Structuraly diverged sex chromosomes have volved in many animals as well as in some plants. Sex chromosomes represent a specific genomis region(s) with locally suppressed recombination. As a consequence, repetitive DNA accumulates on the Y (W) chromosome structure in selected populations in teo closely realted dioecious species, Silene latifoila and S. dioica. Using flow cytometry, NGS analysis and bioinfrmatics we are investigating which elements are major players in genome size variability and Y chromosome expansion. Both intra- and interspecies differences in repeat distributions in the genome as well as in the Y chromosome will shed light on evolutionary forces regulating genome sie dynamics.