Karyotype,DNA replication and origin of sex chromosomes in Anopheles atroparvus

Anopheles atroparvus has two pairs of autosomes similar in length and morphology and two sex chromosomes with equal, heterochromatic, late replicating long arms with homologous C-, G-, and Q-bands. The short arm of the Y is shorter than that of the X and both are euchromatic. The mean number of chiasmata per cell in the male is 3.2. During mitosis there is a high grade of somatic pairing but X and Y, which form a heteropycnotic mass in the interphase nucleus, have a differential behaviour. The chronology of DNA replication was studied in spermatogonia and brain cells by autoradiography. It is hypothesized that the present sex chromosomes of A. atroparvus evolved by accumulation of sex determining factors and gene deterioration resulting in heterochromatinization of the long arms, followed by structural rearrangements.—The homology of the two sex chromosomes requires limited dosage compensation which is achieved either as in Drosophila by modifier genes or by accumulation on the short arm of the X, only of female determining factors which do not require dosage compensation.     

On the cytotaxonomy of phasmids (Phasmatodea)

Summary A wide diversity in chromosome complement is found in two species of phasmids of the primitive group Prisopini—Prisopus ariadne Hebard and Prisopus berosus Westwood. P. ariadne has a diploid male complement of 28, comprising 13 pairs of relatively large mediokinetic autosomes and Neo XY sex chromosomes. P. berosus, 2n =49, has relatively small autosomes most of which are mediokinetic, and retains the XO—XX sex mechanism. Chromosomal polymorphism in this species is suggested by the presence of an unequal pair of autosomes and a structural differentiation in the X in one of two males studied.The relative amount of DNA per nucleus in male germ cells (Peulgen cytophotometry) shows a significant difference in total chromosomal content between the complements of the two species.These data are discussed with reference to the cytotaxonomy of phasmids.Supported in part by research grant G-4370 from the National Institutes of Health, Public Health Service.     

The chromosomes of mantids (Orthoptera; Manteidae) in relation to taxonomy

Summary The distinguishing cytological characters, including chromosome number and sex chromosome mechanism, have been tabulated for all mantids thus far studied cytologically—including 16 species (starred in Table 1) not hitherto examined. Data on 46 species in 17 subfamilies are reviewed.The XO—XX sex chromosome mechanism represents the ancestral condition for the family and has been retained in 14 of the subfamilies sampled.The distribution of the X¹X²Y—X¹X¹X²X² mechanism, previously known in the subfamilies Manteinae and Choeradodinae, is extended to include Mellierinae, Acromantinae and Vatinae.Cytological evidence indicates a monophyletic origin of the compound sex chromosomes and necessitates a reconsideration of the taxonomic association of XO and X¹X²Y forms in the same subfamily, specifically of Miomantis and Callimantis with the other Manteinae, and Aethalocroa with Vates and Phyllovates in the Vatinae.The pre-metaphase stretch is established as a regular stage in male meiosis in 14—and is absent in 3—of the species checked for this character. It should prove of taxonomic value when a wider sampling of the lower categories is available.The prolongation of parallel pairing of homologous chromosomes in male meiosis appears to be characteristic of the Manteidae as a whole. No correlation is apparent between the degree of its expression and taxonomic category. Since it provides a mechanism, additional or alternative to chiasmata, for the post-pachytene association of homologues, its establishment permits the evolution of non-chiasmate meiosis in the group.Two structural types of bivalents—one with, one without chiasmata—are found. Present evidence favors the multiple origin of the non-chiasmate type within the family, and since both types may coexist in the karyotype of the same sex of a single species, presence or absence of chiasmata cannot be considered a valid taxonomic criterion.     

DNS-Reduplikationsmuster bei den Riesen-Geschlechtschromosomen von Microtus agrestis

Summary The X-chromosome of Microtus agrestis (2 n=50), comprising about 20 per cent of the homogametic haploid (AX) set, is the largest X-chromosome reported so far in placental mammals. It is four times the size of the X possessed by a great majority of mammals, including the human and the mouse. The Y-chromosome is also enormous, almost three-fifths the size of the X.The present cytological study concerned somatic interphase and prophase nuclei as well as the DNA replication pattern revealed by labeling cultured bone marrow cells with tritiated thymidine.In the male nuclus, the entire Y as well as the long arm and proximal part of the short arm of the X are late labeling and positively heteropycnotic. In the female, one entire X is late labeling and condensed, while the other X shows the same labeling pattern as the male X. Thus the pattern of inactivation of this huge X is such that in each diploid nucleus of both sexes, the amount of euchromatic X-chromosome material is the same as it is in the majority of placental mammals in which the X comprises about five per cent of the haploid set.

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Wesentliche Teile der vorliegenden Arbeit werden von Gertraud Flinspach als Dissertation der Medizinischen Fakultät der Universität Freiburg i.Br. vorgelegt.     

DNA synthesis in the neo-X neo-Y sex determination system of Dichroplus bergi (Orthoptera: Acrididae)

DNA replication in the neo-X neo-Y sex determining system was studied by means of tritiated thymidine and autoradiography. Asynchronous replication was found in the X arm of the neo-X and the long arm of the neo-Y. In addition, striking asynchrony was also found for short isopycnotic homologous regions at the distal end of the autosmal arm of neo-X and the short arm of neo-Y to which pairing during meiosis is restricted. These short regions are asynchronous with respect to the heterochromatic segments as well as to the remaining proximal region of the autosomal euchromatic arm of neo-X. This difference in replication pattern within the same chromosome arm may be related to a differentiation between regions which are homozygous in both sexes and regions which are hemizygous in males.This work was supported by U.S. Atomic Energy Commission Contract N AT (30-1) 3517 to Prof. F. A. Saez.     

Molecular analysis of aberrations of Xp and Yq

Summary Three cases of Y chromosomal aberrations were studied using a panel of Y-specific DNA sequences from both Yp and euchromatic Yq. One case was a phenotypic male fetus with a Y-derived marker chromosome. The short arm of this chromosome was intact, but most of its long arm was missing. The second case had a 46,Xyq- karyotype with portions of euchromatic Yq, including the spermatogenesis region, missing. The third case was a phenotypic female with a 46,XXp+ karyotype. The extra material on the Xp+ chromosome was derived from the heterochromatic, and part of the euchromatic, portion of Yq. Application of X-specific DNA sequences demonstrated that the distal portion of the short arm of the translocation X chromosome was deleted (Xpter—p22.3). The three examples demonstrate the importance of diagnostic DNA analysis in cases of marker chromosomes, and X and Y chromosomal aberrations. In addition, the findings in the patients facilitate further deletion mapping of euchromatic Yq.     

The meiotic chromosomes of man

Summary Information was obtained on the chromosome number, and the behavior of autosomes as well as of the sex chromosomes in meiosis in human male germ cells derived from 25 Japanese patients, 4 to 79 years in age, who were hospitalized mostly due to epididymitis, prostate cancer, undescended testes or infertility.In 16 out of the 25 specimens, the chromosome numbers, 46 in 2n and 23 in n, were consistently established together with an XY sex-determining mechanism based on spermatogonial and spermatocyte divisions. No reliable counts were obtained from the remaining 9 cases, because of that they provided no cells for precise investigation.The X and Y chromosomes during the leptotene stage were observed as two separate heteropycnotic bodies lying along the inner wall of the nucleus, while at pachytene they formed a sex-vesicle after homologous pairing. At the diplotene, diakinesis and first metaphase the X and the Y appeared as an isopycnotic bivalent showing an end-to-end association, though there were some cells in which they remained as two separate entities free from contact. Evidence was presented that the X and the Y seemed to associate with each other at the distal end of the short arm of each element.One or sometimes two smallest autosomal bivalents tended to show rather precociously a chiasma-terminalization at the first metaphase.The metaphase chromosomes of the second spermatocytes were evident by the haploid number as well as by their widely diverged chromatids with a characteristic spiral configuration.The testicular materials under study contained in most cases polyploid cells with a considerable frequency in spermatogonia as well as in first and second spermatocytes. Giant sperm heads were observed not infrequently, mostly being abnormal in shape. No significant correlation was obtained between the frequency of polyploid cells and the age of patients so far studied.Contribution No. 679 from the Zoological Institute, Faculty of Science, Hokkaido University, Sapporo. — It is our pleasure to dedicate this paper to Professor Dr. Hans Bauer, Max-Planck-Institut für Meeresbiologie, Tübingen, in honor of his sixtieth birthday.     

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.     

Extensive DNA sequence homologies between the human Y and the long arm of the X chromosome.

It has been proposed that sequence homology should exist between the short arms of the human sex chromosomes, in the regions pairing at meiosis. Out of 40 clones picked at random from a collection of non-repetitive DNA sequences derived from the human Y chromosome, we have found nine sequences which show very high homology with sequences located on the X chromosome. All nine probes originate from the euchromatic part of the Y chromosome. All the homologous sequences are located within the Xq12-Xq22-24 region. None of them map to the short arm of the X chromosome. We conclude that an important part of the euchromatic region of the Y chromosome is homologous to the middle of the X chromosome long arm, possibly as a result of recent translation event(s).     

Meiotic pairing constraints and the activity of sex chromosomes

The state of activity and condensation of the sex chromosomes in gametocytes is frequently different from that found in somatic cells. For example, whereas the X chromosomes of XY males are euchromatic and active in somatic cells, they are usually condensed and inactive at the onset of meiosis; in the somatic cells of female mammals, one X chromosome is heterochromatic and inactive, but both X chromosomes are euchromatic and active early in meiosis. In species in which the female is the heterogametic sex (ZZ males and ZW females), the W chromosome, which is often seen as a condensed chromatin body in somatic cells, becomes euchromatic in early oocytes. We describe an hypothesis which can explain these changes in the activity and condensation of sex chromosomes in gametocytes. It is based on the fact that normal chromosome pairing seems to be essential for the survival of sex cells; chromosomal anomalies resulting in incomplete pairing during meiosis usually result in gametogenic loss. We argue that the changes seen in the sex chromosomes reflect the need to avoid pairing failure during meiosis. Pairing normally requires structural and conformational homology of the two chromosomes, but when the regions is avoided when these regions become heterochromatinized. This hypothesis provides an explanation for the changes found in gametocytes both in species with male heterogamety and those with female heterogamety. It also suggests possible reasons for the frequent origin of large supernumerary chromosomes from sex chromosomes, and for the reported lack of dosage compensation in species with female heterogamety.     

Replication pattern of the X and Y chromosomes in partially synchronized human lymphocyte cultures

The replication pattern of the X and Y chromosomes at the beginning of the synthetic phase was studied in human lymphocyte cultures partially synchronized by the addition of 5-fluoro-2-deoxyuridine (FUdR). The data were evaluated statistically by an analysis of the distribution of silver grain counts over the X and Y chromosomes. —In cells from normal females, one of the X chromosomes began replication later than any other chromosomes of the complement. The short arm of the late replicating X chromosome started replication earlier than the long arm. The telomeric region of the short arm was a preferential site of DNA synthesis at the beginning of replication. —In partially synchronized lymphocyte cultures from a patient with the XXY syndrome, the Y chromosome started replication together with the late replicating X chromosome. The Y chromosome most frequently replicated synchronously with the short arm of the X. The centromeric region of the Y chromosome initiated synthesis before the telomeric region and appeared to replicate synchronously with the telomeric region of the short arm of the X. These findings are discussed with reference to the pairing of the X and Y chromosomes at meiosis.Supported in part by the National Institute of Health Research Grant HD-01979 and National Foundation Birth Defects Research Grant CRCS-40. Dr. Knight was a predoctoral fellow under National Institute of Health Training Program HD-00049-09.     

Distance segregation and compound sex chromosomes in mantispids (Neuroptera: Mantispidae)

The sex chromosomes segregate precociously in prometaphase I of male meiosis, without prior synapsis or any physical connection, in 4 species of American mantispids (Neuroptera: Mantispidae). Segregational movements are interpolar, and are implemented through chromosomal fibers. Univalent autosomes, present from diakinesis on in several species, are capable of a similar distance segregation in prometaphase. The sex chromosomes are XX —XY , as is characteristic of the Order, with the exception of Entanoneura phthisica in which both elements are compound —X¹X²X³Y¹Y²Y³in the male, and X¹X¹X²X²X³X³ in the female. In tetraploid sectors of gonial origin in testes of this species no sex bivalents are formed; a distance segregation of 6 sex univalents to each pole is effected, but — as observed in the one individually identifiable pair — segregation separates complete homologues, Y¹ from Y¹, X¹ from X¹, etc. In all species the male meiotic spindle is formed by the collocation of individual chromosomal spindle units within which bivalents become deformed; the timing and degree of deformation vary with the species. In karyotype the American species conform to a common pattern with the known Japanese and European species; diploid numbers range only from 18 to 22, and each complement carries the family insigné of one pair of disproportionately large autosomes in a set of small and rather uniformly sized chromosomes.     

Molecular Cytogenetic Characterization of the Dioecious Cannabis sativa with an XY Chromosome Sex Determination System

Hemp (Cannabis sativa L.) was karyotyped using by DAPI/C-banding staining to provide chromosome measurements, and by fluorescence in situ hybridization with probes for 45 rDNA (pTa71), 5S rDNA (pCT4.2), a subtelomeric repeat (CS-1) and the Arabidopsis telomere probes. The karyotype has 18 autosomes plus a sex chromosome pair (XX in female and XY in male plants). The autosomes are difficult to distinguish morphologically, but three pairs could be distinguished using the probes. The Y chromosome is larger than the autosomes, and carries a fully heterochromatic DAPI positive arm and CS-1 repeats only on the less intensely DAPI-stained, euchromatic arm. The X is the largest chromosome of all, and carries CS-1 subtelomeric repeats on both arms. The meiotic configuration of the sex bivalent locates a pseudoautosomal region of the Y chromosome at the end of the euchromatic CS-1-carrying arm. Our molecular cytogenetic study of the C. sativa sex chromosomes is a starting point for helping to make C. sativa a promising model to study sex chromosome evolution.     

Chromosome banding in Amphibia

The chromosomes of the South American marsupial frogs Gastrotheca fissipes, G. ovifera, G. walkeri and Flectonotus pygmaeus were analyzed by means of conventional and various banding techniques. The karyotypes of G. ovifera and G. walkeri are characterized by highly differentiated XY/XX sex chromosomes. Whereas the X chromosomes and autosomes contain large amounts of constitutive heterochromatin, extremely little heterochromatin is located in the Y chromosomes. This is in contrast to all previously known amphibian Y chromosomes and the Y chromosomes of most other vertebrates. In the male meiosis of G. walkeri, the euchromatic segments of the heteromorphic XY chromosomes show the same pairing configuration as the autosomal bivalents. The karyotype of F. pygmaeus is remarkable for the unique presence of telocentric chromosomes and the high frequency of interstitially located chiasmata in the meiotic bivalents. The evolution of the karyotypes and sex chromosomes, the structure of the various classes of heterochromatin and the data obtained from meiotic analyses of the marsupial hylids are discussed.     

Chromosomenverhältnisse,konstitutives Heterochromatin und Geschlechtsbestimmung bei einigen Arten der GattungChrysomya (Calliphoridae,Diptera)

Compared with the strongly monogenic blowfliesChrysomya albiceps andC. rufifacies investigated previously, the closely related speciesC. chloropyga, C. putoria andC. varipes turned out to be amphogenic, i.e. each female of these species produces both sexes in nearly equal frequencies.C. chloropyga, C. putoria andC. varipes possess similar karyotypes consisting of five pairs of long identifiable autosomes and one pair of smaller sex chromosomes, which are homomorphic (XX) in the female and heteromorphic (XY) in the male. In all species the Y is shorter than the X. Among the cytologically known amphogenicChrysomya speciesC. varipes has the smallest heterochromosomes. Male meiosis inC. chloropyga, C. putoria andC. varipes is achiasmatic. During mitosis the autosomes show a high degree of somatic pairing but heterochromosomes, which form a heteropyknotic mass in the interphase nuclei, usually separate in the course of early mitotic prophase. A comparative study of C-banding patterns in mitotic and meiotic chromosomes ofC. chloropyga, C. putoria, C. varipes andC. rufifacies revealed that constitutive heterochromatin is present in almost all chromosomes. In the five pairs of large chromosomes it is confined to short regions adjacent to the kinetochores. An interstitial C-band has been observed only in the shorter arm of autosome no. 3 inC. putoria. With the exception of the tiny Y chromosome ofC. varipes, which seems to be completely euchromatic, all the other sex chromosomes of the amphogenicChrysomya species as well as the small chromosome no. 6 ofC. rufifacies possess longer or shorter C-segments. The karyotype ofC. varipes resembles closely that of the monogenic speciesC. albiceps andC. rufifacies thus indicating that the monogenic species could have evolved from an ancestral species closely related toC. varipes. The occurrence of two exceptional XO females and one exceptional XXY male among normal XX and XY animals in a wild stock ofC. chloropyga demonstrates that the Y chromosome of C. chloropyga and most probably of all amphogenicChrysomya species bears an epistatic male determining factor.

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Herrn Professor Dr. G. Krause zum 70. Geburtstag gewidmet     

A complex sex-chromosome system in the hare-wallaby Lagorchestes conspicillatus Gould

Among specimens of the spectacled hare-wallaby Lagorchestes conspicillatus Gould (Marsupialia, family Macropodidae) 4 males had 15 chromosomes and 2 females 16 chromosomes. The sex chromosomes are X₁X₁X₂X₂ in the female and X₁X₂Y in the male, the Y being metacentric and both X chromosomes are acrocentric. In about 96% of sperm mother cells at meiosis the sex chromosomes form a chain trivalent and in more than 99% of these this orients convergently so that the X₁ and X₂ move to the same pole. Evidence is presented that L. conspicillatus has evolved from a form with 22 chromosomes including a small X and a minute Y. Autoradiographic studies show that the proximal fifth of the X₁ chromosome replicates late. This is probably the ancestral X chromosome which has been translocated to an autosome. The fate of the original Y is obscure but an hypothesis is proposed that it forms the centromeric region of the Y. A single male had 14 chromosomes and was heterozygous for a translocation involving the centric fusion of two acrocentric autosomes. In about 30% of sperm mother cells the autosomal trivalent did not disjoin regularly but, despite this, all secondary spermatocytes observed at metaphase 2 had balanced complements of chromosomes. It is assumed that unbalanced secondary spermatocytes died before reaching metaphase.     

Y;autosome translocations and mosaicism in the aetiology of 45,X maleness: assignment of fertility factor to distal Yq11

Summary Three 45,X males have been studied with Y-DNA probes by Southern blotting and in situ hybridization. Southern blotting studies with a panel of mapped Y-DNA probes showed that in all three individuals contiguous portions of the Y chromosome including all of the short arm, the centromere, and part of the euchromatic portion of the long arm were present. The breakpoint was different in each case. The individual with the largest portion (intervals 1–6) is a fertile male belonging to a family in which the translocation is inherited in four generations. The second adult patient, who has intervals 1–5, is an azoospermic, sterile male. These phenotypic findings suggest the existence of a gene involved in spermatogenesis in interval 6 in distal Yq11. The third case, a boy with penoscrotal hypospadias, has intervals 1–4B. In situ hybridization with the pseudoautosomal probe pDP230 and the Y chromosome specific probe pDP105 showed that Y-derived DNA was translocated onto the short arm of a chromosome 15, 14, and 14, respectively. One of the patients was a mosaic for the 14p+ translocation chromosome. Our data and those reported by others suggest the following conclusions based on molecular studies in eight 45,X males: The predominant aetiological factor is Y;autosome translocation observed in seven of the eight cases. As the remaining case was a low-grade mosaic involving a normal Y chromosome, the maleness in all cases was due to the effect of the testis determing factor, TDF. There is preferential involvement of the short arm of an acrocentric chromosome (five out of seven translocations) but other autosomal regions can also be involved. The reason why one of the derivative translocation chromosomes becomes lost may be that it has no centromere.     

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.     

The neo-X neo-Y sex pair in Acrididae,its structure and association

In most of the fifty described cases of neo-X neo-Y sex determining systems in Acrididae the pairing regions during meiosis are limited to distal regions. A comparative study on the structure and pairing mechanisms of Dichroplus silveiraguidoi (2n=8); Dichroplus bergi (2n=22) and Dichroplus vittatus (2n=20) has been undertaken. — The sex bivalents of these three grasshoppers are different: the neo-X centromere is associated with the neo-Y telomere in D. silveiraguidoi; in D. bergi the neo-X is related through the short arm telomere to the centromere of neo-Y and both members of the sex pair are associated by the telomeres in D. vittatus. Centromeric and telomeric C-band positive blocks are present in both members of the pair in the three species. D. silveiraguidoi also presents an interstitial block in the neo-X. These blocks are brightly fluorescent with quinacrine mustard and Hoechst 33258 at low concentration (0.05 g/ml). The region of neo-X corresponding to the primitive X takes an intermediate staining during the early meiotic prophase with C-banding and Hoechst 33258. — The structure of the sex bivalent and the particular staining of the X region are discussed in relation to the available information on the presence of different types of DNA in this segment. The possibility that the neo-X interstitial block of D. silveiraguidoi plays a role in preventing the spreading of heterochromatinization along the chromosome is also discussed. The classical interpretation of the neo-X neo-Y association during meiotic prophase as the result of a terminalized chiasma is considered in the light of optic and electronmicroscopic data. Other possible mechanisms of relationship between both chromosomes are also presented by these three orthopteran species.     

The Chromosomes of some octodontids with special reference to Octodontomys (Rodentia; Hystricomorpha)

The chromosomes of three species (Octodontomys gliroides, Octodon degus and Ctenomys talarum) of octodontid hystricomorph rodents are compared. — The diploid numbers are 38, 58 and 48 respectively. No polymorphic chromosomes were noted in the specimens available. The sex chromosomes are heteromorphic and pair end to end in meiosis. — The karyotypes are compared on the basis of the extreme specialisation of the habitat of the three species. The asymmetric karyotype of Octodontomys can be compared more readily with that of a Ctenomys ancestor than with that of its present-day relative, Octodon.