Platypus have four male-specific chromosomes that segregate together at meiosis

Abstract

More than 200 years after the discovery of the platypus, monotreme chromosomes remain deeply puzzling. Karyotypes of both sexes were claimed to contain a set of chromosomes for which no homologues could be identified. At male meiosis these presumably translocated chromosomes assemble in a multivalent chain involving the X chromosome. Such a system of translocation heterozygosity is unprecedented in vertebrates, although similar systems are found naturally in some plants and a few social insects. Mice heterozygous for autosomal translocations can be deliberately bred, but have severe meiotic defects and produce high proportions of aneuploid sperm. The number, identity and homology relationships of the translocated chromosomes have been controversial over the past 30 years, as has their role in sex determination and their segregation to form balanced gametes. In order to solve the complex chromosome system in platypus we have generated chromosome paints for 20 platypus chromosomes and hybridised these paints on male and female metaphase chromosomes as well as meiotic cells and sperm. Five paints show a different hybridisation pattern on male and female chromosomes. Four chromosomes (including the X) were identified that are present in one copy in males and in two copies in females. Additionally, four male-specific chromosomes have been identified that detect no homologues in females, but share sequences with other unpaired chromosomes. One of these has complete homology to the short arm of the X. BAC clones from one of the male specific chromosomes contain male-specific sequences. Position of these chromosomes at different meiotic stages revealed a consistent order of chromosomes within the meiotic chain. Co-localisation of these elements on mature sperm provides the first direct evidence that two different sperm types are produced as a result of alternate segregation in platypus.