Resolution and evolution of the duck-billed platypus karyotype with an X1Y1X2Y2X3Y 3X4Y4X5Y5 male sex chromosome constitution

Willem Rens*, Frank Grützner, Patricia C.M. O'Brien, Helen Fairclough, Jennifer A.M. Graves, Malcolm A. Ferguson-Smith

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    130 Citations (Scopus)


    The platypus (2n = 52) has a complex karyotype that has been controversial over the last three decades. The presence of unpaired chromosomes and an unknown sex-determining system especially has defied attempts at conventional analysis. This article reports on the preparation of chromosome-specific probes from flow-sorted chromosomes and their application in the identification and classification of all platypus chromosomes. This work reveals that the male karyotype has 21 pairs of chromosomes and 10 unpaired chromosomes (E1-E10), which are linked by short regions of homology to form a multivalent chain in meiosis. The female karyotype differs in that five of these unpaired elements (E1, E3, E5, E7, and E9) are each present in duplicate, whereas the remaining five unpaired elements (E2, E4, E6, E8, and E10) are absent. This finding indicates that sex is determined by the alternate segregation of the chain of 10 during spermatogenesis so that equal numbers of sperm bear either one of the two groups of five elements, i.e., five X and five Y chromosomes. Chromosome painting reveals that these X and Y chromosomes contain pairing (XY shared) and differential (X- or Y-specific) segments. Y differential regions must contain male-determining genes, and X differential regions should be dosage-compensated in the female. Two models for the evolution of the sex-determining system are presented. The resolution of the longstanding debate over the platypus karyotype is an important step toward the understanding of mechanisms of sex determination, dosage compensation, and karyotype evolution.

    Original languageEnglish
    Pages (from-to)16257-16261
    Number of pages5
    JournalProceedings of the National Academy of Sciences of the United States of America
    Issue number46
    Publication statusPublished - 16 Nov 2004


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