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Abstract
Sex chromosomes degenerate and evolve independently in the absence of recombination. Homologous chromosomes pair during meiosis and exchange genetic information in the form of crossovers and non-crossovers. However, this poses a challenge to sex chromosome in terms of pairing as well as double stranded break repair as they are typically only homologous at the small, shared region called pseudoautosomal region (PAR). Threespine sticklebacks (Gasterosteus aculeatus) have a young sex chromosomes system which is approximately 14 million years old and retains much of homology between sex chromosomes. While ancient sex chromosomes only pair at PAR and rarely form breaks outside PAR, we sought to address how young sex chromosomes navigate meiosis. In this dissertation the patterns of pairing, double stranded break formation on sex chromosomes and the mechanisms of repair of such breaks is explored. The sex chromosomes despite inversions and divergence were shown to pair fully and undergo synaptic adjustment in late prophase of meiosis I, revealing that the degeneration and divergence of Y chromosome is not sufficient to abrogate pairing in this system. The sex chromosomes also were shown to form double stranded breaks at the same rate and spatial pattern as autosomes. This indicates that DSBs are not suppressed in our system which is in contrast with ancient sex chromosomes on mice where DSB suppression on sex chromosomes is well documented. DSBs on sex chromosome systems were often found to be repaired through gene conversion between sex chromosomes in more recently formed strata or the regions where recombination was halted. Together these results show that meiotic nature of young sex chromosomes is similar to autosomes and ongoing genetic exchange between the sex chromosomes.