In mammals, sperm have evolved various shapes and contain elaborate acrosome and tail structures for efficient motility and fertilization. The MHC class I and class II genes are found on GGA 16, while framework genes that localize to the human class III interval map to GGA 17 ( Fillon et al. We have recently mapped MHC genes on platypus Y3×3 and Y4×5 ( Dohm et al. In human, the 3.6-Mb MHC complex is located on chromosome 6 ( Horton et al. Sequencing MHCs from different mammalian species have led to the identification of approximately 220 genes located within an interval of 3.5– 4 Mbp, which have important roles for example in immunity and reproduction. The major histocompatibility complex (MHC) is of central importance for adaptive and innate immunity in vertebrates. Another unusual feature of monotreme sex chromosomes is that they contain MHC genes.
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The recent sequencing of the entire platypus genome revealed that there is no homology between the mammalian X, and most of the genes from the human X that have been mapped in platypus reside on chromosome 6 ( Veyrunes et al. Surprisingly, the platypus sex chromosome system shows homology with the chicken Z ( Grutzner et al. The sex chromosomes of the chain undergo alternate segregation to form male-determining sperm with five Y chromosomes and female-determining sperm with five X chromosomes ( Grutzner et al. The platypus ( Ornithorhynchus anatinus) sex chromosome system is remarkably different to both therian mammals and birds in that there are ten sex chromosomes that form a meiotic chain in prophase I ( Grutzner et al. Comparative mapping revealed that the chicken Z and mammalian X do not share homology and most of the genes from the eutherian X are found on chicken chromosomes 1 and 4 ( Kohn et al. Male chickens have two Z chromosomes and female mammals two X chromosomes. In chicken, the heterogametic sex is the female (ZW), whereas in mammals it is the male (XY). Sex chromosome systems are different in birds and mammals. Recently, we described the localization of MHC genes on the smallest platypus chromosomes, which show homology to chicken MICs 16 and 17 ( Dohm et al. However, this suggestion was based on chromosomes size alone and has been challenged by Van Brink as both platypus and echidna show a continuum of chromosome size, while in reptiles there is a bimodal distribution of chromosomes that clearly separate as MICs and MACs ( Van Brink 1959). Monotremes are the only mammalian species that may share microchromosomes with reptiles ( Matthey 1949 White 1973). In interphase nuclei, MICs are found in the nuclear interior, while macrochromosomes (MACs) reside towards the nuclear periphery ( Habermann et al. One of the most obvious differences in genome organization between mammals and reptiles is the presence of a distinct set of small very gene-rich microchromosomes (MICs) and large gene-poor macrochromosomes ( McQueen et al. A general theme of genome organization in the interphase nucleus is that gene-rich chromosomes are organized preferentially towards the interior of the nucleus whereas gene-poor chromosomes are located more towards the exterior ( Cremer et al. This suggests that in some therian mammals a more anterior position of the X chromosome has evolved independently.Ī rapidly increasing body of evidence shows that higher-order organization in the nucleus is essential for a genome to function properly ( Cremer et al.
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Chicken and platypus autosomes sharing homology with the human X chromosome located centrally in both species suggesting that this is the ancestral position. In platypus, most sex chromosomes cluster in the posterior region of the sperm nucleus, presumably the result of postmeiotic association of sex chromosomes. The distance of loci correlated with the total length of sperm nuclei, suggesting that chromatin extension depends on sperm elongation. 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 use of genomic clones allowed us to determine chromosome orientation and chromatin compaction 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. We used platypus and chicken genomic clones to investigate genome organization in sperm. They have elongated sperm like chicken and a complex sex chromosome system with homology to chicken sex chromosomes. Monotremes (platypus and echidnas) are the most basal group of living mammals. In mammals, chromosomes occupy defined positions in sperm, whereas previous work in chicken showed random chromosome distribution.