Synaptonemal complex analysis of hybrid cattle. II. Bos indicus x Bos taurus F1 and backcross hybrids

The levels of meiotic chromosome pairing abnormalities observed in six Australian F1 Bos indicus x Bos taurus cattle crosses (mean = 23%) were significantly higher than those of the full-blood breeds (9%). The abnormal configurations in the F1 hybrids included partial pairing failure, multivalents, interlocks, and inversion pairing. Abnormal configurations were also present, but at lower frequency, in backcross hybrid bulls. The main types of abnormal configurations and the levels of XY-autosomal associations and autosomal asynapsis observed were unlikely to cause significant fertility problems in the hybrids.     

Synaptonemal complex analysis of hybrid cattle. III. Meiotic pairing mechanisms in F1 Brahman x Hereford hybrids

The mechanisms of homologous chromosome pairing were studied in synaptonemal complex (SC) spreads of F1 Brahman (Bos indicus) x Hereford (Bos taurus) cattle. The most common SC abnormalities were bivalents with partial pairing failure and interlocks. While C-band polymorphisms could underlie most of the SC abnormalities observed in the full-blood cattle, other causes seem also to be contributing in the hybrids. The pattern of the abnormalities indicates that genic differences between the species were probably involved. Pachytene substaging data suggest that in some spreads, early pachytene bivalents with partial pairing failure may achieve complete synapsis or may be converted to interlocks by late pachytene.     

Synaptonemal complex analysis of spermatocytes of Talpa occidentalis (Insectivora, Mammalia): autosomal synapsis and substaging of zygonema and pachynema

Spermatocytes from the mole, Talpa occidentalis, a species that includes both XX males and intersexes, were surface-spread and silver-stained to substage meiotic prophase from early zygonema through pachynema. In zygonema, only the Z2 and Z3 substages were found. This stage differed in comparison with such species as the Chinese hamster, laboratory mouse, and deer mouse, which belong to orders other than Insectivora. Pachynema, in which five substages were established (P1-P5), seems to be a more homogeneous stage, and remarkable differences with respect to the above-mentioned species were not found. Synaptic adjustment was demonstrated in X-Y pairing. Nonhomologous pairing was evident at the Y-centromeric region and considered likely in the proximal arm of this chromosome. In addition to sequencing the events taking place during zygonema and pachynema in males from a wild population in which some members show sex reversal, our finding represents the first attempt to substage zygonema and pachynema in an Insectivore species, thus contributing to current knowledge of the nature and degree of variability in the mammalian synaptic process.     

Synaptonemal complex analysis of an autosomal trisomy in the horse.

Synaptonemal complex analysis by electron microscopy of a trisomy 28 in a male horse demonstrated a trivalent or a bivalent plus a univalent in primary spermatocytes. Two of the chromosomes making up the trivalent were, most often, completely paired with each other and only partially paired or associated with the third one. Half of the spermatocytes analysed demonstrated heterologous pairing or association between the free axis of the trivalent and the sex bivalent. The pairings remained, to a large extent, into diakinesis-metaphase I. In most pachytene cells one autosomal bivalent showed proximal asynapsis and paired often, heterologously, with the trivalent or the sex bivalent. The horse demonstrated azoospermy, which was due, at least in part, to degeneration at both the spermatocyte and spermatid levels.     

Synaptonemal complex analysis of mouse chromosomal rearrangements

Synaptonemal complex (SC) analysis by electron microscopy of spermatocytes in surface microspreads was carried out in mice heterozygous for two paracentric inversions: either In(1)1RK or In(2)5Rk. Characteristic SC inversion loops are formed at synapsis in bivalents carrying the rearrangements. Although all loops were observed to be eliminated by late pachytene through synaptic adjustment, every spermatocyte at early pachytene contained a fully synapsed loop. Cells in the earliest stage of pachytene contained the longest loops and thus had undergone minimal adjustment. The SC estimates of inversion lengths and breakpoint positions in such cells corresponded well with those from mitotic chromosome banding and could be correlated with genetic maps of chromosomes # 1 and # 2, thus demonstrating the basis for the mapping of pachytene chromosomes. The regularity of loop formation and reproducibility of the SC analysis are reflected in the constant relative positions of the estimated breakpoints. The method is sensitive enough to reflect small, real, interstitial length differences between meiotic and mitotic chromosomes. The results demonstrate the feasibility and precision of detection and quantitative characterization of inversions at early meiotic prophase by SC analysis.This paper is warmly dedicated to Prof. Dr. Wolfgang Beermann, on the occasion of his 60th birthday     

Synaptonemal complex analysis of mouse chromosomal rearrangements

Electron microscopy of surface-spread spermatocytes from mice heterozygous for a tandem duplication shows the heteromorphic synaptonemal complex (SC) to comprise two lateral elements of unequal length, the longer of which is buckled out in a characteristic loop, representing the unsynapsed portion of the duplication. The loop is a regular feature of late zygotene-early pachytene nuclei; it is longest at these early stages, but, through equalization of the two axes as a consequence of synaptic adjustment, it is replaced by a normal appearing SC at late pachytene. Because equalization, as indicated by a decrease in the percent difference between axes, may begin shortly after completion of synapsis, estimates of duplication segment length are restricted to a sample selected for least adjustment. — Although the mean position of the loop is constant at various pachytene substages, individual positions vary widely from cell to cell, consistent with the behavior expected of a duplication, but not of a deletion or an inversion. The length of the segment that is duplicated is estimated to be 22% of the normal chromosome, the midpoint of the segment is mapped at 0.61 of the chromosome distal to the kinetochore, and the ends of the segment are mapped at 0.50 to 0.72. Measurements of G-banded mitotic chromosomes give comparable values: duplication length, 24%; midpoint, 0.60, and segment ends, 0.48 and 0.71. This agreement constitutes further validation of the SC/spreading method for detecting and analyzing chromosomal rearrangements at pachytene and substantiates the fidelity with which the axes and SCs represent the behavior of chromosomes in synapsis.     

Synaptonemal complex analysis of mouse chromosomal rearrangements

Two paracentric inversions in the mouse, In(1)1 Rk and In(2)5 Rk, have been studied in surface microspreads of spermatocytes from heterozygotes. At zytogene, synaptic initiation occurs independently in three regions: within the inversion, and without, on either side. Synaptonemal complex (SC) formation is restricted to homologous regions, resulting in inversion loops in all early pachytene spermatocytes. An adjusting phase then occurs during pachytene in which the inversion loop is reduced by desynapsis of homologously synapsed SC, followed immediately by non-homologous synapsis with the alternate pairing partner, progressing from the ends toward the middle. Adjustment occurs during the first half of pachytene, but is not closely synchronized with sub-stage. It is complete by late pachytene, the loop having been eliminated in all cases and replaced by straight SCs in which the inverted region is heterosynapsed. Synapsis in the adjustment phase is evidently permitted only after the homosynaptic phase, and is indifferent to homology. It may lead to heterosynapsis, as in the inversion region, or to synapsis of homologous regions not synapsed at zytogene. The anaphase bridge frequency, a measure of crossing over within the inversion, is about 34% for both inversions studied, indicating that such crossovers do not block adjustment, that crossing over probably occurs before or during the adjustment period, and that there is some crossover suppression. The last could be the consequence of blocking by desynapsis/heterosynapsis. Synaptic adjustment appears to be a general phenomenon that occurs to varying extents in different forms. A hypothetical scheme for two phases of synapsis is proposed: at zytogene, a basic propensity for indifferent SC formation is limited by a restricting condition to synapsis between homologous regions. Subsequently, the restriction is lifted, whereupon synaptic instability is resolved by desynapsis, followed by resynapsis that is indifferent to homology, but that results in a topologically more stable structure.     

Pachytene pairing and metaphase I configurations in a tetraploid somatic Lycopersicon esculentum x L. peruvianum hybrid.

In the tetraploid somatic hybrid between the diploid Lycopersicon species L. esculentum (tomato) and L. peruvianum, synaptonemal complexes formed quadrivalents in 73 of the 120 sets of four chromosomes (60.8%) in 10 cells studied in detail at pachytene. Of these, 43 had one pairing partner exchange, 22 had two, and 8 had three, very close to a Poisson distribution. The points of pairing partner exchange were concentrated at the middle of the two arms. The frequency per arm corresponded with physical arm length. There was a sharp drop around the centromere, and pericentric heterochromatin had a slightly lower probability of being involved in pairing partner exchange than euchromatin. The chromosomes align before pairing and there are several points of pairing initiation, with concentrations at or near the ends and the centromere. From zygotene to late pachytene the quadrivalent frequency decreased considerably. At late pachytene it was lower than expected with the observed high frequency of pairing partner exchange. Pairing affinity between species was only slightly lower than affinity within species, in spite of considerable genetic differentiation. The frequency of recombination nodules increased from early to late zygotene and then decreased strongly to full pachytene. There is a highly significant negative correlation between percent pairing and SC length. At metaphase I the frequency of quadrivalents was 0.444, and branched quadrivalents were rare, probably caused by interference and restriction of chiasma formation to distal euchromatin. Metaphase I quadrivalent frequency is a relatively good indication of pairing affinity in this material.     

Synaptonemal complex analysis of a three-breakpoint translocation in a subfertile bull.

Somatic chromosome analysis of a subfertile Brown Swiss bull demonstrated a three-breakpoint translocation involving chromosomes 1, 8, and 9 in G- and R-banded karyotypes. Based on standard bovine chromosome nomenclature, the translocation was defined as t(1;8;9)(q43;q13;q26). Synaptonemal complex analysis of the chromosome aberration by electron microscopy revealed a hexavalent configuration in 52 of 53 pachytene cells. Twenty-seven cells (51%) had a completely paired hexavalent configuration showing distinctly nonhomologous pairings between normal and/or translocated chromosomes involved in the exchanges. Thirteen cells showed a hexavalent configuration with centrally unpaired chromosome segments but with completely paired terminal arms. In 13 cells (including one at zygotene) the translocation chromosomes formed an open hexavalent, and in one cell there were two completely paired trivalents. Thirty-two cells at diakinesis-MI demonstrated 28 configurations, including one large hexavalent. Testicular histology, testis size, and seminal characteristics were normal.     

Synaptonemal complex analysis of heteromorphic trivalents in Lemur hybrids

Synaptonemal complexes (SCs) in surface spread pachytene spermatocytes of Lemur resemble those in other mammals and are of two types: metacentric (or submetacentric) and acrocentric, with a very short second arm. In autosomal SC and mitotic karyotypes of Lemur fulvus (2n=60) a 11 proportionality in relative length is observed as in other mammals. In an intraspecific lemur hybrid (2n=55) obtained by mating L. fulvus rufus (2n=60) x L. fulvus collaris (2n=51), G-band patterns show that 10 single acrocentric mitotic chromosomes correspond to the arms of 5 single metacentrics, implying homology. It is inferred that the metacentrics have evolved by centric (Robertsonian) fusion of the acrocentrics. In the SC karyotype of the hybrid all SCs are normal except for five which have the configurations expected of metacentric-acrocentric trivalents. Similarly, in L. f. collaris (2n= 51), with one unpaired metacentric and two unpaired acrocentrics, one such SC trivalent is present in the complement. In an SC trivalent, each of the acrocentric long axes is synapsed with an arm of the metacentric axis, confirming the homology predicted from banding similarities. At late zygotene, the acrocentric short arms, which are non-homologous, are the last to pair, demonstrating that synapsis of the homologous arms occurs first. At later pachytene the acrocentric short arms are fully synapsed, producing a short SC side arm. This subsequent non-homologous synapsis is taken to be an instance of the synaptic adjustment phenomenon which has been shown to lead to non-homologous synapsis in a duplication and several inversions in the mouse. The kinetochore of the metacentric is the same size as those of the acrocentrics, and thus is unlikely to have arisen by true centromeric fusion, but rather by a translocation. The kinetochores of the acrocentrics always lie together on the same side of the metacentric kinetochore (cis configuration), implying a single pairing face on the metacentric axis. The observed trivalent configuration may well constitute a prerequisite for proper meiotic disjunction in metacentric-acrocentric heterozygotes. Such a mechanism is consistent with fertility regularly observed in such hybrid lemurs.     

Synaptonemal complex analysis of X-7 translocations in male mice

The synaptonemal complexes of surface-spread spermatocytes of mice heterozygous for one of two reciprocal translations (R3 and R5) between the X and chromosome 7 have been examined by light and electron microscopy (EM). The break points of R3 were determined to be at 70% of chromosome 7, as measured from the centromere, and at 22% of the X. Translocation quadrivalents were formed almost exclusively. The break points of R5 were at 21% of chromosome 7 as measured from the centromere, and at 83% of the X. There was little indication that the break in the X interfered with sex-chromosome synapsis between the 7X and Y. Univalent Y's were not observed in R3, and only seldom observed (8–14%) in R5. However, in contrast to R3, R5 formed quadrivalents relatively rarely (20% in the EM study of 100 nuclei), and heteromorphic bivalents of 7X-Y and X7-7 quite frequently (72%). Possible causes of this high bivalent frequency are discussed. Light-microscope (LM) analysis alone was found to be inadequate for interpreting synaptic configurations (quadrivalents vs. bivalents) in R5. The LM analysis was further complicated by the occurrence of nonhomologous synapsis in the heteromorphic bivalents of R5, a phenomenon easily recognized and interpreted in the EM portion of the study.     

Meiosis in triploid Lolium. I. Synaptonemal complex formation and chromosome configurations at metaphase I in aneuploid autotriploid L. multiflorum.

A spreading technique for synaptonemal complexes (SCs) was applied to pollen mother cells of two aneuploid genotypes of autotriploid Lolium multiflorum (2n = 3x + 1 = 22). In the earliest nuclei analyzed the axial elements are in six groups of 3 and one group of 4. Most groups have formed multivalents with from one to five pairing partner exchanges, but there are also groups that have formed bivalents and univalents. Some axial elements have formed triple associations, in one case for the length of the trivalent. Unsynapsed axial elements remain aligned with their homologous SCs into pachytene, but this alignment is abolished as these axes pair heterologously among themselves until the entire axial element complement is synapsed. At metaphase I most chromosomes are associated as trivalents and quadrivalents.     

联会复合体:减数分裂的结构基础

减数分裂是有性生殖生物产生单倍体配子的特殊分裂方式,其第一次分裂(减数分裂I)过程中同源染色体的行为是最突出的特征。在减数分裂I,同源染色体间形成的联会复合体通过促进和调控程序性DNA双链断裂的形成和修复,确保同源染色体正确的识别、配对、重组和分离,从而为减数分裂I的顺利完成提供保障。本综述对联会复合体的组成和功能研究进展进行了回顾,探讨了联会复合体的组装如何影响程序性DNA双链断裂的修复和交叉互换的形成,并总结了与人类生殖障碍相关的联会复合体成分突变,还对该领域未来研究方向进行了展望。     

Synaptonemal complex analysis in spermatocytes of diploid and triploid Chinese shrimp Fenneropenaeus chinensis

A modified surface spreading technique for synaptonemal complex (SC) analysis was tested to assess the process of chromosome synapsis in spermatocytes of diploid and induced triploid Fenneropenaeus chinensis. Spermatocytes of diploid shrimp showed typical morphological characteristics of eukaryote SC, with complete synapsis of bivalents. No recognizable bivalent associated with sex chromosomes was observed in spermatocytes of diploid shrimp. However, differences in morphology of SC, including unsynapsed univalents, bivalents, totally paired trivalents with non-homologous synapsis, partner switches and triple synapsis were identified at early pachytene stage of triploid spermatocytes. Triple synapsis was especially common at late pachytene stage in spermatocytes of triploid shrimp. The observed abnormal synapsis behavior of chromosomes in spermatocytes indicated that triploid male shrimp may find it difficult to develop normal haploid sperm.     

Synaptonemal complex analysis of the Holstein-Friesian, Piemontese and Simmental breeds of Bos taurus taurus

The synaptonemal complex (SC) of specimens of Bos taurus taurus from the Holstein-Friesian, Piemontese, and Simmental breeds, was analysed. The analysis included quantification of the frequency of various types of abnormalities in the SC, and the frequency of cells with SC abnormalities. All animals had 29 autosomal bivalents and one sexual bivalent and the most frequently recorded abnormality was pairing failure. The number of cells with abnormalities in the Holstein-Friesian breed was 29.41%, in the Piemontese breed was 30.00% and in the Simmental breed it was 29.54%. The subspecies Bos taurus taurus had 29.63% of cells showing abnormalities with 57.33% of these abnormalities occurring in zygotene and 42.67% occurring in pachytene. Statistical analyses showed that there were no significant differences in the number of cells with SC abnormalities among the breeds studied. The frequency of cells with abnormalities, and the efect on the fertility of the Holstein-Friesian, Piemontese and Simmental breeds are discussed.     

Synaptonemal complex analysis of B-chromosome behavior in meiotic prophase I in the East-Asiatic mouse Apodemus peninsulae (Muridae, Rodentia)

The mitotic and meiotic chromosomes of four male East-Asiatic mice, Apodemus peninsulae, having three to seven chromosomes in addition to the standard karyotype (2n = 48), were investigated. B-chromosomes were represented by medium-sized metacentric and dotlike chromosomes. Mosaicism of bone marrow cells due to a numerical variation of accessory chromosomes was established for the males examined. Capacity of B-chromosomes to form axial elements and synaptonemal complexes in meiotic prophase I was revealed by electron microscopy. The occurrence of univalents of different morphology, bivalents, and multivalents, corresponding to B-chromosomes, was demonstrated. An increase in the number of B-chromosomes was found in spermatocytes at zygotene-pachytene relative to the number in bone marrow cells, which may be evidence of B-chromosome accumulation in the germ cell line of the East-Asiatic mouse.     

Synaptonemal complex formation and meiosis in the resting sporangium ofBlastocladiella emersonii

Summary Blastocladiella emersonii Cantino etHyatt (Phycomycetes, Blastocladiales) has aBrachyallomyces type of life cycle (sensu Emerson 1941) or aBlastocladiella type of life cycle (sensu Karling 1973), with a regular formation of zoosporangia and resting sporangia and no sexual stages.Synaptonemal complex formation (a characteristic stage in meiotic prophase—pachytene) occurs during development of the resting sporangium inB. emersonii.During resting sporangium germination, two meiotic nuclear divisions give rise to nuclei which have approximately one-fourth of the nuclear volume of the diplotene nucleus. The life cycle ofBlastocladiella emersonii resembles that ofCatenaria anguillulae. The time of diploidization (n2 n) has not been determined.