Sex Chromosome Complex Research Articles

Rapid Sex Identification in Spotted Knifejaw (Oplegnathus punctatus) Using tmem88 Gene Structural Variation Markers.

Spotted knifejaw (Oplegnathus punctatus) is an economically important marine cultured species exhibiting a unique complex sex chromosome system (X1X1X2X2 in females and X1X2Y in males), with males possessing one fewer chromosome (2n = 47) than females (2n = 48) and an abnormally large Y chromosome. Additionally, males demonstrate significant growth advantages over females. Rapid and accurate sex identification is essential for effective culture management, selective breeding, and population control. In this study, we identified a homologous region of the tmem88 gene containing large DNA insertion markers on the X and Y chromosomes through whole-genome sequencing of O. punctatus. The X1 chromosome harbors a 278bp DNA fragment, whereas the Y chromosome contains a 1472bp fragment, resulting in a 1194bp size difference indicative of structural variation in the non-coding region of the tmem88 gene. We developed a rapid detection method based on this variation, utilizing a pair of primers that amplify two distinct bands (278bp and 1472bp) in male (X1X2Y) individuals and a single 278bp band in female (X1X1X2X2) individuals when analyzed by agarose gel electrophoresis. This method enables efficient and accurate sex differentiation in O. punctatus, significantly reducing the time required for identification and enhancing detection efficiency. This study provides a valuable tool for the rapid identification of sex in O. punctatus, facilitating improved breeding strategies and supporting the large-scale production of high-quality fry.

Sex assignment and psychosexual peculiarities of individuals with different forms of androgen insensitivity syndrome: A qualitative study.

A mismatch between chromosomal, gonadal, and phenotypic sexes in individuals with androgen insensitivity syndrome (AIS) creates problems in sex assignment and psychosexual identification. To identify psychosexual and sex assignment peculiarities of individuals with different forms of AIS. In this qualitative study, 41 individuals with AIS aged between 15 and 31 yr who referred to the Universe Center for Reproductive Medicine Tbilisi, Georgia between 2016 and 2021 were studied. All individuals underwent clinical, genealogical, hormonal, ultrasonographic, and cytogenetic examinations. In-depth interviews and medical records assessed psychosexual profiles and sex assignment histories. 32 cases were diagnosed with the complete form of AIS (CAIS), 8 individuals with the partial form (PAIS), and one with a mild form (MAIS). Individuals with CAIS and PAIS were assessed at birth and raised as girls. Individuals with CAIS and female psychosexual disposition were referred to us due to amenorrhea. Adolescent individuals with PAIS assessed as girls referred to us due to masculinization detected in puberty. An individual with MAIS was assessed at birth and raised as a boy with male genitalia. All individuals with AIS had typical hormonal data and sex chromosome complex for men. 20 sexually active individuals with CAIS had penile-vaginal contact with the man. None of the individuals with CAIS and PAIS thought about gender reassignment after being diagnosed, only the individual with MAIS aimed for male-to-female transition. Psychosexual identification remains a significant challenge in AIS management. Detection of female psychosexual disposition in one participant that is unusual to MAIS may be associated with somatic mosaicism of the androgen receptor gene.

Evolution and dosage compensation of nucleolar organizing regions (NORs) mediated by mobile elements in turtles with female (ZZ/ZW) but not with male (XX/XY) heterogamety.

Understanding the evolution and regulation of nucleolar organizing regions (NORs) is important to elucidate genome structure and function. This is because ribosomal gene (rDNA) copy number and activity mediate protein biosynthesis, stress response, ageing, disease, dosage compensation and genome stability. Here, we found contrasting dosage compensation of sex-linked NORs in turtles with male and female heterogamety. Most taxa examined exhibit homomorphic rRNA gene clusters in a single autosome pair (determined by 28S rDNA fluorescence in situ hybridization), whereas NORs are sex-linked in Apalone spinifera, Pelodiscus sinensis and Staurotypus triporcatus. Full-dosage compensation upregulates the male X-NOR (determined via silver staining-AgNOR) in Staurotypus (who lacks Y-NOR) compared with female X-AgNORs. In softshell Apalone and Pelodiscus, who share homologous ZZ/ZW micro-chromosomes, their enlarged W-NOR is partially active (due to 28S rDNA invasion by R2 retroelements), whereas their smaller Z-NOR is silent in females but active in both male-Zs (presumably because the W-NOR meets cellular demands and excessive NOR activity is costly). We hypothesize that R2 disruption favoured W enlargement to add intact 28S-units, perhaps facilitated by reduced recombination during sex chromosome evolution. The molecular basis of the potentially adaptive female Z-silencing is likely intricate and perhaps epigenetic, as non-ribosomal Z genes are active in Apalone females. Yet, Emydura maquarii exhibit identical heteromorphism in their autosomal NOR (R2 invaded 28S-units and the small-autosome NOR is silent), suggesting that the softshell turtle pattern can evolve independent of sex chromosome evolution. Our study illuminates the complex sex chromosome evolution and dosage compensation of non-model systems that challenges classic paradigms.

Evolution, Expression and Meiotic Behavior of Genes Involved in Chromosome Segregation of Monotremes.

Chromosome segregation at mitosis and meiosis is a highly dynamic and tightly regulated process that involves a large number of components. Due to the fundamental nature of chromosome segregation, many genes involved in this process are evolutionarily highly conserved, but duplications and functional diversification has occurred in various lineages. In order to better understand the evolution of genes involved in chromosome segregation in mammals, we analyzed some of the key components in the basal mammalian lineage of egg-laying mammals. The chromosome passenger complex is a multiprotein complex central to chromosome segregation during both mitosis and meiosis. It consists of survivin, borealin, inner centromere protein, and Aurora kinase B or C. We confirm the absence of Aurora kinase C in marsupials and show its absence in both platypus and echidna, which supports the current model of the evolution of Aurora kinases. High expression of AURKBC, an ancestor of AURKB and AURKC present in monotremes, suggests that this gene is performing all necessary meiotic functions in monotremes. Other genes of the chromosome passenger complex complex are present and conserved in monotremes, suggesting that their function has been preserved in mammals. Cohesins are another family of genes that are of vital importance for chromosome cohesion and segregation at mitosis and meiosis. Previous work has demonstrated an accumulation and differential loading of structural maintenance of chromosomes 3 (SMC3) on the platypus sex chromosome complex at meiotic prophase I. We investigated if a similar accumulation occurs in the echidna during meiosis I. In contrast to platypus, SMC3 was only found on the synaptonemal complex in echidna. This indicates that the specific distribution of SMC3 on the sex chromosome complex may have evolved specifically in platypus.

Platypus and echidna genomes reveal mammalian biology and evolution

Egg-laying mammals (monotremes) are the only extant mammalian outgroup to therians (marsupial and eutherian animals) and provide key insights into mammalian evolution1,2. Here we generate and analyse reference genomes of the platypus (Ornithorhynchus anatinus) and echidna (Tachyglossus aculeatus), which represent the only two extant monotreme lineages. The nearly complete platypus genome assembly has anchored almost the entire genome onto chromosomes, markedly improving the genome continuity and gene annotation. Together with our echidna sequence, the genomes of the two species allow us to detect the ancestral and lineage-specific genomic changes that shape both monotreme and mammalian evolution. We provide evidence that the monotreme sex chromosome complex originated from an ancestral chromosome ring configuration. The formation of such a unique chromosome complex may have been facilitated by the unusually extensive interactions between the multi-X and multi-Y chromosomes that are shared by the autosomal homologues in humans. Further comparative genomic analyses unravel marked differences between monotremes and therians in haptoglobin genes, lactation genes and chemosensory receptor genes for smell and taste that underlie the ecological adaptation of monotremes.

Evolution of multiple sex-chromosomes associated with dynamic genome reshuffling in Leptidea wood-white butterflies

Sex-chromosome systems tend to be highly conserved and knowledge about their evolution typically comes from macroevolutionary inference. Rapidly evolving complex sex-chromosome systems represent a rare opportunity to study the mechanisms of sex-chromosome evolution at unprecedented resolution. Three cryptic species of wood-white butterflies—Leptidea juvernica, L. sinapis and L. reali—have each a unique set of multiple sex-chromosomes with 3–4 W and 3–4 Z chromosomes. Using a transcriptome-based microarray for comparative genomic hybridisation (CGH) and a library of bacterial artificial chromosome (BAC) clones, both developed in L. juvernica, we identified Z-linked Leptidea orthologs of Bombyx mori genes and mapped them by fluorescence in situ hybridisation (FISH) with BAC probes on multiple Z chromosomes. In all three species, we determined synteny blocks of autosomal origin and reconstructed the evolution of multiple sex-chromosomes. In addition, we identified W homologues of Z-linked orthologs and characterised their molecular differentiation. Our results suggest that the multiple sex-chromosome system evolved in a common ancestor as a result of dynamic genome reshuffling through repeated rearrangements between the sex chromosomes and autosomes, including translocations, fusions and fissions. Thus, the initial formation of neo-sex chromosomes could not have played a role in reproductive isolation between these Leptidea species. However, the subsequent species-specific fissions of several neo-sex chromosomes could have contributed to their reproductive isolation. Then, significantly increased numbers of Z-linked genes and independent neo-W chromosome degeneration could accelerate the accumulation of genetic incompatibilities between populations and promote their divergence resulting in speciation.

Differential cohesin loading marks paired and unpaired regions of platypus sex chromosomes at prophase I

Cohesins are vital for chromosome organisation during meiosis and mitosis. In addition to the important function in sister chromatid cohesion, these complexes play key roles in meiotic recombination, DSB repair, homologous chromosome pairing and segregation. Egg-laying mammals (monotremes) feature an unusually complex sex chromosome system, which raises fundamental questions about organisation and segregation during meiosis. We discovered a dynamic and differential accumulation of cohesins on sex chromosomes during platypus prophase I and specific reorganisation of the sex chromosome complex around a large nucleolar body. Detailed analysis revealed a differential loading of SMC3 on the chromatin and chromosomal axis of XY shared regions compared with the chromatin and chromosomal axes of asynapsed X and Y regions during prophase I. At late prophase I, SMC3 accumulation is lost from both the chromatin and chromosome axes of the asynaptic regions of the chain and resolves into subnuclear compartments. This is the first report detailing unpaired DNA specific SMC3 accumulation during meiosis in any species and allows speculation on roles for cohesin in monotreme sex chromosome organisation and segregation.

Rapid Karyotype Evolution in Lasiopodomys Involved at Least Two Autosome - Sex Chromosome Translocations.

The generic status of Lasiopodomys and its division into subgenera Lasiopodomys (L. mandarinus, L. brandtii) and Stenocranius (L. gregalis, L. raddei) are not generally accepted because of contradictions between the morphological and molecular data. To obtain cytogenetic evidence for the Lasiopodomys genus and its subgenera and to test the autosome to sex chromosome translocation hypothesis of sex chromosome complex origin in L. mandarinus proposed previously, we hybridized chromosome painting probes from the field vole (Microtus agrestis, MAG) and the Arctic lemming (Dicrostonyx torquatus, DTO) onto the metaphases of a female Mandarin vole (L. mandarinus, 2n = 47) and a male Brandt's vole (L. brandtii, 2n = 34). In addition, we hybridized Arctic lemming painting probes onto chromosomes of a female narrow-headed vole (L. gregalis, 2n = 36). Cross-species painting revealed three cytogenetic signatures (MAG12/18, 17a/19, and 22/24) that could validate the genus Lasiopodomys and indicate the evolutionary affinity of L. gregalis to the genus. Moreover, all three species retained the associations MAG1bc/17b and 2/8a detected previously in karyotypes of all arvicolins studied. The associations MAG2a/8a/19b, 8b/21, 9b/23, 11/13b, 12b/18, 17a/19a, and 5 fissions of ancestral segments appear to be characteristic for the subgenus Lasiopodomys. We also validated the autosome to sex chromosome translocation hypothesis on the origin of complex sex chromosomes in L. mandarinus. Two translocations of autosomes onto the ancestral X chromosome in L. mandarinus led to a complex of neo-X1, neo-X2, and neo-X3 elements. Our results demonstrate that genus Lasiopodomys represents a striking example of rapid chromosome evolution involving both autosomes and sex chromosomes. Multiple reshuffling events including Robertsonian fusions, chromosomal fissions, inversions and heterochromatin expansion have led to the formation of modern species karyotypes in a very short time, about 2.4 MY.

Sex Determination, Sex Chromosomes, and Karyotype Evolution in Insects.

Insects harbor a tremendous diversity of sex determining mechanisms both within and between groups. For example, in some orders such as Hymenoptera, all members are haplodiploid, whereas Diptera contain species with homomorphic as well as male and female heterogametic sex chromosome systems or paternal genome elimination. We have established a large database on karyotypes and sex chromosomes in insects, containing information on over 13000 species covering 29 orders of insects. This database constitutes a unique starting point to report phylogenetic patterns on the distribution of sex determination mechanisms, sex chromosomes, and karyotypes among insects and allows us to test general theories on the evolutionary dynamics of karyotypes, sex chromosomes, and sex determination systems in a comparative framework. Phylogenetic analysis reveals that male heterogamety is the ancestral mode of sex determination in insects, and transitions to female heterogamety are extremely rare. Many insect orders harbor species with complex sex chromosomes, and gains and losses of the sex-limited chromosome are frequent in some groups. Haplodiploidy originated several times within insects, and parthenogenesis is rare but evolves frequently. Providing a single source to electronically access data previously distributed among more than 500 articles and books will not only accelerate analyses of the assembled data, but also provide a unique resource to guide research on which taxa are likely to be informative to address specific questions, for example, for genome sequencing projects or large-scale comparative studies.

Identification of mediator complex 26 (Crsp7) gametologs on platypus X1 and Y5 sex chromosomes: a candidate testis-determining gene in monotremes?

The basal lineage of monotremes features an extraordinarily complex sex chromosome system which has provided novel insights into the evolution of mammalian sex chromosomes. Recently, sequence information from autosomes, X chromosomes, and XY-shared pseudoautosomal regions has become available. However, no gene has so far been described on any of the Y chromosome-specific regions. We analyzed sequences derived from Y-specific BAC clones to identify genes with potentially male-specific function. Here, we report the identification and characterization of the mediator complex protein gametologs on platypus Y5 (Crspy). We also identified the X-chromosomal copy which unexpectedly maps to X1 (Crspx). Sequence comparison shows extensive divergence between the X and Y copy, but we found no significant positive selection on either gametolog. Expression analysis shows widespread expression of Crspx. Crspy is expressed exclusively in males with particularly strong expression in testis and kidney. Reporter gene assays to investigate whether Crspx/y can act on the recently discovered mouse Sox9 testis-specific enhancer element did reveal a modest effect together with mouse Sox9 + Sf1, but showed overall no significant upregulation of the reporter gene. This is the first report of a differentiated functional male-specific gene on platypus Y chromosomes, providing new insights into sex chromosome evolution and a candidate gene for male-specific function in monotremes.

Non‐random autosome segregation: A stepping stone for the evolution of sex chromosome complexes?

AbstractA new study in Caenorhabditis elegans shows that homologous autosomes segregate non‐randomly with the sex chromosome in the heterogametic sex. Segregation occurs according to size, small autosomes segregating with, and large autosomes segregating away from the X‐chromosome. Such sex‐biased transmission of autosomes could facilitate the spread of sexually antagonistic alleles whose effects favor the fitness of one sex at the expense of the other. This may provide a first step toward the evolution of new sex determination systems.

MECP2 duplications in six patients with complex sex chromosome rearrangements

Duplications of the Xq28 chromosome region resulting in functional disomy are associated with a distinct clinical phenotype characterized by infantile hypotonia, severe developmental delay, progressive neurological impairment, absent speech, and proneness to infections. Increased expression of the dosage-sensitive MECP2 gene is considered responsible for the severe neurological impairments observed in affected individuals. Although cytogenetically visible duplications of Xq28 are well documented in the published literature, recent advances using array comparative genomic hybridization (CGH) led to the detection of an increasing number of microduplications spanning MECP2. In rare cases, duplication results from intrachromosomal rearrangement between the X and Y chromosomes. We report six cases with sex chromosome rearrangements involving duplication of MECP2. Cases 1-4 are unbalanced rearrangements between X and Y, resulting in MECP2 duplication. The additional Xq material was translocated to Yp in three cases (cases 1-3), and to the heterochromatic region of Yq12 in one case (case 4). Cases 5 and 6 were identified by array CGH to have a loss in copy number at Xp and a gain in copy number at Xq28 involving the MECP2 gene. In both cases, fluorescent in situ hybridization (FISH) analysis revealed a recombinant X chromosome containing the duplicated material from Xq28 on Xp, resulting from a maternal pericentric inversion. These cases add to a growing number of MECP2 duplications that have been detected by array CGH, while demonstrating the value of confirmatory chromosome and FISH studies for the localization of the duplicated material and the identification of complex rearrangements.

A distinct chromosome race of the common shrew (Sorex araneus Linnaeus, 1758) within the Arctic Circle in European Russia

A unique arm chromosome combination detected in the common shrew karyotype from a Karelian locality within the Arctic Circle (Kandalaksha State Nature Reserve, European Russia) is interpreted as a new chromosome race Poyakonda. According to the standard nomenclature of G-banded chromosomes of Sorex araneus the karyotype contains the species specifi c bi-armed autosomes af, bc, jl, tu and the typical complex sex chromosome system (two metacentrics XX in the female and one metacentric/two acrocentrics XY1Y2 in the male). A group of race specifi c chromosomes includes 4 metacentric combinations (hn, ip, kq, mo) and two acrocentric chromosomes (g and r). In all 5 shrew specimens examined the same homozygous autosome complement 2Na=20 was found. Data available from neighbouring regions in Russia and, in particular, from adjacent Fennoscandian countries reveal complex relationships between karyotypes of common shrews from the European North.

105. MEIOTIC ACROBATS: MONOTREME SEX CHROMOSOME ORGANISATION DURING SPERMATOGENESIS

Monotremes feature an extraordinarily complex sex chromosome system which shares extensive homology with bird sex chromosomes but no homology to sex chromosomes of other mammals (1,2,3). At meiotic prophase I the ten sex chromosomes in platypus (nine in echidna) assemble in a sex chromosome chain. We previously identified the multiple sex chromosomes in platypus and echidna that form the meiotic chain in males (1,2,4). We showed that sex chromosomes assembly in the chain in a specific order (5) and that they segregate alternately (1). In secondary spermatocytes we observed clustering of X and Y chromosomes in sperm (6). Our current research investigates the formation of the synaptonemal complex, recombination and meiotic silencing of monotreme sex chromosomes. Meiotic sex chromosome inactivation (MSCI) has been observed in eutherian mammals, marsupials and birds but has so far not been investigated experimentally in monotremes. We found that during pachytene the X5Y5 end of the chain closely associates with the nucleolus and accumulates repressive chromatin marks (e.g. histone variant mH2A). In contrast to the differential accumulation of mH2A we observe extensive loading of the cohesin SMC3 on sex chromosomes in particular during the pachytene stage of meiotic prophase I. We have also used markers of active transcription and gene expression analysis to investigate gene activity in platypus meiotic cells. I will discuss how these findings contribute to our current understanding of the meiotic organisation of monotreme sex chromosomes and the evolution of MSCI in birds and mammals. (1) Grützner et al. (2004), Nature 432: 913–917.(2) Rens et al. (2007), Genome Biology 16;8(11): R243.(3) Veyrunes et al. (2008), Genome Research, 18(6): 995–1004.(4) Rens et al. (2004), Proceedings of the National Academy of Sciences USA. 101 (46): 16 257–16 261.(5) Daish et al. (2009), Reprod Fertil Dev. 21(8): 976–84.(6) Tsend-Ayush et al. (2009), Chromosoma 118(1): 53–69.

Observations on the behavioral pattern of the sex chromosome complex during spermatogenesis in man

Spermatocytes from testis biopsies that were squashed and stained according to the differential staining method gave, according to preliminary observations of the sex complex in pachytene, structural details that displayed variations in appearance in conjunction with the stage of pachytene. The possibility is discussed that detailed studies of the sex vesicle in pachytene might eventually complement clinical diagnosis in males with sex chromosome aberrations. Studies of sex bivalents in diakinesis and early metaphase I showed that the association between the X and Y chromosomes is not terminal, as it was maintained by several workers earlier, but that the medial part of the long arm of the Y chromosome is associated with the distal part of the short arm of the X chromosome. This association was not a point association, but one that occurred between chromosome segments. As a consequence of this, it seems that the existence of homologous segments in the X and Y chromosomes is highly probable.

Location, Location, Location! Monotremes Provide Unique Insights into the Evolution of Sex Chromosome Silencing in Mammals

Platypus and echidnas are the only living representative of the egg-laying mammals that diverged 166 million years ago from the mammalian lineage. Despite occupying a key spot in mammalian phylogeny, research on monotremes has been limited by access to material and lack of molecular genetic resources. This has changed recently, and the sequencing of the platypus genome has promoted monotremes into a generally accessible tool in comparative genomics. The most extraordinary aspect of the monotreme genome is an amazingly complex sex chromosomes system that shares extensive homology with bird sex chromosomes and no homology with sex chromosomes of other mammals. This raises important questions about dosage compensation of the five pairs of sex chromosomes in females and meiotic silencing in males, and we are only beginning to unravel possible mechanisms and pathways that may be involved. The homology between monotreme and bird sex chromosomes makes comparison between those species worthwhile, also as they provide a well-defined example where the same sex chromosomes changed from female heterogamety (chicken) to male heterogamety (monotremes). We summarize recent research on monotreme and chicken sex chromosomes and discuss possible mechanisms that may contribute to sex chromosome silencing in monotremes.

Platypus chain reaction: directional and ordered meiotic pairing of the multiple sex chromosome chain in Ornithorhynchus anatinus

Monotremes are phylogenetically and phenotypically unique animals with an unusually complex sex chromosome system that is composed of ten chromosomes in platypus and nine in echidna. These chromosomes are alternately linked (X1Y1, X2Y2, ...) at meiosis via pseudoautosomal regions and segregate to form spermatozoa containing either X or Y chromosomes. The physical and epigenetic mechanisms involved in pairing and assembly of the complex sex chromosome chain in early meiotic prophase I are completely unknown. We have analysed the pairing dynamics of specific sex chromosome pseudoautosomal regions in platypus spermatocytes during prophase of meiosis I. Our data show a highly coordinated pairing process that begins at the terminal Y5 chromosome and completes with the union of sex chromosomes X1Y1. The consistency of this ordered assembly of the chain is remarkable and raises questions about the mechanisms and factors that regulate the differential pairing of sex chromosomes and how this relates to potential meiotic silencing mechanisms and alternate segregation.

Higher-order genome organization in platypus and chicken sperm and repositioning of sex chromosomes during mammalian evolution

In mammals, chromosomes occupy defined positions in sperm, whereas previous work in chicken showed random chromosome distribution. Monotremes (platypus and echidnas) are the most basal group of living mammals. They have elongated sperm like chicken and a complex sex chromosome system with homology to chicken sex chromosomes. We used platypus and chicken genomic clones to investigate genome organization in sperm. In chicken sperm, about half of the chromosomes investigated are organized non-randomly, whereas in platypus chromosome organization in sperm is almost entirely non-random. The use of genomic clones allowed us to determine chromosome orientation and chromatin compaction in sperm. We found that in both species chromosomes maintain orientation of chromosomes in sperm independent of random or non-random positioning along the sperm nucleus. The distance of loci correlated with the total length of sperm nuclei, suggesting that chromatin extension depends on sperm elongation. In platypus, most sex chromosomes cluster in the posterior region of the sperm nucleus, presumably the result of postmeiotic association of sex chromosomes. Chicken and platypus autosomes sharing homology with the human X chromosome located centrally in both species suggesting that this is the ancestral position. This suggests that in some therian mammals a more anterior position of the X chromosome has evolved independently.

The Status of Dosage Compensation in the Multiple X Chromosomes of the Platypus

Dosage compensation has been thought to be a ubiquitous property of sex chromosomes that are represented differently in males and females. The expression of most X-borne genes is equalized between XX females and XY males in therian mammals (marsupials and “placentals”) by inactivating one X chromosome in female somatic cells. However, compensation seems not to be strictly required to equalize the expression of most Z-borne genes between ZZ male and ZW female birds. Whether dosage compensation operates in the third mammal lineage, the egg-laying monotremes, is of considerable interest, since the platypus has a complex sex chromosome system in which five X and five Y chromosomes share considerable genetic homology with the chicken ZW sex chromosome pair, but not with therian XY chromosomes. The assignment of genes to four platypus X chromosomes allowed us to examine X dosage compensation in this unique species. Quantitative PCR showed a range of compensation, but SNP analysis of several X-borne genes showed that both alleles are transcribed in a heterozygous female. Transcription of 14 BACs representing 19 X-borne genes was examined by RNA-FISH in female and male fibroblasts. An autosomal control gene was expressed from both alleles in nearly all nuclei, and four pseudoautosomal BACs were usually expressed from both alleles in male as well as female nuclei, showing that their Y loci are active. However, nine X-specific BACs were usually transcribed from only one allele. This suggests that while some genes on the platypus X are not dosage compensated, other genes do show some form of compensation via stochastic transcriptional inhibition, perhaps representing an ancestral system that evolved to be more tightly controlled in placental mammals such as human and mouse.

Cytogenetic Variations In Clarias Species (Clariidae: Surulifromis) Of The Anambra River Using Leucocytes Culture Techniques

Cytogenetic variations among four Clarias species from Anambra river, Nigeria were studied using leucocytes culture techniques. Heterogeneity in chromosome number (2n = 48 in Clarias ebriensis and C. albopunctatus to 2n = 56 in C. anguillaris and C. gariepinus) and Karyotype morphologies occurred among the clariids. The chromosomes were characterized by a high proportion of meta-submetacentric chromosomes and low proportion of acrocentric chromosomes. The females karyotype morphologies exhibited a heteromorphic pair suspected to be the sex chromosome complex. The following formulae were established for the male clariids; C. ebriensis 6m + 22sm + 20a FN =76; C. albopunctatus 4m ± 22sm + 22a FN = 74; C. gariepinus 8m + 24a FN = 88; and C. anguillaris 8m + 26m + 22a FN = 90. The female karyotype morphologies were C. ebriensis 6m + 23sm + 19a FN = 77, C. albopunctatus 4m ± 23sm + 21a FN = 75, C. gariepinus 8m + 25sm + 23a FN = 89 and C. anguillaris 8m + 27sm + 21a FN = 91. A generic chromosomal number of 2n = 54 + 4 for the clariids was suggested. The almost uniform karyotype morphologies and the closeness of the chromosome numbers around the generic chromosome number may suggest success with which the clariids may hybridize in nature.