in Neocurtilla (Gryllotalpa) hexadactyla: an ultrastructural study

During meiosis I in males of the mole cricket Neocurtilla (Gryllotalpa) hexadactyla, the univalent X1 chromosome and the heteromorphic X2Y chromosome pair segregate nonrandomly; the X1 and X2 chromosomes move to the same pole in anaphase. By means of ultrastructural analysis of serial sections of cells in several stages of meiosis I, metaphase of meiosis II, and mitosis, we found that the kinetochore region of two of the three nonrandomly segregating chromosomes differ from autosomal kinetochores only during meiosis I. The distinction is most pronounced at metaphase I when massive aggregates of electron-dense substance mark the kinetochores of X1 and Y chromosomes. The lateral position of the kinetochores of X1 and Y chromosomes and the association of these chromosomes with microtubules running toward both poles are also characteristic of meiosis I and further distinguish X1 and Y from the autosomes. Nonrandomly segregating chromosomes are typically positioned within the spindle so that the kinetochoric sides of the X2Y pair and the X1 chromosome are both turned toward the same interpolar spindle axis. This spatial relationship may be a result of a linkage of X1 and Y chromosomes lying in opposite half spindles via a small bundle of microtubules that runs between their unusual kinetochores. Thus, nonrandom segregation in Neocurtilla hexadactyla involves a unique modification at the kinetochores of particular chromosomes, which presumably affects the manner in which these chromosomes are integrated within the spindle.     

Male and female meiosis in grasshoppers II. Chorthippus brunneus

The frequencies and distribution patterns of chiasmata in the autosomal bivalents of Chorthippus brunneus are very similar in male and female meiosis. The mean cell chiasma frequency of oocytes is however significantly higher than that of spermatocytes, but this difference is almost certainly due to the incremental effect of the X bivalent on the chiasma frequencies of oocytes. The implications of these findings in terms of chiasma control are briefly discussed.     

Polyploidy and reproductive patterns in the root-knot nematode Meloidogyne hapla

A total of 32 populations and egg mass isolates of Meloidogyne hapla obtained from various geographical areas were studied cytologically and with respect to their mode of reproduction. In 29, maturation of oocytes is by regular meiosis. The reduced chromosome number at metaphase I is 17 in 18 populations, 16 in 8, and 15 in 3 populations. Reproduction in all these populations is by cross-fertilization, although nonfertilized eggs can develop by parthenogenesis. In the latter case, the two groups of telophase chromosomes of the second maturation division become enclosed in the same pronucleus, thus reestablishing the somatic chromosome number. Maturation of spermatocytes in three populations studied is by regular meiosis and the reduced chromosome number appears to be equal to that of the oocytes. In the remaining three populations, no synapsis takes place and the somatic number of 45 chromosomes is observed at metaphase of the single maturation division of both oocytes and spermatocytes. Reproduction is by obligatory mitotic parthenogensis. It is postulated that the basic chromosome number for the genus is nine and that the facultatively parthenogenetic populations are tetraploid, whereas, the obligatorily parthenogenetic populations are pentaploid. A preliminary scheme of the phylogeny in the family Heteroderidae is given.     

Pattern and frequency of nondisjunction in oocytes from the Djungarian hamster are determined by the stage of first meiotic spindle inhibition

In order to study the mechanisms of nondisjunction at meiosis I in oocytes gonadotropin-stimulated Djungarian hamsters were treated at two stages [4.5 and 6 h post human chorionic gonadotropin (HCG)] during the preovulatory period with 1000 mg/kg Carbendazim (MBC). The compound, known to bind fast but reversibly to mammalian tubulin, was chosen to investigate whether the stage at which spindle function is inhibited affects the pattern of nondisjunction. Ovulated oocytes were cytologically prepared and scored for hyperhaploidy, diploidy and presegregation. Application at an early spindle phase, 4.5 h post HCG, to females stimulated with a low gonadotropin dose [3 IU pregnant mares serum (PMS); 2 IU HCG] caused a high frequency of nondisjunction (40.6%) with a more or less nonspecific pattern of malsegregated bivalents. Treatment at a late stage of spindle function (6 h post HCG) resulted in a less frequent (22.5%) but highly preferential malsegregation of those A-D group bivalents thought earlier to be late segregators. On the other hand, oocytes from females primed with a high (10 IU PMS and HCG) gonadotropin dose, a treatment assumed to delay meiosis by approximately 1.5 h, responded to MBC treatment at the late stage (6 h) with a nonspecific pattern and a high frequency (71.2%) of nondisjunction. The latter result is comparable to that in which MBC was given at the early stage (4.5 h) and after a low gonadotropin dose. The high nondisjunction response additionally indicates that spindles in hypergonadotropic stimulated oocytes are more susceptible and/or that the concentration of the inhibitor is higher in such oocytes. Only few oocytes with presegregation (3.1%; 0.0%; 1.7%) and few diploid oocytes (3.3%; 1.5%; 3.2%) with complete inhibition of meiosis I were observed. We conclude, that in Djungarian hamsters (1) the segregation of bivalents at meiosis I is asynchronous with the large A-D bivalents segregating last, (2) the phase in which spindle function is inhibited determines the pattern of nondisjunction, and (3) the resumption of meiosis I — from dictyotene to metaphase II — does not follow a rigidly timed programme but depends on the conditions of follicular maturation.     

Equal frequencies of recombination nodules in both sexes of the pigeon suggest a basic difference with eutherian mammals.

The total number of recombination nodules (RNs) in the autosomal synaptonemal complexes (SCs) is statistically equivalent in oocytes and spermatocytes from the domestic pigeon Columba livia. The distribution on RNs along the three longest autosomes is also equivalent in oocytes and spermatocytes. The numbers of RNs show a linear relationship when plotted against SC length both in oocytes and spermatocytes. On the other hand, the ZW pair shows a single and strictly localized RN near the synaptic termini, but the ZZ pair shows unrestricted location of RNs (average 3.8). The ZW and ZZ pairs of the pigeon are euchromatic and do not show specific chromatin packing at pachytene in either sex. The lack of sex-specific differences in the number and location of RNs in the autosomal bivalents of C. livia and previous data on the chicken, suggest that the regulation of crossing-over is basically different in birds and mammals.