The multiple sex chromosomes of platypus and echidna are not completely identical and several share homology with the avian Z

Background

Sex-determining systems have evolved independently in vertebrates. Placental mammals and marsupials have an XY system, birds have a ZW system. Reptiles and amphibians have different systems, including temperature-dependent sex determination, and XY and ZW systems that differ in origin from birds and placental mammals. Monotremes diverged early in mammalian evolution, just after the mammalian clade diverged from the sauropsid clade. Our previous studies showed that male platypus has five X and five Y chromosomes, no SRY, and DMRT1 on an X chromosome. In order to investigate monotreme sex chromosome evolution, we performed a comparative study of platypus and echidna by chromosome painting and comparative gene mapping.

Results

Chromosome painting reveals a meiotic chain of nine sex chromosomes in the male echidna and establishes their order in the chain. Two of those differ from those in the platypus, three of the platypus sex chromosomes differ from those of the echidna and the order of several chromosomes is rearranged. Comparative gene mapping shows that, in addition to bird autosome regions, regions of bird Z chromosomes are homologous to regions in four platypus X chromosomes, that is, X₁, X₂, X₃, X₅, and in chromosome Y₁.

Conclusion

Monotreme sex chromosomes are easiest to explain on the hypothesis that autosomes were added sequentially to the translocation chain, with the final additions after platypus and echidna divergence. Genome sequencing and contig anchoring show no homology yet between platypus and therian Xs; thus, monotremes have a unique XY sex chromosome system that shares some homology with the avian Z.     

The gastric mucosa of two monotremes: the duck-billed platypus and echidna

The gastric mucosa of both the echidna and platypus is aglandular and the lining epithelium is stratified squamous. The latter exhibits three principle layers: stratum germinativum, stratum spinosum, and stratum corneum. The cytoplasm of cells composing the first two strata of both species shows bundles of tonofibrils and numerous free ribosomes. Cells of the stratum spinosum in the platypus also show numerous dense granules limited to the peripheral cytoplasm. The stratum spinosum of both species is comprised of fusiform-shaped cells whose adjacent cell membranes show extensive interlocking. The stratum spinosum of the echidna in addition shows numerous intercellular bridges. Cells of the stratum corneum become flattened and elongate and in the echidna nuclei near the surface appear to degenerate. Cells comprising the stratum corneum of the platypus exhibit well preserved nuclei and contain scattered large granules of varying electron density. Prior to sloughing, cells near the surface of both species show a separation of adjacent cell membranes. True keratinization is not found in the gastric lining epithelium of either species and the epithelium lining of the stomach of the echidna more closely represents a form of parakeratosis. Delicate papillae containing capillaries extend considerable distances into the overlying epithelium of both species and are thought to contribute to its nutrition.     

Antibody response to sheep red blood cells in platypus and echidna

There is limited information regarding the kinetics of antibody responses exhibited by the platypus and the echidna in response to a T cell dependent antigen. In this preliminary study a platypus, an echidna and a rabbit were inoculated with sheep red blood cells to compare their antibody responses and kinetics. The antibody titres, produced by the platypus and echidna, were less than those elicited in the rabbit. Furthermore, the echidna and platypus exhibited a weak secondary response. This was most likely due to a failure of the platypus and echidna to undergo the characteristic IgM to IgG isotype switch following second antigen exposure. The conformational structure of these antibodies may differ from eutherian antibodies. This was further supported by a heat sensitivity experiment that indicated that these antibodies are more labile than rabbit immunoglobulins and therefore structurally less stable.     

Gene mapping studies confirm the homology between the platypus X and echidna X1 chromosomes and identify a conserved ancestral monotreme X chromosome

The identification of the sex chromosomes in the three extant species of Prototherian mammals (the monotremes) is complicated by their involvement in a multivalent translocation chain at the first division of male meiosis. The platypus X chromosome, identified by the presence of two copies in females and one in males, has been found to possess a suite of genes that have been mapped to the X chromosomes of all eutherian and metatherian mammals. We have extended gene mapping studies to a member of the only other extant monotreme family, the echidna, which has a G-band equivalent X₁ chromosome, as well as a smaller X₂. We find that the five human X-linked genes (G6PD, GDX, F9, AR and MCF2) map to the echidna X₁ chromosome in locations equivalent to those on the platypus X. These results confirm that the echidna X₁ is the original X chromosome in this species, and identify a conserved ancestral monotreme X chromosome.     

Immunoglobulin genetics of Ornithorhynchus anatinus (platypus) and Tachyglossus aculeatus (short-beaked echidna)

In this paper, we review data on the monotreme immune system focusing on the characterisation of lymphoid tissue and of antibody responses, as well the recent cloning of immunoglobulin genes. It is now known that monotremes utilise immunoglobulin isotypes that are structurally identical to those found in marsupials and eutherians, but which differ to those found in birds and reptiles. Monotremes utilise IgM, IgG, IgA and IgE. They do not use IgY. Their IgG and IgA constant regions contain three domains plus a hinge region. Preliminary analysis of monotreme heavy chain variable region diversity suggests that the platypus primarily uses a single VH clan, while the short-beaked echidna utilises at least 4 distinct VH families which segregate into all three mammalian VH clans. Phylogenetic analysis of the immunoglobulin heavy chain constant region gene sequences provides strong support for the Theria hypothesis. The constant region of IgM has proven to be a useful marker for estimating the time of divergence of mammalian lineages.     

Chromosome chains and platypus sex: kinky connections

Mammal sex determination depends on an XY chromosome system, a gene for testis development and a means of activating the X chromosome. The duckbill platypus challenges these dogmas.(1,2) Gutzner et al.(1) find no recognizable SRY sequence and question whether the mammalian X was even the original sex chromosome in the platypus. Instead they suggest that the original platypus sex chromosomes were derived from the ZW chromosome system of birds and reptiles. Unraveling the puzzles of sex determination and dosage compensation in the platypus has been complicated by the fact that it has a surplus of sex chromosomes. Rather than a single X and Y chromosome, the male platypus has five Xs and five Ys.     

Iron(III) binding proteins of echidna (Tachyglossus aculeatus) and platypus (Ornithorhynchus anatinus)

Iron (III) binding proteins are isolated from echidna (Tachyglossus aculeatus multiaculeatus) and platypus (Ornithorhynchus anatinus) milk and blood. On the basis of several criteria it is shown that the milk proteins are not lactoferrins, but are transferrins similar to the corresponding transferrins from the blood. The heterogeneity of the proteins, particularly the echidna milk transferrin, is, at least in part, due to different levels of sialic acid. Their N-terminal sequences (30 residues) are determined and compared with those of other transferrins and lactoferrins. The role of the proteins is discussed.     

The platypus genome unraveled

The genome of the platypus has been sequenced, assembled, and annotated by an international genomics team. Like the animal itself the platypus genome contains an amalgam of mammal, reptile, and bird-like features.     

Meiosis and chromosome painting of sex chromosome systems in Ceboidea

The identity of the chromosomes involved in the multiple sex system of Alouatta caraya (Aca) and the possible distribution of this system among other Ceboidea were investigated by chromosome painting of mitotic cells from five species and by analysis of meiosis at pachytene in two species. The identity of the autosome #7 (X2) involved in the multiple system of Aca and its breakage points were demonstrated by both meiosis and chromosome painting. These features are identical to those described by Consigliere et al. [1996] in Alouatta seniculus sara (Assa) and Alouatta seniculus arctoidea (Asar). This multiple system was absent in the other four Ceboidea species studied here. However, data from the literature strongly suggest the presence of this multiple in other members of this genus. The presence of this multiple system among several species and subspecies that show high levels of chromosome rearrangements may suggest a special selective value of this multiple. The meiotic features of the sex systems of Aca and Cebus apella paraguayanus (Cap) are strikingly different at pachytene, as the latter system is similar to the sex pair of man and other primates. The relatively large genetic distances between species presently showing this multiple system suggest that its origin is not recent. Other members of the same genus should be investigated at meiosis and by chromosome painting in order to know the extent and distribution of this complex sex-chromosome system.     

The sex chromosome that refused to die

Chromosomes that harbor dominant sex determination loci are predicted to erode over time--losing genes, accumulating transposable elements, degenerating into a functional wasteland and ultimately becoming extinct. The Drosophila melanogaster Y chromosome is fairly far along this path to oblivion. The few genes on largely heterochromatic Y chromosome are required for spermatocyte-specific functions, but have no role in other tissues. Surprisingly, a recent paper shows that divergent Y chromosomes can substantially influence gene expression throughout the D. melanogaster genome.1 These results show that variation on Y has an important influence on the deployment of the genome.     

Transition in sexual system and sex chromosome evolution in the tadpole shrimp Triops cancriformis

Transitions in sexual system and reproductive mode may affect the course of sex chromosome evolution, for instance by altering the strength of sexually antagonistic selection. However, there have been few studies of sex chromosomes in systems where such transitions have been documented. The European tadpole shrimp, Triops cancriformis, has undergone a transition from dioecy to androdioecy (a sexual system where hermaphrodites and males coexist), offering an excellent opportunity to test the impact of this transition on the evolution of sex chromosomes. To identify sex-linked markers, to understand mechanisms of sex determination and to investigate differences between sexual systems, we carried out a genome-wide association study using restriction site-associated DNA sequencing (RAD-seq) of 47 males, females and hermaphrodites from one dioecious and one androdioecious population. We analysed 22.9 Gb of paired-end sequences and identified and scored >3000 high coverage novel genomic RAD markers. Presence–absence of markers, single-nucleotide polymorphism association and read depth identified 52 candidate sex-linked markers. We show that sex is genetically determined in T. cancriformis, with a ZW system conserved across dioecious and androdioecious populations and that hermaphrodites have likely evolved from females. We also show that the structure of the sex chromosomes differs strikingly, with a larger sex-linked region in the dioecious population compared with the androdioecious population.     

The sex chromosome system can influence the evolution of sex‐biased dispersal

Sex‐biased dispersal is a much‐discussed feature in literature on dispersal. Diverse hypotheses have been proposed to explain the evolution of sex‐biased dispersal, a difference in dispersal rate or dispersal distance between males and females. An early hypothesis has indicated that it may rely on the difference in sex chromosomes between males and females. However, this proposal was quickly rejected without a real assessment. We propose a new perspective on this hypothesis by investigating the evolution of sex‐biased dispersal when dispersal genes are sex‐linked, that is when they are located on the sex chromosomes. We show that individuals of the heterogametic sex disperse relatively more than do individuals of the homogametic sex when dispersal genes are sex‐linked rather than autosomal. Although such a sex‐biased dispersal towards the heterogametic sex is always observed in monogamous species, the mating system and the location of dispersal genes interact to modulate sex‐biased dispersal in monandry and polyandry. In the context of the multicausality of dispersal, we suggest that sex‐linked dispersal genes can influence the evolution of sex‐biased dispersal.     

Effects of sex chromosome dosage on corpus callosum morphology in supernumerary sex chromosome aneuploidies

Background

Supernumerary sex chromosome aneuploidies (sSCA) are characterized by the presence of one or more additional sex chromosomes in an individual’s karyotype; they affect around 1 in 400 individuals. Although there is high variability, each sSCA subtype has a characteristic set of cognitive and physical phenotypes. Here, we investigated the differences in the morphometry of the human corpus callosum (CC) between sex-matched controls 46,XY (N =99), 46,XX (N =93), and six unique sSCA karyotypes: 47,XYY (N =29), 47,XXY (N =58), 48,XXYY (N =20), 47,XXX (N =30), 48,XXXY (N =5), and 49,XXXXY (N =6).

Methods

We investigated CC morphometry using local and global area, local curvature of the CC boundary, and between-landmark distance analysis (BLDA). We hypothesized that CC morphometry would vary differentially along a proposed spectrum of Y:X chromosome ratio with supernumerary Y karyotypes having the largest CC areas and supernumerary X karyotypes having significantly smaller CC areas. To investigate this, we defined an sSCA spectrum based on a descending Y:X karyotype ratio: 47,XYY, 46,XY, 48,XXYY, 47,XXY, 48,XXXY, 49,XXXXY, 46,XX, 47,XXX. We similarly explored the effects of both X and Y chromosome numbers within sex. Results of shape-based metrics were analyzed using permutation tests consisting of 5,000 iterations.

Results

Several subregional areas, local curvature, and BLDs differed between groups.Moderate associations were found between area and curvature in relation to the spectrum and X and Y chromosome counts. BLD was strongly associated with X chromosome count in both male and female groups.

Conclusions

Our results suggest that X- and Y-linked genes have differential effects on CC morphometry. To our knowledge, this is the first study to compare CC morphometry across these extremely rare groups.

     

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.