1. Research School of Biological Sciences, Australian National University, GPO Box 475, Canberra, Australian Capital Territory 2601, Australia
2. Centre for Veterinary Science, Department of Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge CB3 0ES, UK
3. Department of Biological Sciences, The University of Newcastle, Callaghan, New South Wales 2308, Australia
4. These authors contributed equally to this work

Correspondence to: Frank Grützner¹ Email: frank.gruetzner@anu.edu.au
The sequences reported in this paper have been deposited in EMBL under accession numbers AJ744847, AJ744848, AJ744849.

Abstract

Two centuries after the duck-billed platypus was discovered, monotreme chromosome systems remain deeply puzzling. Karyotypes of males¹, or of both sexes^{2, 3, 4}, were claimed to contain several unpaired chromosomes (including the X chromosome) that form a multi-chromosomal chain at meiosis. Such meiotic chains exist in plants⁵ and insects⁶ but are rare in vertebrates⁷. How the platypus chromosome system works to determine sex and produce balanced gametes has been controversial for decades^{1, 2, 3, 4}. Here we demonstrate that platypus have five male-specific chromosomes (Y chromosomes) and five chromosomes present in one copy in males and two copies in females (X chromosomes). These ten chromosomes form a multivalent chain at male meiosis, adopting an alternating pattern to segregate into XXXXX-bearing and YYYYY-bearing sperm. Which, if any, of these sex chromosomes bears one or more sex-determining genes remains unknown. The largest X chromosome, with homology to the human X chromosome, lies at one end of the chain, and a chromosome with homology to the bird Z chromosome lies near the other end. This suggests an evolutionary link between mammal and bird sex chromosome systems, which were previously thought to have evolved independently.

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