MARSUPIAL SEX CHROMOSOME BEHAVIOUR DURING MALE MEIOSIS

Abstract

The absence of homology between sex chromosomes in marsupials strongly influences their behaviour during male meiosis. The highly differentiated X and Y chromosomes perform a precise and specific meiotic program that includes pairing and segregation, but lacks the usual mechanisms of synapsis, recombination and chiasma formation that occur in the autosomes and also in the sex chromosomes of eutherian mammals. The most relevant feature of marsupial male meiosis is the development of a synaptonemal complex-derived structure called the dense plate (DP). This structure maintains the association between the asynaptic and achiasmatic sex chromosomes during the first meiotic division, contributing to the proper segregation of sex chromosomes into daughter cells. Comparison of the meiotic mechanism present in marsupials with those present in some eutherian mammals opens new perspectives concerning the origin of sex chromosomes and sex chromosome segregation in the ancestor of marsupials and placental mammals. Similarly, recent characterisation of the mechanisms involved in the inactivation of sex chromosomes during marsupial meiosis has led to the idea that somatic inactivation of sex chromosomes in mammals may have originated from the more ancient and conserved mechanism of meiotic sex chromosome inactivation. This clearly places marsupial meiosis at the heart of the discussion concerning sex chromosome evolution and the origin of gene dosage compensation in mammals.