尼罗罗非鱼染色体的C带、Ag带和减数分裂联会复合体的研究

以Sumner法和界面铺张——硝酸银技术,对尼罗罗非鱼(Tilapia nilotica)染色体C带、Ag染带及减数分裂前期精母细胞联会复合体(SC)进行了显微和亚显微结构观察。 尼罗罗非鱼的2n=44,核型可分为三个组:第一组为4对亚中着丝粒染色体;第二组为17对亚端着丝粒染色体;第三组为具1对端着丝粒的特大染色体。 结构异染色质主要分布于着丝粒附近,其中Nos.6、8、15亚中着丝粒染色体短臂全部深染。带有银染核仁组织者(Ag-NORs)染色体的数目为2—6条,NORs均位于6、8、15亚中着丝粒染色体短臂。 银染色可清楚地显示尼罗罗非鱼的联会复合体(SC)结构和减数分裂行为。SC组型与有丝分裂染色体的组型有较好的一致性。     

四种鱼的血清、体表黏液SDS-PAGE分析

鱼类的血清和体表黏液在其适应复杂水体环境的生理生化反应中起着重要的作用。为了解鱼类血清和体表黏液的蛋白组分与含量的差异,采用SDS-聚丙烯酰胺凝胶电泳(SDS-PAGE)对体表黏液丰富的宽体沙鳅Sinibotia reevesae、中华沙鳅S.superciliars、长薄鳅Leptobotia elongata及南方鲇Silurus meridionalis四种鱼的血清与体表黏液的蛋白组分展开研究,共获得158条蛋白区带,其中血清67条,体表黏液122条,其分子量在10.92~194.48 k D之间。聚类分析显示宽体沙鳅血清蛋白与中华沙鳅的相似度最高(0.67),与南方鲇的相似度最低(0.40)。四种鱼的血清蛋白组分的差异体现了其在进化上的亲缘关系。而体表黏液蛋白组分中中华沙鳅与南方鲇的相似度最高(0.53),与长薄鳅的最低(0.47),未呈现出与血清类似的遗传分化规律。     

中国刺鳅属鱼类分类整理及两新种和一新纪录(鲈形目,刺鳅科)(英文)

系统整理了中国刺鳅属鱼类Mastacembelus,记述2新种,腹纹刺鳅M.strigiventus Zhouet Yang,sp.nov.和三叶刺鳅M.triolobus Zhouet Yang,sp.nov.及中国1新纪录种,云斑刺鳅M.oatesii(Boulenger)。新种模式标本分别保存于中国科学院昆明动物研究所(KIZ)鱼类标本库和西南林业大学(SWFC)。腹纹刺鳅与三叶刺鳅、大刺鳅的区别在于:背鳍、臀鳍与尾鳍基部大部愈合,但具缺刻相区分;体侧前部具4～5条褐色纵条纹,最下1条常断续,全部纵纹至肛门前方渐成网格交叉或断续;腹面亦具1条明显褐色纵纹,有时分歧形成小网格。三叶刺鳅区别于腹纹刺鳅及大刺鳅的主要特征包括:背鳍条、臀鳍条和尾鳍条数目均少;背鳍、臀鳍与尾鳍仅在基部相连,在端部分开,能明显区分;除背部的黑色大斑块外,体无六角状环纹或锯齿状纹,腹面亦无斑纹;头长为头宽3.5倍以下,为吻长2.8倍以下。云斑刺鳅以下面组合特征区别于三叶刺鳅和腹纹刺鳅:背鳍条,臀鳍条数目均少,尾鳍条数目相对较多;背鳍、臀鳍与尾鳍仅基部愈合;体侧具云状斑,背部具14～15个褐色斑块,腹面无纵纹或网眼斑;头长为头宽4.0倍以上,为吻长3.0倍以上。     

西藏高原鳅、细尾高原鳅和异尾高原鳅消化道结构的比较

运用解剖学、组织学方法比较研究西藏高原鳅(Triplophysa tibetana)、细尾高原鳅(Triplophysa stenura)和异尾高原鳅(Triplophysastewarti) 3种高原鳅的消化道结构。结果表明: (1)3种高原鳅的消化道均由口咽腔、食道、胃、肠组成。胃“U”型, 无幽门盲囊, 肠道短, 可分为前、中、后三个部分。异尾高原鳅胃长与消化道长比值最大, 比肠长最短, 为0.51±0.07, 与西藏高原鳅比肠长0.64±0.08和细尾高原鳅比肠长0.70±0.06, 差异显著。(2)异尾高原鳅胃黏膜层相对高度大于西藏高原鳅和细尾高原鳅, 肌肉层相对厚度也比其他2种鱼厚。前肠黏膜层为异尾高原鳅相对高度最大, 肌肉层为西藏高原鳅相对最厚。中肠与后肠黏膜层相对厚度由大到小为异尾高原鳅>细尾高原鳅>西藏高原鳅。综上所述, 3种高原鳅的消化道结构均符合肉食性鱼类特征, 推测异尾高原鳅的结构特征适于消化更多的动物性饵料。     

青海省条鳅属鱼类一新种和一新亚种的描述

Herzenstein(1888)将采自青海湖、黄河上游和柴达木等地的背鳍最后不分支鳍条为硬刺、腹腔内有发达的游离膜质鳔和尾柄侧扁的条鳅,命名为硬刺条瞅Nemachilus sclero-pterus,并以青海湖的标本为模式标本。我们比较了不同地区的硬刺条鳅,发现Herzens-tein描述的硬刺条鳅中还包括了另一个不同的种——拟硬刺条瞅Nemachilus pseudosclero-     

赣南桃江和九曲河夏季大刺鳅资源现状评估

大刺鳅（Mastacembelus armatus）主要分布于我国南方各大水系,现阶段野生资源急剧减少,已被福建、广东、湖南等省列为重点保护野生水生动物。为了解赣南地区大刺鳅在主要分布水域桃江和九曲河中的资源状况,于2020年夏季对该水域鱼类和大刺鳅资源状况进行了调查,共采集鱼类34种隶属于4目11科31属。调查结果显示：所有样点本土鱼类中黄颡鱼（Pelteobagrus fulvidraco）、大刺鳅、鲫（Carassius auratus）为优势物种,桃江中外来物种莫桑比克罗非鱼（Oreochromis mossambicus）占主要优势;随着水利工程建设、外来物种入侵、栖息生境破坏及过度捕捞,大刺鳅资源量呈显著下降趋势。调查分析表明,修复自然河流生境,恢复洄游通道;增强本土物种保护意识,加强外来物种治理;严格禁捕,科学增殖放流对大刺鳅资源保护具有重要意义。     

斑纹薄鳅(Leptobotia zebra)应该为斑纹沙鳅(Sinibotia zebra)

斑纹薄鳅(Leptobotia zebra)最初是由Wu(1939)描述的一个新种,当时定名为斑纹沙鳅(Botia zebra),后来Chen(1980)根据眼下刺不分叉将其改归为薄鳅属的物种。本研究通过对线粒体DNA细胞色素b基因序列的测定和分析,发现斑纹薄鳅和薄鳅属(除斑纹薄鳅)物种间的平均遗传距离为0.177,和中华沙鳅属物种美丽沙鳅(Sinibotia pulcher)的平均遗传距离仅为0.057。系统发育分析发现斑纹薄鳅并未和薄鳅属的物种聚在一起,而是和中华沙鳅属物种美丽沙鳅聚在一起形成姐妹群。进一步对斑纹薄鳅进行形态学特征检视,发现该物种具有颊部裸露无鳞、颏部具一对纽状突起等中华沙鳅属鱼类的特征,但又具有眼下刺简单不分叉的薄鳅属鱼类的特征。结合分子数据分析的结果,将斑纹薄鳅订正为中华沙鳅属的物种,其命名为斑纹沙鳅(Sinibotia zebra)。另外,对沙鳅科鱼类属的划分标准及形态特征的演化也进行了讨论。     

鲱形目和鲈形目七种鱼的核型分析

本文利用体内注射秋水仙素、低渗和空气干燥法,分析了７种海产鱼的染色体组型;鲱形目的斑和青鳞小沙丁,鲈形目的花鲈、皮氏叫姑鱼、黄姑鱼、小黄鱼和黄鳍刺假虎鱼。并结合前人的文献,讨论了鲱形目的核型演化,将鲈形目几种鱼的核型与前人的报道作了比较。     

3种不同食性淡水鱼类谷胱甘肽S-转移酶Pi、Mu、Theta型cDNA序列的克隆分析及表达

鱼类谷胱甘肽S-转移酶(glutathione S-transferase,GST)是鱼类一种重要的Ⅱ相去毒酶,在催化毒素与还原谷胱甘肽(GSH)加合去毒代谢过程中具有关键作用。采用RT-PCR及RACE法,分离、克隆得到草鱼、尼罗罗非鱼pi、mu、theta型GST(GSTpi、GSTmu、GSTtheta)基因、鲢鱼GSTmu、GSTtheta基因的cDNA部分序列并推测各自对应的氨基酸序列。氨基酸序列同源性比较和系统进化分析均表明,鲢鱼、草鱼、尼罗罗非鱼与鱼类GST同源性较高,与哺乳类、鸟类、两栖类GST同源性较低,可能与鱼类GST基因在水环境毒素去毒代谢中承担的特殊功能有关。而不同种鱼类GSTtheta的同源性明显要较GSTpi、GSTmu的同源性低,可能与不同淡水鱼类食性及对毒素耐受性不同有关。用实时荧光定量PCR(RT-PCR)检测三种鱼肝脏中三型GST基因组成型表达水平,发现三种鱼各型之间皆有一定差异,尼罗罗非鱼肝脏整体GSTs基因表达很低,GSTtheta显著低于草鱼(P<0.05),GSTmu显著低于鲢鱼(P<0.05)。本研究为从分子水平上研究不同型谷胱甘肽S-转移酶基因在不同食性淡水鱼类体内代谢去毒过程中的作用提供了基础。     

寄生于罗非鱼的刺纹车轮虫种群分析及其寄主偏好性研究

文章在刺纹车轮虫Trichodina centrostrigata Basson, Van As & Paperna, 1983的重描述基础上, 基于形态学特征、分子特征及系统发育分析, 对其种群多样性及寄主偏好性进行了研究。结果表明, 刺纹车轮虫三个种群: Trichodina centrostrigata (KP295473)、Trichodina centrostrigata (W)和Trichodina centrostrigata (Q)在历经4年的时间里, 未发生明显的种群分化; 其次, 刺纹车轮虫与寄生于海水寄主的车轮虫聚支, 表明二者具有较近的亲缘关系; 再次, 刺纹车轮虫主要寄生于鲈形目的广盐性罗非鱼, 少数寄生于其他淡水鱼类, 具有较强的寄生特异性; 最后, 此研究中的寄主罗非鱼对于刺纹车轮虫具有100%的高感染率, 表明刺纹车轮虫为罗非鱼养殖业中一类潜在病害性微生物。     

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

Relative length is a constant and distinctive characteristic for each autosomal SC, despite variations in absolute length from cell to cell. Arm ratio is distinctive for each SC except for two of the three sub-acrocentrics, and serves, together with relative length, for identification. The constancy of relative length and arm ratios indicates biological stability and lack of physical distortion in these spread preparations. There is a 11 relationship between relative lengths of autosomal SCs and mitotic autosomes; their arm ratios are also similar. These close parallels provide strikingly similar SC and somatic karyotypes. Variability was observed in sub-acrocentric arm ratios and in lengths of unpaired X and Y axes, correlated with the presence of constitutive heterochromatin. — Utilizing progressive differentiations of the X and Y chromosomes for staging, it is demonstrated that autosomal SCs decrease in length from late zygotene to mid-pachytene, and then increase at late pachytene. Within a nucleus, synchrony of length changes is maintained. It is concluded that the factors governing autosomal SC length are regular for any given bivalent from cell to cell, and may be related to those that control somatic autosome length relationships. — The X and Y axes differ quantitatively as well as qualitatively from autosomal SCs. The SC portion of the X and Y is constant in length through most of pachytene; the unpaired axes shorten and lengthen, but not in proportion to autosomal SCs. X and Y relative lengths and arm ratios vary throughout pachytene and do not maintain proportionality with somatic values. The evidence suggests, but does not prove, that the long arm of the X is paired with the short arm of the Y. — Twists occur in autosomal SCs at increasing frequencies throughout pachytene but cannot account for length changes. The number of twists per SC is directly proportional to SC length. Intertwining of SCs is random and proportional to SC length. End-to-end associations of autosomal SCs appear to be random; however, the ends of the X and Y are less often involved in such connections. — The length of axial material in all chromosomes at pachytene, expressed as an equivalent length of DNA double helix, represents 0.013% of the diploid DNA complement.     

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     

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 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.     

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.     

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.     

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.     

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.     

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     

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.