Poster Presentation The Annual Scientific Meeting of the Endocrine Society of Australia and the Society for Reproductive Biology 2014

Non-random segregation of platypus chromosome 6 at male meiosis (#320)

Deborah TOledo Flores 1 , Frank Gr├╝tzner 1
  1. University of Adelaide, North Adelaide, SA, Australia

Chromosome heteromorphism can lead to non-random segregation at meiosis (Wang et al., 2010). A common example of autosomal hetermorphism is observed in chromosomes carrying the Nucleolar Organiser Region (NOR), which is a region of tandemly repeated rDNA clusters. Differences in the number of clusters, different transcriptional activity and DNA methylation levels between homologous NORs lead to size heteromorphisms. In the platypus, chromosome 6 contains a large heteromorphic NOR. Interestingly, this chromosome is homologous to the therian X-chromosome. Therefore, we were very interested in investigating whether this size difference may lead to non-random segregation of chromosome 6. Firstly, we observed that the heteromorphism exists in both sexes being more prominent in females. 5-Aza-dC treatment showed that chromosome 6 hypermethylation contributes to the heteromorphism in females but not in males and silver staining indicated that both NORs are active but revealed a gap in the males NOR. This showed that sex specific differences exist between the NORs on the chromosome 6 homologs. In our segregation studies on male meiosis we found that the chromosome 6 carrying the short NOR segregates preferentially with X-bearing sperm whereas Y-bearing sperm carries the short and long chromosome 6 in equal proportions. Negative selection maybe acting against the X-bearing sperm that carries the long chromosome 6. Supporting this idea, we have observed consistently lower counts of X-bearing sperm than Y-bearing sperm in three male individuals. Together this study provides the first evidence of non-random segregation of an autosome in the platypus as well as sex-specific NOR differences. The mechanisms behind this segregation bias as well as its exact link with the observed NOR differences remain to be investigated.

  1. Wang, J., Chen, P.J., Wang, G.J., and Keller, L. (2010). Chromosome size differences may affect meiosis and genome size. Science 329, 293.