六种鱼的精母细胞联会复合体的电镜观察

我们以界面铺张——硝酸银染色技术,对鲈形目三种鱼(尼罗罗非鱼、莫桑比克罗非鱼、刺鳅)和鲤形目(鱼句)亚科三种鱼(花(鱼骨)、黑鳍鳈、麦穗鱼)的精母细胞联会复合体进行了电镜观察研究。系统考察了鱼类常染色体SC的亚显微结构、形成过程和配对行为,比较分析了刺鳅的性染色体SC的异配形态和行为,并绘制了鲈形目三种鱼的SC组型模式图。     

Synaptonemal complex karyotyping in spermatocytes of the Chinese hamster (Cricetulus griseus)

Using the Counce-Meyer spreading technique, in over 70 spermatocytes it was possible consistently to obtain whole, flattened nuclei containing complete sets of pachytene SCs. The SCs are visible in both the phase and electron microscopes. Each SC is morphologically intact, preferentially stained, and attached to the nuclear envelope by a dense, terminal plaque. It is thus possible to trace each SC for its entire length. Also, a structure representing the kinetochore is clearly visible in each autosomal SC. Karyotypes comparable to the somatic karyotype can be constructed by arranging SCs according to length and kinetochore position. The observed regularity of SC morphology implies structural stability sufficient to withstand the stresses imposed by the procedure.— A coarse network of closely packed nuclear annuli connecting SC attachment plaques often provides end-to-end associations and may tend to immobilize SCs during processing.— Three kinds of perturbation of SC structure are encountered. Twists in the SC frequently occur, but no regular pattern or correspondence with chiasma distribution is observed. SCs occasionally hook around each other without disruption, but in two instances the unpaired axis of the X apparently was interlocked within an autosomal SC. Stretching of the SC is infrequent; it is conspicuous when it occurs and is usually associated with other obvious distortions of the nucleus.— Distinctive morphologies of the X and Y chromosomes facilitate their identification in all preparations. — During zygotene, autosomal synapsis, i.e., the formation of SCs from the pairing of single axial elements, initiates at distal ends and terminates at the kinetochore region; neither initiation nor termination is synchronous among all autosomes.     

Ultrastructure and behavior of the achiasmatic,telosynaptic XY pair of the sand rat (Psammomys obesus)

The behavior of the X and Y chromosomes in somatic and testicular cells of the sand rat (P. obesus) has been investigated with light and electron-microscope procedures. The Y chromosome has been identified as the fourth longest of the complement, both by C-banding and by its meiotic behavior. The X chromosome is the longest of the complement and carries two major C-heterochromatic blocks, one in the distal part of the long arm and the other forming most of the short arm. During presynaptic stages in spermatocytes, separate C-heterochromatic blocks, representing the sex chromosomes, are observed in the nuclei. An XY body is regularly formed at pachytene. During first meiotic metaphase the X and Y chromosomes show variable associations, none of them chiasmatic. Second meiotic metaphases contain, as in other mammals, a single sex chromosome, suggesting normal segregation between the X and the Y. — Electron microscopic observations of the autosomal synaptonemal complexes (SCs) and the single axes of the X and Y chromosomes during pachytene permit accurate, statistically significant identification of each of the largest chromosomes of the complement and determination of the mean arm ratios of the X and Y axes. The X and Y axes always lie close to each other but do not form a SC. The ends of the X and Y axes are attached to the nuclear envelope and associate with each other in variable ways, both autologously (X with X or Y with Y) and heterologously (X with Y), with a tendency to form a maximum number (four) of associated ends. Analysis of 36 XY pairs showed no significant preference for any single specific attachment between arm ends. The eighth longest autosomal bivalent is frequently partially asynaptic during early pachytene, and only at that time is often near or touching one end of the X axis. — It is concluded that while axis formation and migration of the axes along the plane of the nuclear envelope proceed normally in the X and Y chromosomes, true synapsis (with SC formation) does not occur because the pairing region of the X chromosome has probably been relocated far from the chromosome termini by the insertion of distal C-heterochromatic blocks.     

Synaptonemal complexes and associated structures in microspread human spermatocytes

Human spermatocytes processed with a modified microspreading technique which involves the use of sodium dodecyl-sulphate (SDS) have been used to construct synaptonemal complex (SC) karyotypes. Twenty two pachytene spermatocytes were selected for length quantitation. The mean values of relative lengths and centromeric indexes of each SC agree closely with values obtained by three-dimensional reconstructions (Holm and Rasmussen, 1977), except for SCs #4–5, 6–7 and 19–20. Absolute lengths are consistently longer in spreads (10.7% longer than in sections, on average). The mean total length of the SC complement is 258.7 m. Six morphological types of XY paris have been described. On the basis of the relationships between the XY pair, nucleolar development and autosome behavior, these six XY types are assumed to develop in succession. Type O XY pairs occur during late zygotene, types I and II XY pairs occur during early to midpachytene, and types III, IV and V occur during later pachytene substages. Alignment of the X and Y axes is observed at late zygotene, and formation of the SC occurs in relation with type I XY pairs. Progressive desynapsis occurs in types II and III. Splitting and fusion of the X and Y axes attain a maximum in types IV and V. The breakdown of the dense bodies associated with the X and Y axes occurs during stage V. — Bar-like structures, having a mean length of 2,100 Å are associated with SCs in all the pachytene substages defined by the XY types. The average number of bars per nucleus is 46.2 (SD=8.4, N=20), and the average SC length per bar is 5.57 m. The distribution along the SCs of 923 bars shows that near-termini locations are preferred (SC length per bar, 2.98 m) and centromere regions are avoided (SC length per bar, 16.9 m). — On the basis of these data, bars are similar to recombination nodules described in other organisms. The availability of a standard SC karyotype for microspreads and a temporal sequence given by the XY pair provide a basis for rapid screening of chromosome aberrations in human testicular biopsies.     

Synaptonemal complex karyotyping in spermatocytes of the Chinese hamster (Cricetulus griseus)

Synaptonemal complexes (SCs), X and Y axes, and various nucleolar structures stain preferentially with silver in surface microspread preparations and are analyzable by both light and electron microscopy. Central elements, kinetochore region material and nuclear annuli which stain with ethanolic phosphotungstic acid are seldom visible after silver staining. SCs can be characterized by length measurements equally well in light and electron micrographs, from which stages of pachytene can also be determined by differentiation of the axes of the XY pair. By electron microscopy, the lateral elements appear as single strands at zygotene and early pachytene, then become double in a plane perpendicular to that of the SC and appear denser and thicker until late pachytene when they become progressively more attenuated and again appear single. These transitions are difficult to explain in terms of separation of associated chromatids. Identification of various silver stained bodies as nucleoli is supported by their orange-red fluorescence with acridine orange. SCs, X and Y axes and associated sex body material are, with a few exceptions, virtually indistinguishable from the background yellow-green fluorescence of the chromatin. Comet-shaped nucleolar bodies are regularly associated with five (in one animal) or six (in two animals) SCs; their positions along particular SCs identifiable by relative lengths indicate these bodies to be expressions of nucleolus organizer regions. They first appear at leptotene in association with unpaired axes and undergo progressive changes through late pachytene, at which time they redistribute their contents coincident with disappearance of the SCs. A characteristic nucleolar double dense body appears at zygotene; unlike the comet-shaped nucleoli, it is unassociated with other nuclear structures, and is assumed to arise from coalescence of previously existing smaller dense bodies. — The silver staining method described is remarkable for the speed and simplicity with which large numbers of spermatocyte nuclei are obtainable for light and electron microscopy. The fidelity of the light microscopic counterpart of the electron microscopic image has been directly assessed at different stages of pachytene. For cytogenetic analysis, critical information often lies beyond the limits of light optical resolution; the correlated electron microscopy required for verification is easily obtained with this method.This paper is warmly dedicated to Professor Hans Bauer on the occasion of his seventy-fifth birthday and as our expression of gratitude and admiration for his lasting contributions to chromosome biology     

Electron microscopic observations on the synaptonemal complex of spermatocytes of the Giant Panda (Ailuropoda melanoleuca)

Surface-spread and silver-stained preparations of spermatocytes from a giant panda were observed by electron microscopy for synaptonemal complex karyotyping. Ten pachytene spermatocyte nuclei were selected for length quantitation of SC. The mean relative lengths and centromeric indices of each SC agreed closely with those of the mitotic chromosomes. The pairing between lateral elements of autosomal chromosomes starts at early zygotene and leads progressively along their length to complete pairing at pachytene. The whole Y is paired with 1/3 length of X at mid-pachytene. The morphology of X and Y chromosome axes and the nonhomologous pairing of X and Y is discussed.     

小麂、黑麂、赤麂精母细胞联会复合体的比较研究

本工作以界面铺张——硝酸银染色技术,对小麂(Muntiacus reeuesi)、黑麂(M.crinifrons)和赤麂(M.muntjak)的精母细胞联会复合体(Syna ptonemal complex,SC)进行亚显微结构的比较研究。结果表明: 1.SC的平均相对长度和臂比指数同有丝分裂细胞相应染色体的数值有很好的一致性。根据SC的相对长度和臂比指数绘制了三种麂的SC组型图。雄性黑麂减数分裂前期形成一个复杂的易位多价体,意味着其核型的演化过程涉及两次染色体易位和一次臂间倒位。 2.在减数分裂前期,性染色体的形态和行为同常染色体的有明显差异,如性染色体嗜银性较强,配对延迟等。XY的配对起始于早粗线期,在中粗线期,Y的全长均同X配对;XY-SC开始解体于晚粗线期。 3.在粗线期,X染色体未配对区域出现自身折叠,形成“发夹”状结构。这种“发夹”结构的形成,可能是在性染色体的进化过程中,X染色体通过不对称易位得到的重复片段在减数分裂前期同源配对的一种细胞学表现。     

Autosomal synaptonemal complexes and sex chromosomes without axes in Triatoma infestans (Reduviidae; Hemiptera)

Meiotic and somatic cells at interphase in Triatoma infestans are characterized by the formation of a large chromocenter, which was assumed to contain the whole of the three large pairs of autosomes and the sex chromosomes. Observations with C-banding techniques show that the chromocenter is formed only by the terminal and subterminal heterochromatic blocks of the three large pairs of autosomes and the sex chromosomes. During pachytene the two largest autosomal pairs loop on themselves and their condensed ends form the chromocenter, together with the single heterochromatic end of the third autosomal pair. The X and Y chromosomes seem to associate with these condensed ends by their affinity for C-heterochromatin. During a very short pachytene stage, bivalents and synaptonemal complexes (SCs) are observed. Pachytene is followed by a very long diffuse stage, during which SCs are disassembled, multiple complexes aggregate on the inner face of the chromocenter and finally all complexes disappear and a dense material is extruded to the cytoplasm through the annuli. The 3-dimensional reconstruction of early pachytene chromocenters show 3 SCs entering and tunnelling the chromocenter, while during mid-pachytene 4 SCs enter this mass and a 5th SC is in a separate small mass. The looping of a whole SC which has both ends in the chromocenter was shown by the reconstructions. These data are interpreted as the progressive looping of the two largest bivalents during pachytene, forming finally the association of 5 bivalent ends corresponding to the 5 C-banding blocks of the large autosomal pairs. No single axis or SC that could be ascribed to the sex chromosomes was found. This agrees with the pachytene microspreads, which show only 10 SCs corresponding to the autosomal bivalents. The X and Y chromosomes are enclosed in the chromocenter, as shown by the unravelling chromocenters at diplotene-diakinesis. Thus the sex chromosomes do not form axial condensations, and this fact may be related to the ability of the X and Y chromosomes to divide equationally at metaphase I. SCsThis paper is dedicated to the memory of the late Professor Francisco A. Saez     

Electron microscopy of spread maize pachytene synaptonemal complexes

The Counce-Meyer microspreading technique for animal synaptonemal complexes (SCs) has been adapted to allow spreading of the SCs of maize pachytene microsporocytes for examination in the electron microscope (EM). The spread nuclei were well dispersed and flattened, and unstained SCs could be seen with light microscope (LM) phase optics. After PTA or ammoniacal silver staining, the SCs and kinetochores were readily recognized in the EM. Variable degrees of asynapsis, stretching of the SCs, and nonhomologous synapsis of lateral elements were noted, and cases of interlocking of lateral elements or SCs were not uncommon. Distinct lens-shaped thickenings of one or both lateral elements were observed at numerous sites along the SC in most nuclei. — The yield of well spread, complete nuclei, although not high, was sufficient to allow karyotypes to be prepared, based on relative SC lengths and arm ratios. The karyotypes agreed well with published EM and LM determinations, establishing the accuracy of the spreading technique for maize. However, considerable variation in absolute lengths of the SCs was noted. To evaluate the utility of the technique for cytogenetic investigations, two paracentric inversions, and two reciprocal translocations were spread and examined in the EM. The breakpoints estimated from measurements of spread SCs were in agreement with LM determinations.     

Meiosis in gray voles of the subgenus microtus (rodentia, arvicolinae) and in their hybrids

The results of light and electron microscopic (EM) studies of meiosis in Microtus arvalis males of the karyoform "arvalis" (2n = 46, NFa = 80), in hybrids between the chromosomal forms arvalis and obscurus (2n = 46, NFa = 68), in M. rossiaemeridionalis voles (2n = 54, NFa = 54), and in a hybrid between the species M. rossiaemeridionalis and M. kermanensis (2n = 54, NFa = 54) are presented. SC (synaptonemal complex) karyotypes of the parental forms and the hybrids were constructed on the basis of measurements of the length ofautosomal SCs revealed by the EM analysis in spermatocytes at the stage of middle pachytene. The SC karyotypes of M. arvalis and the hybrids female obscurus x male arvalis consist of 22 synaptonemal complexes of autosomal bivalents and the axial elements of the synaptonemal complexes of the sex chromosomes X and Y. The SC karyotypes of M. rossiaemeridionalis and the hybrid M. rossiaemeridionalis x M. kermanensis consist of 26 synaptonemal complexes of autosomal bivalents and a sex bivalent; they differ only in the length of the Y chromosome axis (Y chromosome in the hybrid was inherited from M. kermanensis). Asynaptic configurations of the autosomal SCs were not observed in the hybrids. The SC axial elements of the X and Y chromosomes in the parental forms and in the hybrids were located close to each other throughout pachytene, but they did not form a synaptic region. The normal synapsis in sterile hybrids (M. rossiaemeridionalis x M. kermanensis) and the behavior of the sex chromosomes in meiosis in fertile and sterile hybrids are discussed in the context of specific features of meiosis and reproductive isolation.     

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.     

The pachytene synaptonemal complex complement of the cat

Surface spreads of pachytene spermatocyte nuclei from two cats were used to construct a synaptonemal complex karyotype for the cat. It was possible to recognise the 18 autosomal synaptonemal complexes by reference to a published light microscopic banded somatic karyotype. Some variation from the somatic karyotype was noted, presumably as a result of differential contraction during prophase I. The X and Y chromosome axes were joined by a synaptonemal complex in many of the nuclei, but the structure of the unpaired portion of the X axis was quite variable. In some nuclei it was highly contracted, while in others it was extended and often was split into two or more axes. In most nuclei the autosomal synaptonemal complexes had numerous axial twists.     

Mitotic and meiotic analysis in Arctocephalus australis (Otariidae)

The karyotype with C-, G- and NOR-banding of Arctocephalus australis is reported for the first time. The chromosomal number is 2n = 36. The X chromosome, identified in G-banded metaphases from males, is metacentric and the Y chromosome is a minute chromosome, also metacentric. Pachytene spermatocytes were used for synaptonemal complexes analysis with a surface spreading technique. A total of 17 autosomal synaptonemal complexes are observed plus the XY pair. During early pachytene, the X and Y axes are thickened and remain unpaired. As pachytene advances, a short SC is formed between the gonosomes, as it is common among eutherian mammals. The particular asymmetrical appearance of the synaptonemal complex in the sex pair is described and compared to other cases among mammals.     

Isolation of synaptonemal complexes from hamster spermatocytes

Nuclei from Chinese hamster testicular cells in suspension were prepared in a sucrose gradient. Following the basic procedure of Blobel and co-workers for separating a fibrous lamina-nuclear pore complex, synaptonemal complexes (SCs) from spermatocytes were isolated free of other nuclear structures, except for fibrillar tufts at the attachment plaques in which annuli were observed. All the major morphological components of the SC appeared to be intact, showing that the structure could survive the procedure and was not dispersed by the removal of DNA with DNase and solubilization of membranes and some proteins with Triton X-100. Isolated sex bodies were also well preserved, as were various structures from other cell types in the mixed cell suspension, such as spermatid manchettes, acrosomal ‘ghosts’, axonemes, etc. While no nuclear matrix was found associated with autosomal SCs, a residual material was present in the sex body, in which the X and Y axes were embedded. The results indicate the feasibility of isolating and fractionating SCs from testicular cell suspensions enriched for pachytene spermatocytes. The association between SC attachment plaques and annuli that is seen in spreads of whole nuclei persists through the isolation procedure and implies an integrated structural relationship.     

Effects of the male contraceptive agent gossypol on meiotic chromosomes of the male rat

Synaptonemal complexes (SCs) of rat spermatocytes were analyzed in silver-stained meiotic preparations 10-24 days after treatment with gossypol acetic acid, 30 mg/kg/day, for 70 days. Gossypol did not affect SC formation or function, as judged by the absence of pairing anomalies, SC fragmentation, or presynaptic arrest. The unpaired lateral axes could be seen at zygotene, and at pachytene normal SCs could be observed. The behavior of the XY axes also appeared to be normal.     

The synaptonemal complex karyotypes of palearctic hamsters,Phodopus roborovskii satunin and P. sungorus pallas

Synaptonemal complex (SC) karyotyping is used to analyse the relationship of two Palearctic hamsters. Male Phodopus sungorus have 13 SCs plus an X-Y pair while P. roborovskii has 16 SCs and an X-Y pair. P. sungorus has four nucleolar organizers as opposed to two in P. roborovskii. Centromeres of the X and Y are juxtaposed in the pairing region of the latter but not in the former. SC lengths and relative arm ratios are also discussed. Comparisons with karyotypes of other hamster genera indicate large intergeneric karyotypic differences and expectedly much smaller variation within each genus. Data on the two Phodopus species suggest that their presently accepted congeneric status warrants further examination.     

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.     

Synaptic behaviour and recombination nodules in the human XY pair

A sample of 90 XY pairs from men with normal karyotypes has been analyzed by measuring their morphological features in electron micrographs of microspread spermatocytes. The classification of human XY types (Solari, 1980) has been given stricter definitions. Stepwise splitting of the axes is seen in types 1 and 2. The development of axial branches and lenhthening of the X axis is seen in type 3. In the two subtypes a and b of type 4 the net-like filamentous array grows in length to a maximum (average=59.7 m) in subtype b. The location of the putative Y kinetochore defines a short arm that measures 22.34% of Y axis length, and the kinetochore of the X axis defines a short arm of 38.15% of the axial length. The average number of excrescences in the X axis is 19.9 and in the Y is 4.3. The frequency of a non-homologous, distal end-joining grows steadily from type 0 to type 3. The average length of the synaptonemal complex (SC) in 51 XY pairs of types 1 and 2 is 1.33 m (SD=0.65) and it corresponds to 25.54% of the Y axis length. Thus, the average SC covers the short arm of the Y and the pericentromeric region. Maximum lengths of this SC may reach up to 81.8% of the Y axis, 30 recombination nodules (RNs) were located in 26 XY pairs, and 90% of the nodules are located in the distal half of the short arm of the Y axis. Thus, RNs are restricted to a segment much shorter than the length of the average SC. A gradient of decreasing probability of recombination may reach up to the centromeric region of the Y chromosome. Some possible consequences of these facts are discussed.     

Ultrastructure of the synaptic autosomes and the ZW bivalent in chicken oocytes

Chromosomal axes of chicken oocytes from pre- and post-hatching chickens were analyzed with a microspreading technique for electron microscopy. At leptotene, chromosomal axes begin to be formed as discontinuous, non-polarized axial segments. During zygotene synaptonemal complex (SC) formation begins at the axial ends attached to the nuclear envelope. Polarization of axial ends is nearly simultaneous with the beginning of SC formation. The complete SC set is found at pachytene and it consists of 38 SC's and an unequal SC which has been identified as the ZW pair. This unequal SC is formed by two axes of different length. The Z and W axes represent 6.2% and 4.5% respectively of the combined length of the SC set plus the Z axis. The unpaired segment of the Z axis shortens markedly from early to mid-pachytene and becomes thicker than the lateral elements of SCs. In the paired region the Z axis forms most of the twists around a straighter W axis, suggesting some extent of non-homologous pairing between the Z and W chromosomes in this region. The existence of partial synapsis of the Z and W axes without heteropycnosis of the sex chromosomes is in marked contrast to partial synapsis in the heteropycnotic XY body of mammalian spermatocytes.     

High-resolution crossover maps for each bivalent of Zea mays using recombination nodules

Recombination nodules (RNs) are closely correlated with crossing over, and, because they are observed by electron microscopy of synaptonemal complexes (SCs) in extended pachytene chromosomes, RNs provide the highest-resolution cytological marker currently available for defining the frequency and distribution of crossovers along the length of chromosomes. Using the maize inbred line KYS, we prepared an SC karyotype in which each SC was identified by relative length and arm ratio and related to the proper linkage group using inversion heterozygotes. We mapped 4267 RNs on 2080 identified SCs to produce high-resolution maps of RN frequency and distribution on each bivalent. RN frequencies are closely correlated with both chiasma frequencies and SC length. The total length of the RN recombination map is about twofold shorter than that of most maize linkage maps, but there is good correspondence between the relative lengths of the different maps when individual bivalents are considered. Each bivalent has a unique distribution of crossing over, but all bivalents share a high frequency of distal RNs and a severe reduction of RNs at and near kinetochores. The frequency of RNs at knobs is either similar to or higher than the average frequency of RNs along the SCs. These RN maps represent an independent measure of crossing over along maize bivalents.