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.     

First evidence of sex chromosome pre-reduction in male meiosis in the Miridae bugs (Heteroptera)

The karyotype and male meiosis of Macrolophus costalis Fieber (Insecta, Heteroptera, Miridae) were studied using C-banding, AgNOR-banding and DNA sequence specific fluorochrome staining. The chromosome formula of the species is 2n = 28(24+X1X2X3Y). Male meiotic prophase is characterized by a prominent condensation stage. At this stage, two sex chromosomes, "X" and Y are positively heteropycnotic and always appeared together, while in autosomal bivalents homologous chromosomes were aligned side by side along their entire length, that is, meiosis is achiasmatic. At metaphase I, "X" and Y form a pseudobivalent and orient to the opposite poles. At early anaphase I, the "X" chromosome disintegrates into three separate small chromosomes, X1, X2, and X3. Hence both the autosomes and sex chromosomes segregate reductionally in the first anaphase, and separate equationally in the second anaphase. This is the first evidence of sex chromosome pre-reduction in the family Miridae. Data on C-heterochromatin distribution and its composition in the chromosomes of this species are discussed.     

Morphology and behavior of the sex chromosomes in meiosis in four vole species of the genus Microtus]

Morphology and behaviour of the X and Y chromosomes of four species of genus Microtus were studied at pachytene, metaphase I and meiotic metaphase. The X chromosomes of the species varied with respect to their size and location of heterochromatic blocks. The axes of X and Y chromosomes of these species as well as Microtus agrestis never formed true synaptonemal complexes at any sub-stage of the pachytene. They approached each other at the start of the pachytene throughout to metaphase I, getting situated closely. At the end of the pachytene, they formed sex vesicle. The X and Y chromosomes kept their proximity during metaphase I, but never formed true bivalents. It is suggested that lack of synapsis of the X and Y chromosomes in the genus Microtus is the final step of evolutionary trend to reduction of the size of the pseudo-autosomal region. The abolition of restrictions on homology between the X and Y chromosomes is supposed to be a cause for the fast divergence in morphology of sex chromosomes in the genus.     

Involvement of synaptonemal complex proteins in sex chromosome segregation during marsupial male meiosis

Marsupial sex chromosomes break the rule that recombination during first meiotic prophase is necessary to ensure reductional segregation during first meiotic division. It is widely accepted that in marsupials X and Y chromosomes do not share homologous regions, and during male first meiotic prophase the synaptonemal complex is absent between them. Although these sex chromosomes do not recombine, they segregate reductionally in anaphase I. We have investigated the nature of sex chromosome association in spermatocytes of the marsupial Thylamys elegans, in order to discern the mechanisms involved in ensuring their proper segregation. We focused on the localization of the axial/lateral element protein SCP3 and the cohesin subunit STAG3. Our results show that X and Y chromosomes never appear as univalents in metaphase I, but they remain associated until they orientate and segregate to opposite poles. However, they must not be tied by a chiasma since their separation precedes the release of the sister chromatid cohesion. Instead, we show they are associated by the dense plate, a SCP3-rich structure that is organized during the first meiotic prophase and that is still present at metaphase I. Surprisingly, the dense plate incorporates SCP1, the main protein of the central element of the synaptonemal complex, from diplotene until telophase I. Once sex chromosomes are under spindle tension, they move to opposite poles losing contact with the dense plate and undergoing early segregation. Thus, the segregation of the achiasmatic T. elegans sex chromosomes seems to be ensured by the presence in metaphase I of a synaptonemal complex-derived structure. This feature, unique among vertebrates, indicates that synaptonemal complex elements may play a role in chromosome segregation.     

End association and segregation of the achiasmatic X and Y chromosomes of the sand rat,Psammomys obesus

In Psammomys obesus there is no pairing between the X and Y chromosomes and no chiasma formation (Solari and Ashley, 1977). It is demonstrated that ends of the axial elements of the X and Y chromosomes come together during pachytene, and regularly form at least one end-to-end junction. This achiasmatic physical connection between the ends of the X and Y persists until anaphase I, thus assuring the normal distribution of the sex chromosomes observed by light microscopy. In addition, there are no differentiations of the axes of the X and Y similar to those observed in other mammalian species thus far examined, a fact that could influence chromatid cohesiveness and disjunction.     

Distance segregation of sex chromosomes in crane-fly spermatocytes studied using laser microbeam irradiations

Univalent sex chromosomes in crane-fly spermatocytes have kinetochore spindle fibres to each spindle pole (amphitelic orientation) from metaphase throughout anaphase. The univalents segregate in anaphase only after the autosomes approach the poles. As each univalent moves in anaphase, one spindle fibre shortens and the other spindle fibre elongates. To test whether the directionality of force production is fixed at anaphase, that is, whether one spindle fibre can only elongate and the other only shorten, we cut univalents in half with a laser microbeam, to create two chromatids. In both sex-chromosome metaphase and sex-chromosome anaphase, the two chromatids that were formed moved to opposite poles (to the poles to which their fibre was attached) at speeds about the same as autosomes, much faster than the usual speeds of univalent movements. Since the chromatids moved to the pole to which they were attached, independent of the direction to which the univalent as a whole was moving, the spindle fibre that normally elongates in anaphase still is able to shorten and produce force towards the pole when allowed (or caused) to do so.     

Identification of the Meiotic Division of Malarial Parasites

Zygotes of Plasmodium berghei were cultured 15–25 h in vitro to yield mature infective ookinetes. Samples taken in the first 5 h of culture were examined by electron microscopy. Meiotic figures were detected in the nuclei of the zygotes. Threadlike leptotene chromatids (chromosomes) condensed from attachment plaques on the nuclear envelope; chromatid pairing followed (zygotene), with synaptonemal complexes subsequently appearing (pachytene). These complexes persisted into metaphase but dissociated when the chromatids rapidly decondensed during anaphase. At telophase of the first meiotic division the kinetochores were retracted toward two small spindle complexes, which were found at widely separated poles in the nuclear envelope. The observations are consistent with a haploid genome of 8–10 chromosomes.     

Immunocytology of chiasmata and chromosomal disjunction at mouse meiosis

Immunocytological and in situ hybridization evidence supports the hypothesis that at meiosis of chiasmate organisms, chromosomal disjunction and reductional segregation of sister centromeres are integrated with synaptonemal complex functions. The Mr 125,000 synaptic protein, Syn1, present between cores of paired homologous chromosomes during pachytene of meiotic prophase, is lost from synaptonemal complexes coordinately with homolog separation at diplotene. Separation is constrained by exchanges between non-sister chromatids, the chiasmata. We show that the Mr 30,000 chromosomal core protein, Cor1, associated with sister chromatid pairs, remains an axial component of post-pachytene chromosomes until metaphase I. We demonstrate that at this time the chromatin loops are still attached to their cores. A reciprocal exchange event between two homologous non-sister chromatids is therefore immobilized by anchorage of sister chromatids to their respective cores. Cores thus contribute to the sister chromatid cohesiveness required for maintenance of chiasmata and proper chromosomal disjunction. Cor1 protein accumulates in juxtaposition to pairs of sister centromeres during metaphase I. Presumably, independent movement of sister centromeres at anaphase I is restricted by Cor1 anchorage. That reductional separation of sister centromeres is mediated by Cor1, is supported by the dissociation of Cor1 from separating sister centromeres at anaphase II and by its absence from mitotic anaphases.     

An XY sex chromosome mechanism in a mantid with achiasmatic meiosis

An Australian mantid, Ima fusca, with 2n male equals 34, shows achiasmatic meiosis in the male, as in other Australian members of the subfamily Iridopteryginae. It is, however, unique among approximately 104 mantid species that have been studied cytologically, in having an XY sex chromosome mechanism. The X and Y chromosomes are not associated as a bivalent in first metaphase, but arrange themselves opposite one another on the spindle and regularly pass to different poles at first anaphase.     

The morphological sequence of XY pairing in the Norway rat Rattus norvegicus

The sequence of XY pairing at meiotic prophase in the Norway rat, Rattus norvegicus, has been studied in spread preparations of spermatocytes obtained from pubertal males. As in most mammals, sex chromosome pairing is delayed in relation to that of the autosomes. At one stage in pachytene, the Y is fully paired in synaptonemal complex association with about one-third of the X. Observation in spread preparations at pachytene and diplotene and in air-dried metaphase I preparations indicates that the long arm of the Y pairs with the short arm of the X. Pairing of the Y with both ends of the X is seen in about 4% of pachytene spermatocytes. The possibility that XY pairing in the rat may be nonhomologous (Ashley 1983) is considered, and the view is expressed that the XY synaptonemal complex may be incomplete in fine structural detail, thus not providing for the effective pairing required in true reciprocal recombination. The same mechanism that excludes crossing over from heterochromatic regions of autosomes may also operate to minimize or prevent crossing over in the sex pair of mammals.     

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.     

Verteilung der Mikrotubuli in Metaphase- und Anaphase-Spindeln der Spermatocyten von Pales ferruginea

One metaphase I spindle, seven anaphase I spindles of different stages, and one metaphase II spindle were sectioned in series. The ultrastructure of chromosomes was examined and microtubules (MTs) were counted. The main results of the study are summarized as follows: 1. The autosomes move at the periphery of the continuous MTs during anaphase while the sex chromosomes move more or less within this group of MTs. 2. In metaphase the antosomes have few coarse surface projections, in anaphase many, but more delicate projections of irregular shape which seem to transform into regular radial lamellae at the end of movement. 3. In metaphase continuous MTs have no contact with the chromosomal surface, while during anaphase movement continuous MTs lie closer to the chromosomes, and finally arrange themselves between the radial surface lamellae. There they show lateral filamentous connections with the chromosomal surface. 4. The MT distribution profiles of metaphase and anaphase are different. While the highest density of MTs is observed in the middle region of the spindle in metaphase, there are two density zones during autosomal movement, each in one half spindle in front of the autosomes. After the autosomes have reached the poles the distribution profile is again similar to the metaphase condition. The MT distribution in metaphase II is the same as in metaphase I. Possible explanations for these observations are discussed in detail. 5. There is an overall decrease in MT content during anaphase. 6. With the onset of anaphase MTs are seen within the spindle mantle, closely associated with mitochondria. — Several theoretical aspects of anaphase mechanism are briefly discussed.     

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     

Inverted meiosis and its place in the evolution of sexual reproduction pathways

Inverted meiosis is observed in plants (Cyperaceae and Juncaceae) and insects (Coccoidea, Aphididae) with holocentric chromosomes, the centromeres of which occupy from 70 to 90% of the metaphase chromosome length. In the first meiotic division (meiosis I), chiasmata are formed and rodlike bivalents orient equationally, and in anaphase I, sister chromatids segregate to the poles; the diploid chromosome number is maintained. Non-sister chromatids of homologous chromosomes remain in contact during interkinesis and prophase II and segregate in anaphase II, forming haploid chromosome sets. The segregation of sister chromatids in meiosis I was demonstrated by example of three plant species that were heterozygous for chromosomal rearrangements. In these species, sister chromatids, marked with rearrangement, segregated in anaphase I. Using fluorescent antibodies, it was demonstrated that meiotic recombination enzymes Spo11 and Rad5l, typical of canonical meiosis, functioned at the meiotic prophase I of pollen mother cells of Luzula elegance and Rhynchospora pubera. Moreover, antibodies to synaptonemal complexes proteins ASY1 and ZYP1 were visualized as filamentous structures, pointing to probable formation of synaptonemal complexes. In L. elegance, chiasmata are formed by means of chromatin threads containing satellite DNA. According to the hypothesis of the author of this review, equational division of sister chromatids at meiosis I in the organisms with inverted meiosis can be explained by the absence of specific meiotic proteins (shugoshins). These proteins are able to protect cohesins of holocentric centromeres from hydrolysis by separases at meiosis I, as occurs in the organisms with monocentric chromosomes and canonical meiosis. The basic type of inverted meiosis was described in Coccoidea and Aphididae males. In their females, the variants of parthenogenesis were also observed. Until now, the methods of molecular cytogenetics were not applied for the analysis of inverted meiosis in Coccoidea and Aphididae. Evolutionary, inverted meiosis is thought to have appeared secondarily as an adaptation of the molecular mechanisms of canonical meiosis to chromosome holocentrism.     

Co-segregation of sex chromosomes in the male black widow spider <Emphasis Type="Italic">Latrodectus mactans</Emphasis> (Araneae,Theridiidae)

During meiosis I, homologous chromosomes join together to form bivalents. Through trial and error, bivalents achieve stable bipolar orientations (attachments) on the spindle that eventually allow the segregation of homologous chromosomes to opposite poles. Bipolar orientations are stable through tension generated by poleward forces to opposite poles. Unipolar orientations lack tension and are stereotypically not stable. The behavior of sex chromosomes during meiosis I in the male black widow spider Latrodectus mactans (Araneae, Theridiidae) challenges the principles governing such a scenario. We found that male L. mactans has two distinct X chromosomes, X₁ and X₂. The X chromosomes join together to form a connection that is present in prometaphase I but is lost during metaphase I, before the autosomes disjoin at anaphase I. We found that both X chromosomes form stable unipolar orientations to the same pole that assure their co-segregation at anaphase I. Using micromanipulation, immunofluorescence microscopy, and electron microscopy, we studied this unusual chromosome behavior to explain how it may fit the current dogma of chromosome distribution during cell division.     

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.     

The behaviour and structure of the sex chromosomes in spermatocytes of Microtus oeconomus

The meiotic behaviour and structure of the sex chromosomes of Microtus oeconomus (2n=30) in Giemsa stained preparations are described. The X-Y pair appears as a sex vesicle at late zygotene. At late pachytene an unfolded sex vesicle is visible. A condensed sex vesicle appears during pre-diffuse diplotene and starts to unfold again during post-diffuse diplotene. At diakinesis and metaphase I the X and Y chromosomes can be recognized in an end-to-end association. During anaphase I, interkinesis and metaphase II the sex chromosomes are heteropycnotic and can therefore easily be recognized during the final stages of meiosis. During spermiogenesis the X and Y chromosomes can be identified in Giemsa stained preparations until the stage of spermatid elongation.     

Meiotic pairing and segregation of achiasmate sex chromosomes in eutherian mammals: the role of SYCP3 protein

In most eutherian mammals, sex chromosomes synapse and recombine during male meiosis in a small region called pseudoautosomal region. However in some species sex chromosomes do not synapse, and how these chromosomes manage to ensure their proper segregation is under discussion. Here we present a study of the meiotic structure and behavior of sex chromosomes in one of these species, the Mongolian gerbil (Meriones unguiculatus). We have analyzed the location of synaptonemal complex (SC) proteins SYCP1 and SYCP3, as well as three proteins involved in the process of meiotic recombination (RAD51, MLH1, and γ-H2AX). Our results show that although X and Y chromosomes are associated at pachytene and form a sex body, their axial elements (AEs) do not contact, and they never assemble a SC central element. Furthermore, MLH1 is not detected on the AEs of the sex chromosomes, indicating the absence of reciprocal recombination. At diplotene the organization of sex chromosomes changes strikingly, their AEs associate end to end, and SYCP3 forms an intricate network that occupies the Y chromosome and the distal region of the X chromosome long arm. Both the association of sex chromosomes and the SYCP3 structure are maintained until metaphase I. In anaphase I sex chromosomes migrate to opposite poles, but SYCP3 filaments connecting both chromosomes are observed. Hence, one can assume that SYCP3 modifications detected from diplotene onwards are correlated with the maintenance of sex chromosome association. These results demonstrate that some components of the SC may participate in the segregation of achiasmate sex chromosomes in eutherian mammals.     

Mitosis with undivided chromosomes

Summary Cases of cell division with single chromatids are discussed in connection with a study on mitosis with undivided chromosomes made on living material of the endosperm of Haemanthus katharinae. Such divisions are known from certain abnormal mitoses in the microspores of a few plant species, and also from the second meiotic division, in which it is possible in numerous materials to study the behaviour of daughter univalents, and, in a few cases, also daughter chromosomes derived from chromosomes that were paired during the first division.The various cases of mitosis with single chromatids show a great variation with respect to the degree of scattering of the chromosomes over the spindle at metaphase. In a few cases there is practically no tendency to form a metaphase plate. In other cases the tendency to form such a plate is more or less pronounced, but also in these cases it is difficult for the chromosomes to form this arrangement. Some of them remain scattered over the spindle. After the metaphase a kind of anaphase usually follows in which the single chromatids, without division, move to the poles, often with other chromosomes lagging in intermediate positions.An approach of chromosomes to the poles may be caused by two different mechanisms in mitoses of this kind and only in a few cases is the information sufficient to show that active centromere movements occur during these anaphases.In many aspects of their behaviour on the spindle, single chromatids are similar to ordinary univalents of the first meiotic division. For this reason the movement mechanics of the chromosomes of the first meiotic division is briefly reviewed.The interpretation is expressed that the structure of the centromere region of a single chromatid shows some similarity to that of a univalent of the first meiotic division and that this may be the reason for their similar behaviour. The chromatid centromere would have a structural multiplicity with respect to its kinetic elements, corresponding to its subdivision in half-chromatids and also to the presence of two or three consecutive chromomeres in its longitudinal direction. As these kinetic elements are arranged close to one another on one side of the narrow cylinder of the centromere constriction, it is difficult for them to orient, towards both poles simultaneously. A single chromatid having a centromere of this kind will show orientation instability and change its orientation between the two unipolar orientations and various more or less bipolar orientations. The movements following these different orientations would cause the scattering of these single chromatids over the spindle. The orientation of ordinary mitotic metaphase chromosomes, consisting of two such chromatids, could often be the consequence of a process of co-orientation similar to that in meiotic bivalents.The anaphase movement of undivided chromosomes, which by active centromere movements are shifted in the polar directions without a separation of daughter components, is discussed with reference to a similar behaviour observed by Dietz in multivalents in Ostracods. These multivalents are stabilized in the equator during metaphase, in spite of the fact that they have two or three centromeres directed towards one pole and a single one towards the other. During anaphase their chromosomes do not separate but the whole configurations are shifted towards that pole towards which the majority of the centromeres are directed (this is followed by another type of movement which does not concern us in this connection). Undivided chromosomes that are oriented with more of their kinetic material towards one of the poles and less towards the other should by the same mechanisms as moved the multivalents be shifted in the equatorial direction during metaphase and in the polar direction during anaphase. The mechanism of these events is obscure. A change in the interpretation given by Dietz is suggested.This paper is dedicated to Professor Franz Schrader on the occasion of his seventieth birthday.     

Meiotic behavior of gonosomically variant females of Akodon azarae (Rodentia, Cricetidae)

The meiotic behavior of sex chromosomes has been investigated in variant females of Akodon azarae, both in pachytene oocytes and metaphase I. In somatic cells, these females have a heteromorphic sex pair, in which the minor chromosome has been previously interpreted as a major deletion of the long arm of the X chromosome (dX). After microspreading for synaptonemal complex analysis, pachytene oocytes show two axes of very different lengths (100:17.1), which correspond to the sex chromosomes X and dX. True synapsis is abnormally restricted (43.3%) between these sex chromosomes; on the other hand, self-synapsis of both the X and dX chromosomes is frequent (60%). Single, nonsynapsed axes or axial segments are thickened. Strong chromatin condensation occurs around nonsynapsed axes or axial segments, giving many of these sex pairs an appearance similar to an XY body ("sex vesicle"). The minor gonosome axis differs from that of the Y chromosome of male meiosis, as the former is shorter (relative to the X) and has a different synaptic behavior. In 17 metaphases I from XdX variant females, only heteromorphic, end-to-end joined sex pairs were observed. These variant females differ from the variant females of the wood lemming Myopus schisticolor in several respects, but a similar mechanism seems to be prevalent in other species of the genus Akodon. Self-synapsis of unequal gonosomes in oocytes is assumed as an escape from functional deterioration, following the hypothesis put forward by others.